The first Neanderchimp?
McBrearty and Jablonski (2005) report on the first discovery of chimpanzee fossil remains. The described fossils are three teeth: left and right upper central incisors and an upper molar, probably M1. They may well represent a single individual, although this cannot be said for certain. The paper also hints at another, "aberrant right upper third molar", but does not describe it.
The teeth are morphologically within the range of living chimpanzees and show no strong differences from them.
The similarity in the array of fossils encountered in K3 and K3' sediments suggests that Middle Pleistocene Pan and Homo lived, or at least died, in broadly similar environmental settings. Taken together, the evidence suggests a locally wooded habitat on the shore of an alternately fresh and saline-alkaline lake, fluctuating lake levels, ephemeral nearshore fluvial channels, a nearby freshwater spring, and a semi-arid climatic regime. These conditions are not unlike those found near the shore of Lake Baringo today, although dense human populations have eliminated much of the woodland that formerly supported chimpanzees and the faunal community of which they were a part.
The paper suggests a mystery: chimpanzees were living 500,000 years ago in a place where there aren't any chimpanzees now. The conclusion seems a bit of a stretch, but for all we know may have been true:
Representatives of both Homo and Pan are present in the same stratigraphic interval of the Kapthurin Formation at sites only 1 km apart, and faunal data suggest that they occupied broadly similar environments in the Middle Pleistocene. This evidence shows that in the past chimpanzees occupied regions in which the only hominoid inhabitants were thought to have been members of the human lineage. Now that chimpanzees are known to form a component of the Middle Pleistocene fauna in the Rift Valley, it is quite possible that they remain to be recognized in other portions of the fossil record there, and that chimpanzees and hominins have been sympatric since the time of their divergence.
However, the paper doesn't probe what I think may be a bigger mystery. Today's east African subspecies of chimpanzees, Pan troglodytes schweinfurthii has a range extending into Uganda, and historically they extended yet farther east. Although this is not as far as the fossil locality, it is not that long a distance. But this comparison assumes that chimpanzees 500,000 years ago had the same range as today.
Genetic evidence suggests that they didn't--at least not the present P. t. schweinfurthii. Gagneux et al. (2001) suggest that a low level of differentiation between east African (P. t. schweinfurthii) and central African (P. t. troglodytes) chimps may indicate that their time of separation is more recent than mtDNA can adequately resolve. And east African chimpanzees are highly restricted in their mtDNA variation compared to other subspecies (Goldberg et al. 1997). Together, these points may suggest a recent colonization of the eastern chimpanzee range by central African chimpanzees. Gagneux et al. (2001:891) suggest that the divergence of these populations may have occurred between 100,000 and 300,000 years ago -- a date based on the negative evidence of the resolving power of mtDNA sequences, not positive evidence.
But if today's P. t. schweinfurthii got into east Africa only recently, then what kind of individual do these fossils represent?
Here's a hypothesis: it's the chimpanzee version of a Neandertal -- a NEANDERCHIMP! An ancient chimpanzee subpsecies that no longer exists once extended across the eastern range of the species -- and in fact, even further east into the Eastern Rift. Sometime within the past 100,000 to 300,000 years, these chimpanzees were replaced by the ancestors of P. t. schweinfurthii, who came from central Africa. And these central African chimpanzees themselves may have come from the western part of the chimpanzee range during the past 500,000 to 1,000,000 years. This hypothesis envisages the spread of modern chimpanzees from a west African source within the past half million years.
On the other hand, we might just accept that alleles have spread into east Africa without the spread of populations. The limited mtDNA diversity of east African chimpanzees might be a marker of selection, rather than population replacement. This would also account for the apparent genetic similiarities between east and central African chimpanzees, as well as the sharing of markers among the west and central African populations. It's not nearly as sexy a model as the proto-Neander-chimp idea, but it does allow this wayward Middle Pleistocene individual to be part of a population ancestral to living chimpanzees. And it does give precedence to nuclear genes (about which we know little) instead of mtDNA (about which we know much, but what we know may well be biased by selection). And it acknowledges the fact that no one has yet sampled DNA diversity along most "subspecies boundaries" in chimpanzees, so we actually don't know if the subspecies are as differentiated as the distant sampling points would indicate by themselves.
So the Neanderchimp model is not the null hypothesis -- gene flow and selection in chimpanzees would be the safe guess here. But it shows a nice symmetry with the case of humans, don't you think? And it raises the central question about this fossil individual very nicely -- what is it, and which chimpanzees are its living relatives?
What about bonobos? Do they figure into this scenario? As far as these fossils go, probably not:
Specific diagnosis of isolated teeth within Pan, however, must be approached with caution, and for this reason we assign the Kapthurin Formation specimens to Pan sp. indet. Non-metric characters that have been suggested as diagnostic criteria for P. troglodytes, such as a more quadrilateral outline shape to the upper central incisor crowns and a better expressed hypocone on the maxillary molars, seem to suggest more similarity for the Kapthurin Formation fossils to P. troglodytes than to P. paniscus, but these features are variably expressed among the living species and subspecies of Pan. Although mean tooth size is known to be significantly smaller in P. paniscus than in P. troglodytes, size ranges overlap (Table 1). Furthermore, apart from the present specimens, we lack a fossil record for the Pliocene and Pleistocene from which to assess past variability within the genus, and it is feasible that the Kapthurin Formation fossils represent members of an extinct lineage within the genus Pan.
But the speciation of bonobos and common chimpanzees occurred sometime between 500,000 and 1.5 million years ago or so, meaning that this divergence probably must be explained by the same kinds of biogeographic phenomena that explain the recent divergence of chimpanzee subspecies.
Only time will tell if the Neanderchimp model will come into vogue. But you heard it here first.
References:
Gagneux P, Gonder MK, Goldberg TL, Morin PA. 2001. Gene flow in wild chimpanzee populations: what genetic data tell us about chimpanzee movement over time and space. Phil Trans R Soc Lond B 356:889-897.
Goldberg TL, Ruvolo M. 1997. Molecular phylogenetics and historical biogeography of east African chimpanzees. Biol J Linn Soc 61:301-324.
McBrearty S, Jablonski NG. 2005. First fossil chimpanzee. Nature 437:105-108. Full text (subscription required)
Mothers' sons
I want to note the new article by Langergraber, Mitani and Vigilant on chimpanzee kinship and social behavior. The paper finds that chimpazee males tend to affiliate with their maternal brothers.
The result of the paper is different from previous work that had shown no significant association between maternal kinship and affiliations for males (e.g., Mitani et al. 2000; Mitani et al. 2002). This discrepancy is surprising, since the other studies involved the same community at Kibale. What the authors found was that the mtDNA was a poor indicator of maternal kin relationships, because haplotypes are more widely shared than within single matrilines. So previous work that typed mtDNA didn't identify maternal kin accurately.
The article creates some unnecessary confusion about kin selection. The authors note that the traditional explanation for cooperation among male chimpanzees emphasizes kin selection as a mechanism:
Cooperation in primates and other animals has frequently been attributed to kin selection, a process whereby individuals cooperate with relatives and gain indirect fitness benefits through the reproduction of kin (6-9). For example, in many Old World cercopithecine monkey species, females remain in their natal groups their entire lives and behave nepotistically toward their sisters and other maternal relatives, whereas males disperse among groups and cooperate less with each other (10). In contrast, male chimpanzees are philopatric and are much more affiliative and cooperative than female chimpanzees (11). Given these observations, it is not surprising that kin selection has historically been assumed to play a central role in the evolution of male chimpanzee cooperation (12, 13). Nevertheless, studies to date do not provide any evidence that male chimpanzees bias their social behavior toward maternal brothers (14 â16), who should be readily recognizable by virtue of the life-long bonds they form with shared mothers (13) (Langergraber et al. 2007:1)
This is a bit confusing, because it is by no means necessary for chimpanzees (or other species) to directly interact with their kin for a behavior to evolve by means of kin selection. What is necessary is that their kin benefit from the behavior. Considering the long life histories of chimpanzees, the long-term fitness effects of behaviors on other individuals are not obvious, so the fact that coalitional behavior does not have a kin bias is not exclusive of kin selection as a mechanism. As an example, an individual who polices the group by suppressing aggressive interactions may benefit his kin without any direct interaction -- or even with antagonistic interactions with his kin.
In any event, the paper finds that maternal brothers do affiliate more with each other than expected by chance. It's comforting to see this result, since maternal brothers have a shared genetic interest in each other. In a game theoretical perspective, the benefits of affiliation ought to be higher for kin, and the fitness costs lower. Considering that chimpanzees recognize their maternal kin, they ought to make this distinction.
That is not to say that a male chimpanzee should always favor his maternal brothers -- they clearly do not do so in all contexts. The indirect fitness benefit from kin selection is weak, compared to the direct fitness benefit of status, and it may be more important to exchange affiliation with other males of equal or higher status or age. Since maternal brothers in chimpanzees are at least 4-5 years apart (and often much more, since sisters may come between!) they are not especially likely to be near each other in rank.
The paper spends a lot of time explaining why paternal sibling effects are not observed. The short answer is that males cannot recognize their paternal brothers. The review of paternal kin recognition and nepotism takes a couple of paragraphs, which might be valuable for those interested.
References:
Langergraber KE, Mitani JC, Vigilant L. 2007. The limited impact of kinship on cooperation in wild chimpanzees. Proc Nat Acad Sci USA (online early) doi:10.1073/pnas.0611449104
Mitani JC, Merriwether DA, Zhang C. 2000. Male affiliation, cooperation and kinship in wild chimpanzees. Anim Behav 59:885-893.
Mitani JC, Watts DP, Pepper JW, Merriwether DA. 2002. Demographic and social constraints on male chimpanzee behavior. Anim Behav 64:727-737.
The chimpanzee sex exchange
Duffy, Wrangham and Silk describe in a short paper the correlation of male mating success and support for the alpha male in a group of chimpanzees. The finding isn't surprising -- it's been long assumed that subordinate male chimpanzees participate in coalitions to attain mating rewards -- but it's a nice quantification. Helping the alpha male maintain his status had a much larger impact on mating success than rank.
References:
Duffy KG, Wrangham RW, Silk JB. 2007. Male chimpanzees exchange political support for mating opportunities. Curr Biol 17:R586-R587. doi:10.1016/j.cub.2007.06.001
Broca's area and chimpanzee communication
Chimpanzees use their own version of Broca's area when they communicate, according to a new PET scan study by Jared Taglialatela and colleagues. The abstract:
Broca's area, a cerebral cortical area located in the inferior frontal gyrus (IFG) of the human brain, has been identified as one of several critical regions associated with the motor planning and execution of language. Anatomically, Broca's area is most often larger in the left hemisphere, and functional imaging studies in humans indicate significant left-lateralized patterns of activation during language-related tasks [1], [2] and [3]. If, and to what extent, nonhuman primates, particularly chimpanzees, possess a homologous region that is involved in the production of their own communicative signals remains unknown. Here, we show that portions of the IFG as well as other cortical and subcortical regions in chimpanzees are active during the production of communicative signals. These findings are the first to provide direct evidence of the neuroanatomical structures associated with the production of communicative behaviors in chimpanzees. Significant activation in the left IFG in conjunction with other cortical and subcortical brain areas during the production of communicative signals in chimpanzees suggests that the neurological substrates underlying language production in the human brain may have been present in the common ancestor of humans and chimpanzees.
I wrote a couple of years ago about the Broca's area homolog in macaques, and the involvement of the area in planning time-sensitive action sequences in people. Those studies clearly foreshadowed the current result, since they provide both a phylogenetic expectation that this brain area evolved early in anthropoid evolution (or earlier), and the functional expectation that motor sequences characteristic of communication depend on it.
There is some uncertainty in the current analysis, because the PET scanning method doesn't localize the increased activity as tightly as they would like:
Although these data indicate that the left IFG is involved in the production of communicative signals in chimpanzees, cytoarchitectonically, it is not clear what cell types fully comprise this region [36]. Therefore, it is not possible to determine whether or not the neuronal metabolic activity reported in this study corresponds to an area within the chimpanzee IFG that contains Brodmann's area 44/45 cells -- those cells that comprise Broca's area in humans. In fact, additional areas of significant activation are observed in the frontal orbital gyrus and the frontal pole (Figure 2). Additional work is needed to explore the significance of these areas of activation.
Nice piece of work. I wouldn't want to be the one to get a chimp into a scanner...
References:
Taglialatela JP, Russell JL, Schaeffer JA, Hopkins WD. 2008. Communicative signaling activates 'Broca's' homolog in chimpanzees. Curr Biol 18:1-6. doi:10.1016/j.cub.2008.01.049
Chimpanzee facial expressions
There's a nice little article on the topic from Reuters:
CHICAGO - The arch of an eyebrow or the curve of a lip tells chimps a lot about each other, a finding that may give scientists new understanding about the evolution of human communication, researchers reported Friday.
Human faces can be easy to read, but sometimes people must look in different places on the face to get an accurate picture.
"What we know from humans is that even a single movement added to an expression can change the entire meaning," said Lisa Parr, director of the Yerkes National Primate Research Center at Emory University in Atlanta. "It can significantly affect the outcome of interactions."
The article describes Parr's work classifying facial expressions with the help of a computer program, and getting chimpanzees to try to identify expressions from cartoon images. It alludes to the difficulty of scoring what is essentially a continuous range of variation in expressions -- one of the reasons why the "analog" system of facial expressions poses interpretive difficulties.
There's no link to the original research, though -- if anybody knows where it is appearing, please let me know!
The Bili chimpanzees
A nice piece in The Guardian about the chimpanzee population near Bili, DRC. The lede is the suspicion of an apparent leopard kill -- that's chimpanzees killing a leopard -- but the other details are interesting:
[Cleve] Hicks said the animals also have what he calls a "smashing culture" - a blunt but effective way of solving problems. He has found hundreds of snails and hard-shelled fruits smashed for food, seen chimps carrying termite mounds to rocks to break them open and also found a turtle that was almost certainly smashed apart by chimps.
Like chimp populations in other parts of Africa, the Bili chimps use sticks to fish for ants, but here the tools are up to 2.5 metres long.
All these observations of chimpanzee behavioral diversity are pretty exciting, since they really provide an interesting model of early hominid diversification. Longstanding subspecies-level populations with strong behavioral differences involving food collection and diet may describe both A. afarensis (along with regional variants) and Late Pliocene Homo.
A mention is in the article about whether they should be considered a new (fifth) chimpanzee subspecies. I'd say that's probably a given at this point; doubtless the Fongoli chimpanzees will become a sixth.
Do they deserve it? Well, I suppose they're at least as distinct as the others behaviorally and anatomically, which isn't saying much. Genetically, it's an open question so far, but I wouldn't be surprised if they were as distinct as central and eastern chimpanzees, and certainly moreso than P. t. vellerosus. It's hard for me to see where ape subspecies taxonomy is going to end, since there are few scientific interests vested against further taxonomizing. Sure, there are conservatives, but none with enough firepower to roll back P. t. vellerosus, apparently. So, more subspecies lie in our future: I would guess two or three more for Bornean orangutan populations and who knows how many Western lowland gorilla populations will qualify?
(via Gene Expression)
When chimpanzees stand
The current (February 2006) issue of AJPA carries an article by Craig Stanford describing the context of bipedal posture for chimpanzees in the Bwindi Impenetrable National Park. When considering how bipedal locomotion evolved in early hominids, it is an essential comparison how chimpanzees (or other hominoids) use bipedal postures. Stanford writes:
As Hunt (1994, 1996) pointed out, hypotheses for the advent of bipedalism that involve behaviors in which prehominids may have frequently engaged offer the most plausible explanations for the adaptive shift from quadrupedal to bipedal posture (Stanford 2006:225).
Stanford was able to observe a large number of episodes of bipedal posture in the study group -- 179 cases in 247 observation hours. I find the context to be the most interesting result:
All 179 instances of bipedalism were recorded while chimpanzees were foraging in large trees. All but one instance occurred as postural rather than locomotor bipedalism, and 96% of all instances occurred in a feeding context....Chimpanzees appeared to forage bipedally most often when feeding in the upper portion of the crown, reaching up to branches emergent in the sunlight, and perhaps containing harder-to-reach ripe fruit. (Stanford 2006:227).
Studies of bipedal posture in wild chimpanzees have been rare, as Stanford reviews, but have typically found fewer instances of bipedality and have included some terrestrial cases. The key finding of all studies appears to be that foraging for fruit is the main reason why chimpanzees occasionally stand.
What do the chimpanzees tell us about early hominids? Here is the suggestion:
The behavior of wild chimpanzees suggests that several aspects of the positional behavior of earliest hominids may have been given less attention that they merit. First, arboreal bipedal posture is not dichotomous with arboreal quadrupedal posture. Bwindi chimpanzees moved fluidly between four-legged, three-legged, and two-legged postures while feeding in tree crowns. Their use of three-dimensional space in tree tops incorporated elements of positional behavior most often seen as binary states. This fluid quadrupedal-bipedal shifting may have occurred in the earliest hominids as well. Arguments about whether early hominids were fully adapted to bipedal walking, or facultatively arboreal, have been carried on for at least three decades (Susman et al., 1984; Lovejoy, 1988). Recent evidence suggests that knuckle-walking may have been employed by the immediate ancestors of the australopithecines (Richmond and Strait, 2000). Chimpanzee bipedal behavior suggests that early hominids likely engaged in a fluid variety of positional behaviors and postures, but provides little evidence for the adaptive advantage of terrestrial knuckle-walking in the last common ancestor of apes and humans (Stanford 2006:230).
Now, humans are fully adapted to bipedal walking, and we are facultatively arboreal (that is, we can climb trees), so there is no reason to think that early hominids were less facultatively arboreal than we are, and I would venture that they were probably a good deal more so.
The fundamental question about early hominids is why they abandoned the ability to be facultative quadrupeds. That is something that chimpanzee positional behavior isn't going to tell us -- after all, chimpanzees take on bipedal posture in ways that don't compromise their quadrupedal abilities.
The chief importance of the chimpanzee comparison is to illustrate the kinds of ways that locomotor diversity occur in hominoids. After a brief discussion of locomotor flexibility in gorillas, Stanford concludes with this:
Rose (1984) argued that there is no reason to view the origin of bipedalism as a progression from "poor biped" to "good biped." Instead, there was likely a diversity of forms of bipedalism in the earliest hominids. One such hominoid example may be Oreopithecus bambolii, a sup-
posed bipedal ape (Kohler and Moya-Sola, 1997). The bipedal evidence from Bwindi, Mahale, and Gombe supports this view of early hominid evolution. Instead of viewing the earliest bipedal adaptation as the lowest
rung on a posture/locomotion evolutionary ladder, it may be that early hominid species evolved a variety of forms of bipedalism in particular ecological contexts (Stanford 2006:230).
I guess that is one possibility to explain evidence of vertical posture in early hominids in the absence of good evidence of bipedality (from postcranial evidence).
The "diversity of forms" argument really suggests a stage during early hominid evolution when the ability to be effective quadrupeds had not yet been lost. Perhaps we will find these quadrupedal hominids. Perhaps we already have. On the other hand, this idea opposes the hypothesis that locomotor evolution may have either caused the origin of the hominid lineage or very closely followed it.
It seems to me that the level of species diversity of early hominids and this locomotor problem may be strongly linked. But I think they might be linked in the opposite direction than one might assume.
Suppose, for instance, that the hominid lineage arose as an adaptive radiation resulting from a significant new adaptation for bipedality. The "adaptive radiation" would be the origin of many new bipedal species spreading and adapting to different ecologies. Early hominid species diversity would be a consequence of their novel locomotor adaptation.
In contrast, if hominids originated as one among many quadrupedal apes in the Late Miocene, they might well have adapted over a long time as quadrupeds within a single ecology to which they remained limited. Perhaps the attainment effective bipedality would have spurred an adaptive radiation, but this event would have followed long after the origin of the hominids. Hominid species diversity might have always been low, or might have remained low until the Late Pliocene.
Now I don't think any of these arguments can be taken very far. It is always possible that bipedality arose early without any consequent adaptive radiation, or that there were multiple bipedal ape lineages other than hominids, or almost any other combination of events. There just isn't fossil evidence that could delimit hypotheses about hominid origins.
But I can't think about diversity without considering the mechanisms for it to have arisen. And while it is possible that many hominoid lineages were experimenting with bipedal posture and locomotion in diverse ways, I can't think of what would have caused the diversification of a large array of hominid species in the absence of bipedality.
References:
Stanford CB. 2006. Arboreal bipedalism in wild chimpanzees: Implications for the evolution of hominid posture and locomotion. Am J Phys Anthropol 129:225-231. Abstract
Bonobo blowback
Following on last month's New Yorker article about bonobos (my comments here), Frans de Waal has penned a response (no permanent link yet; this link may stop working after a week or two). De Waal points out what he sees as a political agenda in Ian Parker's article:
The main message of Parker's piece could of course have been that fieldwork is no picnic, but instead he went for profound revelation: bonobos are not nearly as nice and sexual as they have been made out to be. Given that the bonobo's reputation has been a thorn in the side of homophobes as well as Hobbesians, the right-wing media jumped with delight. The bonobo "myth" could finally be put to rest. Parker's piece was gleefully picked up by The Wall Street Journal and Dinesh D'Souza (yes, the same one who blamed 9/11 on the left), who accused "liberals" of having fashioned the bonobo into their mascot. D'Souza urged them to stick with the donkey.
This might all have been amusing if it weren't for the fact that these are not just political skirmishes. At issue is what we know. Parker presented his trip as a fact-finding mission that had unearthed revolutionary new insights. His message was that bonobos are killer apes, just like their cousins, the chimpanzees. The animal kingdom remained "red in tooth and claw," as it ought to be.
I have to say, I don't buy this idea that the New Yorker is sending out right-wing marching orders. Still, de Waal's point is well-taken -- certainly, there are many who want to push the idea that primates are not peaceful gentle creatures, because it confirms their own view of human nature. Certainly, bonobos are a target for such beliefs, since people have a perception of bonobos that is way beyond their real behavior. Up to now, that perception has fit a stereotype of peace-loving hippie primates. As de Waal points out, that stereotype has gotten them attention, increasing the opportunities to research them. But pushing the stereotype always risks that someone will pop the bubble.
I don't think that Parker's article was very bubble-popping; it was certainly a lot more reasonable than most of the treatments of science in the New Yorker, or Atlantic, or any number of other "thinking" magazines. The shift in the scientific view of bonobos that he describes is genuine. For example, a 2004 book review by Jennifer Rybak includes this passage:
In the process of identifying the variability among chimpanzee and bonobo populations, it becomes clear that the distinction between the two species is not as clear-cut as previously thought. While certain species distinctions still hold true, such as female- vs. male-dominated societies, and differences in tool use, other distinctions are brought into question. This is largely because the taxonomic grouping of these two species has been based on incomplete behavioral data. However, the data presented in this section illustrate that as we learn more about population-specific differences within each species, the distinction between the species becomes more blurred (Figs. 1.2-1.5, p. 23-24). For instance, while Mahale and Gombe chimpanzees are taxonomically similar, and Wamba and Lomako bonobos are similar, the chimpanzee populations of Bossou and Taï forest vary in their placement along the taxonomic spread. As an additional example of these behavioral distinctions exemplified throughout the book, Matsumoto-Oda (Chapter 12) suggests that "the gregariousness seen at Mahale might not be characteristic of chimpanzees in general" (p. 177). Since a great deal of our understanding of these species is based on a few specific groups, it seems that the full breadth of Pan behavior is not yet well understood. Thus, it becomes obvious to the reader that it is difficult to say whether or not chimpanzees and/or bonobos have specific behavioral repertoires, a theme present throughout the remainder of the book.
That's in a review of the 2002 book Behavioural Diversity in Chimpanzees and Bonobos, by Christophe Boesch, Gottfried Hohmann and Linda Marchant. The book's theme is that our perception of bonobo-chimpanzee differences is changing as a result of our greater understanding of behavioral variation in both species. Obviously, this reflects not only changes in our understanding of bonobos but also chimpanzees, as behavioral variation among field sites has become more and more apparent.
To my mind, the most important point that de Waal makes in his essay is that bonobos don't fit any stereotype -- like any other primate, they are aggressive in some contexts. He notes the ways in which he has argued for a more nuanced view in his publications and books. That will, of course, continue, and the challenge is for anthropologists to make the more nuanced view a feature of their own thinking and teaching.
References:
de Waal, F. 2007. Bonobos, left and right: Primate politics heats up again as liberals and conservatives spindoctor science. eSkeptic Aug. 8, 2007.
Boesch C, Hohmann G, Marchant LF. 2002. Behavioural diversity in chimpanzees and bonobos. Cambridge University Press, Cambridge, UK.
Rybak J. 2004. Chimpanzees and bonobos: more similar than we thought? Am J Primatol 63:245-249. doi:10.1002/ajp.20055
A bonobo field report
A couple of readers have pointed me to an exceptional article in the current New Yorker, in which writer Ian Parker travels with Gottfried Hohmann to Lui Kotal, the field site where he studies partially habituated bonobos.
Along with a lot of local color, Parker presents a cogent description of the history of bonobo research. His back-and-forth interviews with Hohmann and Frans de Waal illuminate the differences between field and captive primate research, which give rise to a current disagreement about bonobo nature:
"It was so easy for Frans to charm everyone," Hohmann said of de Waal one afternoon. "He had the big stories. We don't have the big stories. Often, we have to say, 'No, bonobos can be terribly boring. Watch a bonobo and there are days when you don't see anything -- just sleeping and eating and defecating. There's no sex, there's no food-sharing.' " During our first days in camp, the bonobos had been elusive. "Right now, bonobos are not vocalizing," Hohmann said. "They're just there. And if you go to a zoo, if you give them some food, there's a frenzy. It's so different."
Captivity can have a striking impact on animal behavior. As Craig Stanford, a primatologist at the University of Southern California, recently put it, "Stuck together, bored out of their minds -- what is there to do except eat and have sex?" De Waal has argued that, even if captive bonobo behavior is somewhat skewed, it can still be usefully contrasted with the behavior of captive chimpanzees; he has even written that "only captive studies control for environmental conditions and thereby provide conclusive data on interspecific differences." Stanford's reply is that "different animals respond very differently to captivity."
Then there is the most powerful bonobo stereotype -- the sex thing:
When I asked Hohmann about the bonobo sex at Lui Kotal, he said, "It's nothing that really strikes me." Certainly, he and his team observe female "g-g rubbing," which is not seen in chimpanzees, and needs to be explained. "But does it have anything to do with sex?" Hohmann asked. "Probably not. Of course, they use the genitals, but is it erotic behavior or a greeting gesture that is completely detached from sexual behavior?"
A hug? "A hug can be highly sexual or two leaders meeting at the airport. It's a gesture, nothing else. It depends on the context."
This is a book chapter-length article, and worth printing out and reading closely. We could use a half-dozen more like it on different primates.
Meanwhile, if you're interested in reading more about bonobo field research, I can pass along a link to the Lomami blog at Wildlife Direct, where Ashley Vesper writes about his experiences surveying bonobo populations along the Lomami River.
Chimps around the clock
John Noble Wilford reviews recent research on chimpanzee behavior, in the context of last month's chimpanzee behavior conference. It's a nice article; very suitable for classes.
Get this quick anecdote from Matsuzawa's experimental work:
Tetsuro Matsuzawa, a Kyoto primatologist, described a young chimp watching as numbers 1 through 9 flashed on the computer screen at random positions. Then the numbers disappeared in no more than a second. White squares remained where the numbers had been. The chimp casually but swiftly pressed the squares, calling back the numbers in ascending order -- 1, 2, 3, etc.
The test was repeated several times, with the numbers and squares in different places. The chimp, which had months of training accompanied by promised food rewards, almost never failed to remember where the numbers had been. The video included scenes of a human failing the test, seldom recalling more than one or two numbers, if any.
"Humans can't do it," Dr. Matsuzawa said. "Chimpanzees are superior to humans in this task."
The report doesn't include anything really new, but it is a good summary of the kind of current research. My only complaint is the exclusive focus on chimpanzees. It is now clear that many of these interesting behaviors are very widespread among primates -- from tool use by capuchin monkeys to orangutan cultural variability. Understanding these things in chimpanzees will require us to step back away from the particular chimp social context to see the broader scope of their variability.
Fongoli chimpanzees, Pruetz profile
National Geographic sent writer Mary Roach to see the Fongoli chimpanzees, at the field site of primatologist Jill Pruetz. The result is a nice story about the difficulties of establishing and running a field site, and the joys of watching chimpanzees do things no one has ever seen before.
Now here comes Farafa, her baby Fanta on her back and a bushbuck haunch in her jaws. It's a complicated, messy piece of anatomy, with sinew and hide hanging off one end. Tia sees her and stands up to move away. My last glimpse of Tia is with her now bare bone brandished above her head, standing erect, as though reenacting the "dawn of man" scene from 2001: A Space Odyssey.
Fongoli chimps have a flair for the dramatic.
It's a funny thing about the range of a set of observations: New observations can only increase it, never decrease it. People tend to forget that, as long as we are considering the range of behaviors, studying chimpanzees can do nothing but make them overlap with humans more and more.
The Fongoli chimpanzees are in Senegal, at the very western limit of the chimpanzees' range. Their savanna-woodland territory may be the reason for many of their unique behavior patterns, really the impetus for the article. Roach describes a conservation scenario somewhat different than that in much of the current chimpanzee habitat in other parts of Africa:
[T]he animals are accorded a remarkable amount of respect by locals. Kerri Clavette, Pruetz's intern, interviewed villagers about their beliefs regarding chimpanzees and whether they hunted them. Among the region's main tribes -- the Malinke, Bedik, Bassari, and Jahanka -- chimps, compared with monkeys, have an elevated, almost human status. "Chimpanzees came from man, as they have similar hearts," a villager told Clavette. Behaviors normally associated with a baser nature -- such as walking on all fours -- were given a respectful spin: "Chimpanzees walk on their knuckles to keep their hands clean to eat with." Chimpanzee origin myths feature humans running off into the woods for some reason -- war, fear of circumcision, fear of being punished for fishing on Saturday -- and staying there so long that they turn into chimpanzees.
The article includes several anecdotes about the culture of male chimpanzee researchers (that is to say, male researchers of chimpanzees!) and their reluctance to accept certain female behaviors (such as bushbaby skewering) as much like the male counterparts (such as colobus hunting). I think this is to some extent overblown. The literature has made plain for some time that females are the major transmitters of cultural traits in chimpanzees, and so it would be quite silly to deny the importance of females in bearing such traits in the first place. The dispute over the meaning of hunting is mostly semantic -- the real disagreement being whether the motivation for the behavior is food sharing or purely individual foraging.
And as for the dispute over the meaning of the word, "spear," this really seems to indicate the insularity of certain chimpanzee researchers. Sharpened thrusting weapons were the only such implements available for all but the last 80,000 years or so of human evolution. If we don't call such things, "spears," then a lot of archaeologists are going to be confused!
Anyway, it's a great article.
Fongoli savanna cave chimps
Studying chimpanzee behavioral diversity is so important because they do such different things in different parts of their range. The Fongoli field study by Jill Pruetz and colleagues is very important for this reason -- they are studying chimpanzees in a different ecology from any other field site.
As a consequence they are finding some very different behaviors, from the use of sharpened sticks to probe and kill bush babies, to the new announcement that these chimps use caves to keep cool:
The research into chimpanzees' possible use of caves began when Pruetz's field assistant Mboule Camara saw the apes coming from Sakoto cave, the largest cavern within the chimp's home range. The cave is more than 15 feet deep and located at the head of a shallow ravine that was formed through water runoff from a plateau.
To determine why they might use the cave, Pruetz recorded temperature data within the cave as well as at the different habitats the chimpanzees used, such as the woodlands and grasslands. She discovered that chimps most often use the stone cave as shelter during the hottest and driest times of the year, from October to May, findings detailed in an upcoming issue of the journal Primates. It is the first study to document regular cave use by chimps.
The cave use was reported along with some other interesting aspects of their behavior, at the recent Paleoanthropology Society meetings. Here is the abstract from Pruetz and Bertolani, mentioning some of their results:
Chimpanzee (Pan troglodytes verus) Behavioral Responses to Stresses Associated with Living in a Savanna-mosaic Environment: Implications for Hominin Adaptations to Open Habitats
Chimpanzees in the newly-habituated Fongoli community in southeastern Senegal show a unique suite of behavioral adaptations to stresses associated with their savanna environment. These include using caves as shelters during the dry season, soaking in pools of water during the early rainy season, and moving and foraging at night during maximum phases of the moon. Eleven adult males of this 35-member community serve as focal subjects in a long-term study of the ecology and behavior of chimpanzees in a savanna-mosaic environment. The Fongoli chimpanzee home range is predominantly woodland and grassland with small patches of gallery forest. While chimpanzees at Fongoli are species-typical in certain regards, such as including ripe fruit in the diet during all months of the year, they also adjust their behavior to the particular stresses of this dry, hot and open environment. For example, their large home range (>63 km2) is sometimes used cyclically, with the community traveling as one large party, in contrast to the typical chimpanzee fission-fusion pattern. Here, we report on Fongoli chimpanzee activity budgets and ranging behavior during dry versus wet seasons based on over 2500 hours of observational data collected from March 2005-August 2006. Combined with data on temperature in the various habitats within the savanna mosaic, results show that Fongoli chimpanzees minimize energy expenditure during the hottest months and at the hottest time of day by resting more and traveling less in addition to selectively using small patches of closed-canopy habitats, such as gallery forest. Details of how chimpanzees alter their ranging behavior on a larger scale at these times will also be examined. The stresses associated with a savanna-mosaic environment and chimpanzees' behavioral adjustments to them have important implications for our understanding of early hominin behavior in similar environments.
The large size of foraging parties was especially important -- it is a predictable consequence of more open-country activity, as a strategy to reduce predation -- but it should remind us how unlikely it would be for early hominids to have lived in small single-male groups.
These chimpanzees may well live in more open habitat than any hominid before A. afarensis, at least as far as we can tell from the paleoecology.
Ape tool curation skills
An accessible story by Bjorn Carey discusses the paper by Mulcahy and Call, titled "Apes save tools for future use." From the story:
To determine if apes can also perform this type of "mental time travel," Nicholas Mulcahy and Josep Call, researchers at the Max Planck Institute for Evolutionary Anthropology in Germany, enlisted five bonobo chimpanzees and five orangutans for an experiment.
The researchers first led the animals into a room and taught them how to use a tool to get a food reward. Then the apes were directed to a "tool shed" containing tools for reaching grapes and juice bottles. Two of the tools were well suited for the task, while six were not.
After an ape made its selection, it was not allowed access to the goodie dispenser for an hour, so the animal hauled the tool back to a waiting room for storage.
When the researchers allowed the animals to have a go at the dispenser, the apes returned with a suitable tool and retrieved their treat in less than five minutes about 30 percent of the time.
With one bonobo and one orangutan, they ran the test overnight -- they had to take the tool back to their sleeping room and return with the tool in the morning to get the reward.
The clever part of the experiment was one of the controls:
Experiment 4 established the baseline probability of transporting tools in the absence of a future task but using identical reinforcement contingencies as in experiment 3. Two bonobos and two orangutans received the same treatment as in experiment 3, except that no apparatus was set up upon their return to the test room although they were rewarded if they brought the suitable tool back. Subjects solved the task significantly less often (mean = 1.8, SEM = 1.2) than did those in experiment 3 (t6 = 3.91, P = 0.008). In fact, only two of the four subjects brought back the suitable tool at all, and they behaved differently from other successful subjects because after their first successful trial, they failed the next 11 and 14 trials, respectively (Table 1). Subjects in experiment 4 also solved the task significantly less often than those in experiment 1 (t8 = 2.81, P = 0.023), thus ruling out the possible confounding effects of practice, because both groups of subjects were naïve when their respective experiments began.
It seems that if they didn't have to use the tool, they didn't bother to bring the tool.
A question: Why are these kinds of stories always about "how smart" apes are instead of "how dumb" people are? I mean, it would be fairly hard to train people to do this task without talking to them. I think that there would be a good fraction of people who wouldn't get it.
At least, not without a better reward. I guess that even though students pretty consistently fail to bring number 2 pencils on evaluation day, they rarely fail to bring them when there's an exam...
References:
Mulcahy NJ, Call J. 2006. Apes save tools for future use. Science 312:1038-1040. DOI linkChimp spear-hunting
Ann Gibbons reports on the upcoming article in Current Biology:
[Jill] Pruetz's team, working at the Fongoli research site in the wooded savanna of Senegal, observed chimps breaking off green branches and in four cases using their incisors to sharpen the points. The chimps, which typically weigh 26 to 60 kilograms, were hunting nocturnal bush babies, 100- to 300-gram primates that hide by day in holes in trees. In all, Pruetz and Paco Bertolani, a graduate student at Cambridge University, documented 10 different chimps thrusting the tools into holes in 22 instances. "This is habitual," says Pruetz, whose team logged 2500 hours of observations.
Sucks to be a bush baby.
References:
Gibbons A. 2007. Spear-wielding chimps seen hunting bush babies. Science 315:1063. doi:10.1126/science.315.5815.1063
Chimpanzee lets eight cousins drown
Reuters is reporting on a current study by Joan Silk and colleagues in Nature.
Here's the intro:
Chimpanzees share many traits with humans but altruism, it seems, is not one of them, scientists said on Wednesday.
Although chimps live in social groups and co-operate and hunt together, when it comes to helping non-related group members, they don't put up with any monkey business.
When given the opportunity to help themselves and other chimps they often choose the selfish option.
The experimental setup gave the subject an option between two alternatives:
If the subject (hereafter referred to as the actor) chose option 1, the actor obtained a food reward and another chimpanzee simultaneously received an identical reward (hereafter referred to as the '1/1 option'). If the actor chose option 2, the actor obtained the same size and type of food reward, but no food reward was delivered to the other chimpanzee (the '1/0 option'). As a control, actors were presented with exactly the same reward options when there was no other chimpanzee present (Silk et al. 2005:1357).
So it's not a benefit/cost comparison, but a benefit/benefit. Sort of like if you won a house party from VH1, and you could either decide to invite other people or have the party all to yourself.
The chimpanzees didn't choose the "1/1 option" any more often when another chimpanzee was there (and got the reward) than when there was no other chimpanzee there. Those unfeeling primates!
I'm of two minds about the study. On the one hand, I'm not entirely sure how untutored humans would perform on this one. I think my two-year-old twins would pass -- when we are giving out treats, one will insist on an extra treat to bring to her sister. That situation is pretty analogous to the experiment, I think -- it's not like there's any cost to asking for an extra, since we know they are going to take it to the other twin. Nor is it really analogous to "sharing", which they do inconsistently. But we've had to work pretty hard to teach them to give out treats in that way, and they get direct feedback from us and the grateful sibling.
Considering how complex even this simple case is, I'm not too surprised that the chimpanzees would fail to give out the treats to their groupmates. Nobody has taught them how to do it, and there is relatively little direct feedback (although at one study site, the potential recipients sometimes made begging gestures). And I don't think that untutored humans would do it without explanation and feedback -- it's just that humans have a pretty sophisticated verbal and nonverbal ability to give that kind of feedback. So there is a genuine cognitive difference between humans and chimpanzees that may be involved in the result, although it is not perfectly clear that it is "indifference" in the chimpanzees.
On the other hand, look at the claim at the end of the paper:
These results complement observational and experimental studies that indicate that chimpanzees cooperate mainly with kin and reciprocating partners and show no aversion to inequitable exchanges that benefit themselves (Silk et al. 2005:1358).
This raises a question: would chimpanzees give out the treat to their kin? If not, then we're not seeing a failure to be empathetic toward the "unrelated other", we're seeing a failure to be empathetic at all. But we know that chimpanzees do behave preferentially toward kin in many contexts. So if this test failed to show empathy toward kin, it would be a failure of the test, and not a real indication of chimpanzee behavioral capacities.
So I think there are some missing steps here. Coming up with clear psychological demonstrations of the concept of empathy, or altruism, or welfare of other individuals is tough.
References:
Silk JB et al. 2005. Chimpanzees are indifferent to the welfare of unrelated group members. Nature 437:1357-1359. Full text (subscription)
Chimpanzee archaeology
Here's a LiveScience story by Heather Whipps, about the discovery of chimpanzee nutcracking stones dating back to 4300 years ago:
Though there were no chimpanzee remains at the settlement, testing by archaeologists revealed the tool-laden camp was most likely used by the Great Ape. The stones were much bigger than anything a human could use comfortably and bore the residue of nuts that modern chimpanzees like to snack on.
"This is the only case of any prehistoric, non-human Great Ape tool use ever discovered," [archaeologist Julio] Mercader told LiveScience.
That's very cool. The news piece claims that this is great vindication for the idea that chimpanzees developed this ability without exposure to humans. That is, some people had argued that chimpanzees might have been "acculturated" to crack nuts by watching nearby people. Personally, I never thought there was much to that idea, since nutcracking is so widespread among chimpanzees and clearly learned by chimps with minimal or no human contact.
The most interesting part to me is the possibility that archaeologists will develop a search strategy for stone tools older than flaked stone tool manufacture.
The chimpanzee grapevine
Victoria Horner and colleagues (2006) set up two "diffusion chains" of chimpanzees, to see if a learned task could be transmitted faithfully from one chimp to another for several iterations.
Using a powerful three-group, two-action methodology, we found that alternative methods used to obtain food from a foraging device ("lift door" versus "slide door") were accurately transmitted along two chains of six and five chimpanzees, respectively, such that the last chimpanzee in the chain used the same method as the original trained model. The fidelity of transmission within each chain is remarkable given that several individuals in the no-model control group were able to discover either method by individual exploration. A comparative study with human children revealed similar results. This study is the first to experimentally demonstrate the linear transmission of alternative foraging techniques by non-human primates. Our results show that chimpanzees have a capacity to sustain local traditions across multiple simulated generations (Horner et al. 2006:13878)
Essentially, they trained one individual in each of two chimpanzee groups to open a box with a reward inside -- but there were two ways to open the box, and each of these models was taught a different method. Another chimpanzee was given a period of time, with several trials, to observe one of these models opening the box. Then when the learner acquired the method, another chimp became the learner observing the second. And so on.
They found that the two methods were transmitted essentially intact across as many chimpanzees as they tried, with a couple of limits -- some chimpanzees couldn't be paired as model-learner pairs because they were aggressive toward each other, and some just didn't watch the model and learned the task independently. As the paper notes, these cases are interesting because they present limits on the ability of groups to maintain such traditions:
Side branches occurred in both chains because either the model was unsuccessful/unmotivated (RN in FS1, AM in FS2) or aggression occurred bet ween the model and observer (KT to BO in FS1, CY to VV in FS2). The latter highlights the importance of tolerance and reinforces the hypothesis that opportunities for social learning in the wild may be restricted by the level of tolerance between individuals (50) and that not all individuals within a population may be good models for social learning (51) (Horner et al. 2006:13881).
For me, this study helps to clarify some of the constraints on social learning:
It is not the function of diffusion studies to dissect in depth the underlying mechanisms of transmission, although these must be sophisticated enough to ensure the replication of behavior across the generations. However, some limited inferences are suggested by the contrasts deriving from the three-group design. The no-model control condition indicates that for about half the chimpanzees (and children), opening an object like the Doorian fruit can be said to be within their untutored competence. Therefore, it seems reasonable to assume that half the participants from the diffusion chains would also have been able to open the Doorian fruit in a control condition. Their exclusive use of only one of the two available techniques may represent a form of "canalization" (46), whereby a chimpanzee's potentially limitless exploration of a problem is focused around only a subset of behaviors that they see performed by others. Similarly, it is likely that half of the participants in the chains would have failed the control condition, and hence their behavior suggests a more complex social learning mechanism, such as emulation or imitation (28), but further experiments will be required to establish this (Horner et al. 2006:13881).
Of course this is an artificial situation -- with two possible solutions -- but it suggests several of the balancing factors in the learning and transmission of natural behaviors. Simple things ought to be transmitted very readily. But then, if an adaptive behavior is really that simple, then maybe it should be genetically assimilated. One wonders also whether the chimpanzees who failed to pick up the behavior independently would have ever been able to figure it out, or whether they would just remain oblivious to an adaptive resource in the wild. There is also, after all, a possible reason to be bad at trying new things -- that being, that sometimes new things will kill you.
References:
Horner V, Whiten A, Flynn E, de Waal FBM. 2006. Faithful replication of foraging techniques along cultural transmission chains by chimpanzees and children. Proc Nat Acad Sci USA 103:13878-13883. PNAS online
Chimpanzee language antics
Carl Zimmer has an article in Forbes covering recent experiments in chimpanzee vocal communication.
But don't write off those grunts and hoots just yet, at least according to a new study that appears in the Oct. 15 issue of the journal Current Biology. Katie Slocombe and Kaus Zuberbuhler, two primatologists at the University of St. Andrews in Scotland, investigated a particular noise chimpanzees make when they find food, called a "rough grunt." At the Edinburgh Zoo, the scientists fed the chimpanzees two different foods--apples and bread--and recorded the sounds they made. Chimpanzees prefer bread to apples, and Slocombe and Zuberbuhler discovered a corresponding difference in the rough grunts they made for each food. They hit a distinctively high note when they came across the bread, and but made lower and noisier grunts for apples.
It's a short article, supplemented by an entry on the Loom.
There are a number of short interview excerpts in the issue. One has Noam Chomsky discussing spontaneous language innovation in deaf communities. Another from Jane Goodall on the perils of e-mail communication:
I remember when I worked for Lewis [sic] Leakey, first as his secretary. He was very impulsive. He'd get a letter in the mail, and he would open it, and it would be perhaps something from a scientist he thought was quite ridiculous. You could hear him muttering "Bosh! Rubbish!" The poor bit of paper would be scored with his marks, and he'd turn to me and say "Get so and so on the phone!" I got very wise to his moods, so I would pretend the number was engaged, or the man wasn't there, and then an hour or two later, he was rational again.
And other interviews and articles, with Arthur C. Clarke, Wil Wheaton, Desmond Morris, Steven Pinker and many others. Many thanks to the reader who pointed me to the site.
Sex differences in chimpanzee learning
A new article in the New York Times discusses an upcoming paper by Elizabeth Lonsdorf and colleagues in Animal Behavior that examines the way that Gombe chimpanzees learn termite fishing. The paper finds that young females become adept at termite fishing nearly two years before young males.
Dr. Lonsdorf said that typically, when a young male and female are near a mound, "she's really intently termite fishing, and he's spinning himself in circles."
The behavior of both sexes may seem familiar to many parents, she said, adding, "The sex differences we found in the chimps mimic some of the findings from the human child development literature."
She pointed out, however, that at least in the case of chimps, each is doing something important, since the males' play is practice for later dominance behavior.
"They're doing stuff that's really appropriate," she said.
(via Ann Althouse, a rather better-known Madison blogger than I am, and one of my favorites)
The antimalarial properties of chimpanzee geophagy
It's hard to improve on the headline of this story:
Why chimps eat dirt
...
[Sabrina] Krief collected the dirt along with leaves from one of the chimps' favorite foods, the Trichilia rubescens plant. She found that when eaten alone, the leaves had no pharmacological effect, but when combined with soil, the mixture had clear anti-malarial properties.
Scientists previously suspected that animals might eat dirt when stressed or as a source of missing minerals. This new result is the first suggestion that the combination of soil and other foods could have health benefits, Krief said.
The story also discusses the use of high-kaolinite dirts as antidiarrheal treatments in local peoples -- kaolinite being the nameworthy ingredient of Kaopectate, although it is no longer used in that medication.
Anyway, this is a good excuse to use the word "geophagy." Great word.
Chimpanzee genomics
This week's Nature (9/1/05) has a special feature on the chimpanzee genome (subscription required). The introduction is this perspective by Chris Gunter and Ritu Dhand:
We are therefore extremely pleased to present this special section to commemorate the genome of the common chimpanzee, Pan troglodytes. In doing so, we hope to provide a resource for more than just genomics. We introduce the section with a timeline that charts the history of the chimp. This is followed by four Progress pieces that review recent work on chimp culture and behaviour, psychology and neural processing of number systems, as well as a closer look at brain anatomy and neurogenetics at the single-gene level.
On page 69, the Chimpanzee Sequencing and Analysis Consortium reports analysis of the long-awaited draft genome sequence. This is supported on page 101 by Hughes et al., with the sequence of part of the chimpanzee Y chromosome. Comparing the genetic code of humans and chimps will allow us to comb through each gene or regulatory region to find single changes that might have made a difference in evolution, and the authors list some new candidates for further study. Two more research papers by Cheng et al. (page 88) and Linardopoulou et al. (page 94) detail changes in highly variable regions in the human and chimp genomes; additions or deletions of larger chunks of DNA may be as important as single nucleotide changes in shaping our genomes (links deactivated because they aren't functional).
Finally, we need physical evidence to tell us how chimps and humans may have lived millions of years ago. Surprisingly, to date there has been no fossil record of the chimp; on page 105, McBrearty and Jablonski report the first unequivocal fossil evidence of the genus Pan.
I'll be working through several of the papers and posting my comments on them, as well as links to more information. Keep checking back for updates. Together with some interesting stuff coming next week, this is going to be a busy time here!
To put it another way, lefties are more chimp-like
Chimpanzees who use tools, predominantly do so left-handed. So says this AP article, which cites current work by William Hopkins and colleagues:
A three-year study of 17 wild chimps in Gombe National Park, Tanzania, found that 12 of them used their left hands when using sticks to probe for termites. Four were right-handed and one was listed as ambiguously handed.
"Contrary to previous claims, wild chimpanzees show population-level handedness in tool-use," reported the research team led by William D. Hopkins of the Yerkes National Primate Research Center at Emory University in Atlanta. Population-level handedness indicates a preference for one hand in a large group.
What does this tell us about human hand preferences? Not too much. I saw a nice paper by Dean Falk and colleagues at last year's AAPA on the neural correlates of handedness; they're pretty complicated, and it's safe (I think) to say that it will be a long time before anyone understands how they develop. One simple neural correlate has long been thought to be petalial asymmetries, which were once assumed to be unique to humans. We now know that petalial asymmetries are common in hominoids. Perhaps it follows that handedness should also be widespread.
A conventional view would assume that the brain structures associated with hand preference emerged as adaptations for that purpose. Here's one story: people who were opposite-handed (i.e. lefties among humans) were slightly less able to learn important tool-related behaviors (like throwing spears) than others; they faced a (slight) selective disadvantage, and remain at lower frequency for that reason.
Maybe it's true. But then you have to account for all the lefties that are still around. If they really faced a selective disadvantage, they should be long gone. Instead, they're busily supporting Ned Flanders' "Leftorium".
So, one may turn to balancing selection: lefthandedness was bad, but lefties had some compensating advantages. Maybe they were more creative. Maybe they were artistic. It seems to me that there's a lot of grasping at straws here. Since when was being creative better, in terms of reproduction? You might as well tell a story about the adaptiveness of schizophrenia (also, more likely lefties). This is in the category of "any explanation in a pinch" rather than real science.
And on the other side of the coin, where's the evidence that lefthandedness is bad? Sure, they are more likely to cut off digits with power tools (true), and more likely to be bedwetters (also, sadly, true). They are also more easily hypnotized (could I make this stuff up?). But where's the study of spearthrowing? Knapping flint?
Then there are deeper mysteries. Why is left-handedness twice as common in men than in women? Why are the patterns of brain activity related to handedness different in men and women?
It's a puzzle. I may follow up later. In the meantime, I think it would be fun to train a chimpanzee to hold her termite stick up in the air, and say, "Aha, but I am not lefthanded!"
Chimpanzee-human Y chromosome comparisons
Hughes et al. (2005) report on the nature of Y chromosome genomic differences between humans and chimpanzees. The paper is a test of the hypothesis popularized by Bryan Sykes (in his book Adam's Curse: The Science That Reveals Our Genetic Destiny): namely, that in a few million years, the human Y chromosome will disappear entirely because its genes will have become entirely inactive or subsumed on other chromosomes.
The short answer is: not gonna happen.
The long answer has some interesting twists. As it turns out, humans appear to have conserved all of the functional Y chromosome genes that occurred in the human-chimpanzee common ancestor. The paper proposes that this is evidence for a stronger effect of purifying selection in humans than might have been assumed. They were able to confirm this by comparing coding divergence with intron divergence; the coding region sequence divergence between species was significantly low. That leaves an interesting evolutionary question: how strong was purifying selection compared to other chromosomes, and could it have affected standing Y chromosome variation during human evolution? Unknown.
But these genes are ordered very differently on the Y chromosome than the equivalent genes in chimpanzees. In chimps, all the X-degenerate genes are together in one region. In humans, they have been scattered around onto both arms of the Y and intermingled with other sequences.
The study is only of a subset of the Y chromosome: the X-degenerate regions, or the parts that contain genes with X chromosome analogs, but only analogs that are significantly divergent from their X chromosome equivalents. So it does not represent a final answer on many of these questions, but it constitutes what may be the most important source of Y-unique evolution.
Although humans retain all of the functional genes of the common ancestor of the two species, chimpanzees have actually lost several. The authors propose a hypothesis to explain this loss:
Why have X-degenerate genes decayed in the chimpanzee lineage but not in the human lineage? We speculate that X-degenerate gene decay in the chimpanzee lineage may be a by-product of strong positive selection focused elsewhere on the Y chromosome, through a process known as genetic hitchhiking. Because the Y chromosome does not participate in sexual recombination with a chromosome homologue, natural selection acts on the chromosome as a unit. Deleterious mutations in some Y-linked genes can be carried along, even to the point of fixation in a population, by physical linkage to strongly beneficial mutations in other Y-linked genes. In addition to their X-degenerate genes, primate Y chromosomes contain many families of ampliconic genes, which have testes-restricted expression patterns and critical functions in sperm production. Because of this central role in spermatogenesis, the Y chromosome's ampliconic genes may be subject to powerful selective pressures, especially in species such as chimpanzees where females usually mate with multiple males, the sperm of which then compete for a limited number of oocytes. During chimpanzee evolution some X-degenerate genes may have been casualties of selective forces directed at the Y chromosome's ampliconic genes--forces that were not as intense during the evolution of our less promiscuous species (Hughes et al. 2005, references omitted).
Could be true. On the other hand, the fixation of a mutation that deactivates a gene is pretty drastic, even if it is hitchhiking with another favorable variant. It certainly implies that the deactivated genes have little adaptive importance to start with. So this reduction in adaptive importance for the deactivated genes must still be explained. The physical difference between ampliconic and X-degenerate genes may have something to do with it, but it can't be the whole story.
References:
Hughes JF, et al. 2005. Conservation of Y-linked genes during human evolution revealed by comparative sequencing in chimpanzee. Nature 437:100-103. Full text (subscription required)
The killer females
In a short paper, Simon Townsend and colleagues report on several instances of infanticide initiated by female chimpanzees in the Bundongo Forest:
These observations document a systematic pattern of lethal aggression in female chimpanzees. Such infanticidal attacks are neither isolated events by pathological individuals nor mere biproducts of male aggression; they seem to be part of the female behavioural repertoire. In all cases, the remains showed significant bites to the head, almost certainly fatal in two cases, indicating that these were purposeful, not accidental, killings. Equally remarkable, at least one, but possibly more, cases were the result of coalitionary attacks by resident females. While male cooperation in aggressive attacks is well documented for chimpanzees 1 and 2, coalitionary aggression by unrelated females is not.
They speculate that the attacks are related to limited resources: the study group has had a growth in the number of females due to immigration and births, but has not increased as much in the number of males; territorial growth would require a larger male coalition.
References:
Townsend SW, Slocombe KE, Thompson ME, Zuberbüler K. 2007. Female-led infanticide in wild chimpanzees. Curr Biol 17:R355-R356. doi:10.1016/j.cub.2007.03.020
Killer chimpanzees and human warfare
This article from World Science Net popped into my inbox today. It's basically a short news story on an upcoming research presentation by David Watts (Yale University) on the incidence of intergroup chimpanzee violence at Kibale. The article has Watts and Richard Wrangham (Harvard University) in support of the idea that chimpanzee violence is widespread and homologous with human violence, and Brian Ferguson (Rutgers University) speaking against the idea.
The basic observation has been well understood: under certain circumstances, chimpanzee males will kill members of neighboring groups. This occurs most commonly when the victims are caught alone, and repeated instances have in at least one case basically wiped out the males in a targeted group (at Gombe). The new observations at Kibale just add to this record, with Wrangham noting that 49 killings have been documented to date, with two-thirds "either directly seen...or inferred from clear evidence such as chimps prancing around a brutalized corpse." Wrangham offers this as a counterargument to the idea that the chimpanzees have merely disappeared without researchers really knowing if they have been killed, although that argument itself is weak on its face to anyone who has lived on a farm and wondered why the dog suddenly stopped coming for dinner. Maybe sometimes chimps, like dogs, just wander off; but with the dog it is a lot more likely that it has been hit by a car, and I have to imagine that the idea that chimpanzees are just "disappearing" is about as likely as the dog finding a new home that somehow isn't with one of the neighbors.
Really, the central question that most people think is interesting is whether the behavior of chimpanzees has any predictive value for the behavior of ancient humans. Here, I think the data is very weak. The chimpanzee observations clearly show that deliberate killing is within the cognitive range of chimpanzees. If killing is cognitively possible for chimpanzees, we may infer that it would have been so for early hominids also. But this really does nothing to demonstrate that killing would have been in any way adaptive for early hominids. For chimpanzees, the adaptive story has to do with the sizes of groups and types of competition that individuals--especially males--face. And as Wrangham points out, chimpanzees not only have motive, but also opportunity stemming from their fission-fusion community structure, which leaves chimpanzees occasionally isolated as easy prey for packs of marauding males.
Did early hominids similarly have motive and opportunity for killing? If early hominids had more cohesive groups than chimpanzees, they might have been relatively immune from violence even if the motive were present. The relatively cohesive groups of gorillas are suggested as effective defenses against the risk of violence or infanticide from rogue males, so it would not be unusual if early hominids pursued such a strategy. And the advantages of killing or other modes of violence are not assured for early humans. For chimpanzees and other primates like langurs, male coalitions give their members an opportunity to gain mating access through coordinated action, including violence. But the freedom of action of human coalitions is more limited. Cultural inhibitions, driven by the long memories of victims and their kin, present the substantial risks of vengeance or other costs for human killers. It remains an open question whether these inhibitions against violence are more or less effective in small-scale societies like those of ancient humans. Arguably, they are ineffective in almost every society, but they nevertheless present potent reasons for most humans to avoid killing and to coordinate actions to punish killers. So we may question whether the motive to kill was likely to characterize ancient humans.
But on the other hand, biologists like Richard Alexander (University of Michigan) suggest that interactions among early human groups were likely highly competitive. Alexander suggests that this competition was one of the major factors behind the evolution of human cognitive abilities. This idea dovetails with the notion that early human groups may have been significant units of selection, with coordinated actions and altruistic behaviors characterizing within-group interactions, and violent competitions characterizing between-group interactions.
For me, a more interesting issue is the effect of violent behaviors on the pattern of natural selection within ancient human and primate populations. Unlike many modes of competition, killing has a direct selective effect, as competitors are immediately removed from the population. Selective coefficients of 0.01 or less are nonetheless immensely powerful over the course of hundreds of generations. With 49 observed cases of killing among chimpanzees, panicide seems to account for over one percent of chimpanzee deaths, and in the areas where it has been most noted (Gombe, Kibale) its incidence is much higher. The benefits to a coalition of males from killing members of adjacent communities plausibly are very high. At the very least, since males are philopatric, the reduction in size of neighboring groups will provide an assurance of the survival of the males' community, which will contain their own male progeny. Moreover, the weakening of neighboring groups may enable their female offspring to have an easier time dispersing or establishing status in the weakened neighboring groups. And of course there is the possibility that all males in the neighboring group will be exterminated, allowing some or all of the male coalition to extend their mating access to those females. Each of these advantages is potentially powerful, and together they may imply that the slight incidence of killing could nevertheless create a strong selective advantage.
One criticism of the chimpanzee observations is that the chimpanzees may be suffering resource shortages or other extraordinary effects of human presence in the study areas. This might well be true, and Wrangham's argument that the chimp population is growing hardly mitigates the possible effects of overcrowding in a small forest preserve. But even if the behavior is influenced by anthropogenic processes today, this does not mean that the behavior did not occur in the past. Environmental fluctuations and ecological crises have probably recurred many times during the evolution of chimpanzees. Each crisis may not have been as serious as today's human-induced habitat loss, but each might well have influenced population sizes and community structures in ways that made killing a viable behavioral adaptation. An occurrence in populations under stress might well have a high selective effect even if it were normally neutral or maladaptive. And of course it has not been demonstrated that killing is maladaptive in normal circumstances in stable populations.
John Hawks Department of Anthropology
University of Wisconsin—Madison
Copyright © 2007 John Hawks