chimpanzees

Recent University of Michigan Ph.D. Jeremy DeSilva gets some nice press about his work demonstrating that fossil hominins didn't climb like chimpanzees:

"Frankly, I thought I was going to find that early humans would be quite capable, but their ankle morphology was decidedly maladaptive for the kind of climbing I was seeing in chimps," DeSilva told LiveScience. "It kind of reinvented in my mind what they were doing and how they could have survived in an African savannah without the ability to go up in the trees."

This is a good example of the comparative method in paleoanthropology. We can't observe the behavior of extinct species; we can only observe the behavior of their living relatives. We can observe the anatomy of fossil specimens, but testing hypotheses about their behavior requires us to understand the relationship between anatomy and behavior in living species. We've known about the anatomy of fossil hominid ankles for a long time, but it's not so obvious how the anatomical differences between them and chimpanzee ankles relates to behavior.

The paper's abstract:

Whether early hominins were adept tree climbers is unclear. Although some researchers have argued that bipedality maladapts the hominin skeleton for climbing, others have argued that early hominin fossils display an amalgamation of features consistent with both locomotor strategies. Although chimpanzees have featured prominently in these arguments, there are no published data on the kinematics of climbing in wild chimpanzees. Without these biomechanical data describing how chimpanzees actually climb trees, identifying correlates of climbing in modern ape skeletons is difficult, thereby limiting accurate interpretations of the hominin fossil record. Here, the first kinematic data on vertical climbing in wild chimpanzees are presented. These data are used to identify skeletal correlates of climbing in the ankle joint of the African apes to more accurately interpret hominin distal tibiae and tali. This study finds that chimpanzees engage in an extraordinary range of foot dorsiflexion and inversion during vertical climbing bouts. Two skeletal correlates of modern ape-like vertical climbing are identified in the ankle joint and related to positions of dorsiflexion and foot inversion. A study of the 14 distal tibiae and 15 tali identified and published as hominins from 4.12 to 1.53 million years ago finds that the ankles of early hominins were poorly adapted for modern ape-like vertical climbing bouts. This study concludes that if hominins included tree climbing as part of their locomotor repertoire, then they were performing this activity in a manner decidedly unlike modern chimpanzees.

DeSilva's conclusion is straightforward and easy to illustrate. Chimpanzees climb vertical tree trunks pretty much like a logger does. A logger slings a strap around the trunk and leans back on it. Friction from the strap holds him up as he moves his feet upward; spikes on his boots hold him while he moves the strap.

Of course, chimpanzees don't have spikes on their feet, and they don't use a strap. Instead, their arms are long enough to wrap around the trunk, and they can wedge a foot against the trunk by flexing their ankle upward -- dorsiflexing it -- or grip the trunk by bending the ankle sideways -- inverting the foot -- around it. The paper includes a photo that shows the chimpanzee style of climbing clearly:

Photo of chimpanzee climbing a tree, from DeSilva (2008)

You might wonder, yeah so what? Isn't it obvious that chimpanzees climb this way?

Well, it wasn't so obvious which features of the ankle might adapt chimpanzees to this style of climbing. By watching the chimpanzees (and other apes) DeSilva was able to determine the average amount (and range) of dorsiflexion and inversion of the feet while climbing, and could also assess the extent to which dorsiflexion is accomplished at the ankle joint (as opposed to the midfoot). In this case, the observations were pretty obvious -- chimpanzees were habitually flexing their ankles in ways that would damage a human ankle. Then, by examining the bony limits on human ankle flexibility, DeSilva showed that fossil hominins shared the same constraints on ankle movement as recent people. They couldn't have climbed like chimpanzees.

Human climbing

I would say that the ankle-joint observations match the rest of the skeleton. It seems pretty obvious that Australopithecus afarensis and later hominids couldn't possibly have climbed in the chimpanzee-like manner described in DeSilva's paper, because the hominins' arms were too short. If a logger tried to climb with his arms instead of a strap, even spikes on his feet would be relatively ineffective holding him up. Dorsiflexion would be hopeless -- the normal component of force against the tree trunk would be insufficient to prevent slipping.

Humans who aren't loggers use a different strategy to climb vertical tree trunks -- they put a large fraction of the surface area of their legs directly in contact with the trunk. Wrapping legs around and pressing them together gives the necessary friction to hold the body up.

If you're like me, you'll remember this climbing strategy ruefully from gym class, where "rope climbing" is the lowest common denominator of fitness tests. The sad fact is that many otherwise-normal humans fall on the wrong side of the line between mass and muscle power. Straining my groin muscles to the max, I still could never pull my way up a rope.

There's nothing magical about getting a human to climb. Ladders, after all, are relatively easy for the large fraction of the population who can't climb a rope or tree trunk. The trick with a ladder is that friction is organized in a more effective way for our ankle mechanics and arm length. But you don't need to schlep a ladder, if you can manage a little extra arm strength and a low enough body mass.

Early hominin climbing

Australopithecines were light in mass, and from what we can tell, they had strong arms. So they had what it takes for humans today to climb trees effectively -- not like chimpanzees, but like humans. Up to A. afarensis, every early hominin we know about lived in an environment that was at least partially wooded.

In his comments about the paper, DeSilva hypothesizes a trade-off between climbing ability and effective bipedality, so that early hominins could not have effectively adapted to both. I don't think a chimpanzee-like ankle would have been any use with arms as short as australopithecines'. So I don't see the necessity of a trade-off in ankle morphology. A. afarensis -- long before any evidence of stone tool manufacture -- had very non-apelike arms, hands and thumbs.

But there's one significant question that DeSilva omits discussing: StW 573. Clarke and Tobias (1995) describe the foot of StW 573 as having a big toe that is abducted (sticks out) from the foot, intermediate between the chimpanzee and human condition. They conclude:

[W]e now have the best available evidence that the earliest South African australopithecine, while bipedal, was equipped to include arboreal, climbing activities in its locomotor repertoire. Its foot has departed to only a small degree from that of the chimpanzee. It is becoming clear that Australopithecus was not an obligate terrestrial biped, but rather a facultative biped and climber (Clarke and Tobias 1995:524).

DeSilva studied the talus, not the toe. StW 573 has a talus, and although it is not in DeSilva's sample, it probably would place very close to the other hominins in his comparison. Even Clarke and Tobias described its talus as humanlike -- their argument for an intermediate form was based mostly on the toe.

But still, it's hard to believe that australopithecines would retain a chimpanzee-like big toe, if they couldn't use that big toe by inverting or dorsiflexing their foot in any significant way. By all other accounts, an abducted hallux would only impede effective bipedality. It is of no use at all for a human-like pattern of climbing. The only remaining utility would be for small-branch grasping, but small branches would seem unlikely as a support for hominin arboreality.

One possibility is that Clarke and Tobias were simply mistaken. That appears to be the explanation favored by Harcourt-Smith and Aiello (2004:412), who cited Harcourt-Smith's 2002 thesis:

Recent multivariate analyses of the Stw 573 tarsal bones (medial cuneiform, navicular and talus) using geometric morphometric techniques demonstrate that this fossil had a very ape-like talus, a navicular that was intermediate between apes and modern humans, and a human-like medial cuneiform inferring a lack of any hallux opposability (Harcourt-Smith, 2002). This finding contrasts with the findings of Clarke & Tobias (1995), but is does not change the fact that Stw 573 would still have a different combination of morphologies in the foot than does A. afarensis.

This view was also supported by McHenry and Jones (2006), who concluded that all known hominin feet appear to lack any "ape-like ability to oppose the big toe." They also point to the Laetoli footprint trails, most observers of which agree that the big toe was adducted, not abducted.

I tend to favor that explanation -- australopithecines simply didn't have a grasping foot. But they may not have shared the medial longitudinal arch, at least not in the human configuration, and without it one might doubt that their gait featured as strong a toe-off as that of later humans. Who knows?

Meanwhile I can recommend Harcourt-Smith and Aiello's review for those who want to read more about bipedality and climbing in early hominins. It's not the last word but it is a good introduction to the literature.

UPDATE (2009/04/15): A reader writes to suggest also the 1987 paper by Bruce Latimer, James Ohman and Owen Lovejoy. I recommend it for anyone who wants to dig deeper into australopithecine ankle morphology. I've added it to the bibliography below.

References:

DeSilva JM. 2009. Functional morphology of the ankle and the likelihood of climbing in early hominins. Proc Nat Acad Sci USA 106:6567-6572. doi:10.1073/pnas.0900270106

Clarke RJ, Tobias PV. 1995. Sterkfontein Member 2 foot bones of the oldest South African hominid. Science 269:521-524.

Harcourt-Smith WEH, Aiello LC. 2004. Fossils, feet and the evolution of human bipedal locomotion. J Anat 204:403-416. doi:10.1111/j.0021-8782.2004.00296.x

Latimer B, Ohman JC, Lovejoy CO. 1987. Talocrural joint in African hominoids: Implications for Australopithecus afarensis. Am J Phys Anthropol 74:155-175.

McHenry HM. Jones AL. 2006. Hallucial convergence in early hominids. J Hum Evol 50:534-539. doi:10.1016/j.jhevol.2005.12.008

Earlier, I pointed to my new article in Slate, about chimpanzee strength compared to humans. For anthropologists, I thought I might point to a passage in one of John Bauman's articles (1926:7-9), which raises a point I remember well from graduate school:

The last question raised by the strength of the chimpanzee seems to have been completely overlooked in the past. All anatomists place reliance upon the relative development of the various muscle attachment ridges and pits on the bones as a trustworthy indication of the strength of the owner.

Yet anyone who will take the trouble to compare carefully the crest of the ilium of the chimpanzee with that of the human being will notice that the muscle attachment roughnesses are very markedly less prominent in the former than in the latter, yet Suzette's pulls have clearly demonstrated an immense superiority in strength of the lumbar region in the ape. Also with regard to long sustained action, a short time spent in the anthropoidal posture will convince any person that this posture calls for more taxing long sustained action of the lumbar muscles than does the erect posture of the human being.

We certainly can not look to man's erect posture for an explanation of the smooth sharp rim of the hip bone in the anthropoid ape, why then do the usually so reliable muscle attachments fail here to correctly indicate relative strength? The discrepancy is an extremely pronounced, not a trifling one, moreover.

Bauman then generalized to Neandertals:

And finally, how about those interesting Neanderthal men? We customarily base our estimate of their probable strength upon the degree of prominence of their muscle attachments as observed in the fossil bones-but should not the above consideration incline us toward caution in this class of inferences, particularly when the subjects are an ancient race known to have approximated closely to the anthropoidal type in their anatomy--as well as impel the comparative anatomist to a thorough investigation into the reason for this strange discrepancy.

The hip is not a great example, because of the architectural difference between the human and ape pelvis. Still, we can make the same general observation about other bones like the humerus, on which muscle attachments don't convey the information about relative chimpanzee strength.

Pronounced muscle attachment sites are not evidence that Neandertals were weak; within the context of recent hominids the rugosities and robusticity of bones are probably good indicators of muscle mass. But with some increasing evidence for evolution of muscle functional properties on the human lineage, I hesitate to assume that any Pleistocene human muscles interacted with their bony attachments in exactly the same way as ours.

The muscle attachment issue may be especially confusing in Australopithecus, where there is a substantial contrast within species between small individuals like AL 288-1 and larger individuals -- the Maka humerus comes to mind. Here's the comparison, from White et al. (1993):

Maka is the heavily-crested humerus at the top, Lucy's slender and smooth humerus underneath. Straightforward mass difference? Or difference in activity pattern? Or both? This is not such an unusual comparison considering the variability in living hominoids, including people. But it illustrates well the kind of range of muscle marking and cresting that existed in fossil populations. The changes during human evolution would have happened upon this underlying pattern of broad variation.

We can probably assume it's not the functional properties of muscle differing between these two specimens. But what about between Australopithecus and Homo? Or Neandertal to recent human? We already know there are differences in muscle function among human populations, in part corresponding to alpha actinin-3 allele frequencies. Genetics may be starting to make the "expensive tissue" story come down to muscle instead of gut reduction -- if I'm going to make predictions, I would say that MYH16 will not long be alone as a gene corresponding to human muscle reduction.

References:

Bauman JE. 1926. Observations of the strength of the chimpanzee and its implications. J Mammal 7:1-9. JSTOR

White TD, Suwa G, Hart WK, Walter RC, WoldeGabriel G, de Heinzelin J, Clark JD, Asfaw B, Vrba E. 1993. New discoveries of Australopithecus at Maka in Ethiopia. Nature 366:261-265.

I'm not really qualified to give chimpanzee-related advice, but...

...when your friend calls for help because her 200-pound Xanax-drugged chimpanzee has stolen the car keys...

His owner, Sandra Herold, 70, had called a friend over to help when "Travis" began misbehaving at 241 Rock Rimmon Road. The chimp had taken the keys to the car. He was also trying to open car doors, which he apparently did to indicate he wanted to go for a ride. Herold was able to coax Travis back to the house and she gave him some Xanax-laced tea, police said.

When the friend, Charla Nash, 55, of Stamford, arrived minutes later, the 200-pound chimp bolted outside and began brutally attacking Nash as she was getting out of her car, leaving her with severe facial injuries. She was in "very critical condition," according to police, and her condition had not improved considerably Tuesday.

...call Animal Control.

Another of our periodic reminders that chimpanzees are not domesticated animals.

Martin Surbeck and Gottfried Hohmann report in Current Biology that bonobos hunt monkeys, like chimpanzees.

It has been suggested bonobos do not hunt monkeys because aggression was selected against when ecological conditions favored female gregariousness and alliance formation [4]. An alternative view is that insufficient data from multiple bonobo populations, incomplete habituation, and effects of human interference precluded observation of monkey hunting [6]. While more data are required before conclusions can be drawn about the relationship between social traits and hunting behavior, our data raise other questions: Do the observed cases present a novel behavior? What are the environmental and social factors promoting hunting and meat eating at LuiKotale?

So far, evidence for hunting and meat eating by bonobos has largely been based on fresh fecal samples [3]. Only one sample contained the digit of a black mangabey, Cercocebus aterrhimus, but it was not entirely clear if bonobos had hunted the mangabey themselves, or whether they had taken it from another predator. In both Pan species, hunting of mammalian prey is relatively rare and its detection requires frequent, close-range observations. Field studies in the Taï Forest (Ivory Coast) have accumulated one of the largest data sets on monkey hunting by chimpanzees, but it took years before researchers were able to directly observe monkey killing [1]. We tend to believe that improved habituation made our observations possible (rather than the behavior being novel) (Surbeck and Hohmann 2008:R906-R907).

The authors go on to speculate that hunting may be subject to traditional variation in bonobos, since at other field sites the bonobos interact in different ways with monkey species, ranging to mutual grooming. But in their observations, they have five hunts with three successful captures; two of the three individuals who caught monkeys were females.

References:

Surbeck M, Hohmann G. 2008. Primate hunting by bonobos at LuiKotale, Salonga National Park. Curr Biol 18:R906-R907. doi:10.1016/j.cub.2008.08.040