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paleoanthropology, genetics and evolution

mating

  • Primate mating patterns

    Mon, 2013-02-04 00:38 -- John Hawks
    Synopsis: 
    Primate groups are shaped by the pattern of mating competition and interactions

    Ecology, diet, competition, and ease of movement all affect the size of primate groups. The structure of primate groups is primarily affected by the mating system. There are several elements of primate mating systems. In most species, individuals of either one sex or the other disperse from their natal group --- the one they were born in --- when they reach adulthood. This dispersal affects the structure of the groups by breaking some kinds of relationships and preserving others.

    For example, in primate species where maturing females transfer to a new group, males are often left within the group where they are born. This means that males can form relationships as juveniles that last their entire lives. The long-lasting male coalitions in chimpanzees are a side-effect of female dispersal, conditioned by large group sizes and other social factors. In contrast, male savanna baboons transfer to new groups when they reach adulthood. In baboon groups, the female associations are highly structured by kin relations, since mothers and daughters live in the same group as adults.

    Mating conflicts

    Mating is essential to reproduction, and is the contest through which individuals pass their genes into future generations. From an evolutionary perspective, individuals do whatever they can to promote their own chances to reproduce. Sometimes, individuals can promote their own reproduction by inhibiting the chances of others. In other instances, it may be in their best interest to cooperate with other individuals, or to bide their time waiting for higher-ranking individuals to die or lose status.

    The most basic conflict of interest in mating is between males and females. Because of their biological role in carrying and providing nutrition for their offspring, both before and after birth, females must make a large investment in reproduction. Considering the large cost of reproduction, an adaptive mating strategy for a female is to mate with the male whose genes will contribute to the best possible offspring. For this reason, females are typically choosy about which males they will mate with. This element of female choice can give rise to sexual selection, in which males are advantaged by the possession of features that females value.

    In contrast, males may have extreme levels of competition for mates. A single male may be able, through fighting, threat, or intimidation, to prevent other males from having mating access to females, or even to expel all other adult males from the group. If he is successful, the reproductive opportunities for such a male are tremendous. On the other hand, the opportunities of other males fall to zero. This places a tremendous genetic payoff on social competition for mating.

    The intensity and form of mating competition vary from species to species. Some kinds of primates have a sparse diet that is simply unable to sustain the caloric requirements of huge male body sizes. Other primates or may modify the conditions of combat through coordination of activity with other individuals, emphasis on advertising the risks of combat rather than pursuing combat itself, or other means.

    Sexual dimorphism is a difference in size or form between males and females.

    Males and females within a species often differ in size or morphology. For example, male primates almost always have larger canine teeth than females. The canine teeth are important in male mating competition --- males can display their canines as a threat to other males, and at an extreme they can injure or kill other males with these teeth. One indication of the importance of the canines in displays is that they tend to be more dimorphic in species that are active during the day, as opposed to nocturnal species (Leutenegger and Cheverud 1982).

    Sexual dimorphism in body size is very pronounced in many primate species. For example, orangutan males average around twice the body mass of females. The body mass dimorphism is even more extreme for gorillas than for orangutans. Many different factors influence the body size dimorphism in a primate species (Hedrick and Temeles 1989). One of these is mating competition between males --- intense male competition increases the value of male body size. Another factor can be food competition between the sexes --- when males are larger, they may be able to dominate a larger share of valued food resources. Additionally, males can take on important reproductive roles beyond mating, such as protecting young juveniles from predation or infanticide.

    Territorial primate groups maintain their home ranges against incursions by other groups or individuals.

    Mating competition in many primates involves territoriality, when males defend a home range against incursions from other males. Primate groups may be territorial as a result of a single male's action, or the coordinated activity of multiple males. For example, Males of some primate species dominate access to females by preventing other males from coming into their home range. These primate males are said to be territorial. Even small social groups, like the monogamous male-female pairs of gibbons, may be highly territorial. But chimpanzees provide one of the strongest instances of territoriality among primates. Groups of male chimpanzees walk the approximate perimeter of their territory, engaging in violent conflicts with any members of neighboring groups they might encounter (Wrangham 1999). As observed by Jane Goodall (1986) at Gombe in Tanzania, the males of one group of chimpanzees killed all of the adult males and several females and juveniles in a neighboring group over the course of several months.

    Kinds of groups in primates

    Group size, dispersal, and mating competition all contribute to the proportion of males and females found in any given group. Sometimes a single male and female form a group; sometimes a single male and multiple females; sometimes multiple males and females; and occasionally a single female and multiple males.

    Monogamous, or pair-bonded, species have long-lasting mating relationships between a single male and a single female.

    Gibbons tend to form long-lasting associations between a single adult male and a single adult female. These pair-bonded primates occupy territories that they defend against incursions from other individuals. Both males and females make long vocalizations, called songs, to establish their shared territory. They often vocalize together in duets, although in different contexts based on whether threats come from males or female intruders (Geissmann 2000, Mitani 1987). These groups of a single adult male and female and their offspring are called monogamous groups.

    Polygynous groups have a single dominant male and multiple females.

    In many kinds of primates, a single male may dominate a group with multiple females. This is a polygynous group --- a multifemale, single-male group. Polygyny results from strong mating competition among males. Only if a single adult male can repel other males from the group can he reap the powerful reward of mating with many females.

    A well-known species with polygynous groups is the gorilla. Gorilla groups generally have one adult male, up to eight or more females, and their dependent offspring. Solitary males live outside of these polygynous groups and sometimes manage extragroup matings with females. These groups are maintained by strong mating competition --- a dominant male in a group repels other males by threats, intimidation, or violence.

    Even so, there is variability in gorilla societies. In mountain gorillas, multimale groups are common (Robbins 1999). In such groups, the dominant males have a majority of matings, and often harass subordinate males that attempt to mate. But subordinates do have many mating opportunities, demonstrating a social flexibility among gorillas.

    Polyandrous groups have a single dominant female and multiple males.

    Marmosets and tamarins, called callitrichids, are the smallest of the New World monkeys. These monkeys are among the few primates for which twin births --- and sometimes triplets --- are common. This means that females tend to have high energetic requirements in pregnancy, lactation, and caring for young. Callitrichids therefore face unique challenges compared to other primates.

    One way that these monkeys adapt to caring for more young is that older offspring of a female may stay with her for a longer time instead of quickly going off on their own. These older offspring help to watch and sometimes provide food for their younger siblings (Bales et al. 2000).

    Another behavioral adaptation is for a single female to mate with and coexist with multiple males. This kind of mating system is called polyandry. Mating with multiple males reduces the paternity certainty of the males --- a male cannot know if a female conceived her offspring with him or another male. As long as mating opportunities are limited, males may cooperate in a group with a single female on the chance of having offspring with her. These males help to provide food and defense for the young juveniles in polyandrous groups. Polyandry is not universal among callitrichids; it is adopted more when resources and mating opportunities are rare (Goldizen 1988).

    Fission-fusion societies are large multimale, multifemale communities that spend much of their time divided into smaller units that combine in different combinations.

    Some primates coexist in large groups numbering 50 individuals or more. A group this large is always a multimale group --- there is no way for a single male to deter other males from twenty or more adult females. But multiple males sometimes coordinate their behavior to deter neighboring groups. A large multimale group may occupy and defend a large territory, especially where movement costs are relatively low.

    Large primates who live in large groups have a problem: there are very few food patches large enough to feed them all. So even though a large multimale, multifemale group may occupy a substantial territory, it may not be possible for them to feed together much of the time. Chimpanzees live in such large multimale, multifemale groups. Even though members of the group share a dominance hierarchy, social interactions, and relationships, they spend much of their time apart. Smaller groups of individuals --- sometimes a single female and her young, sometimes male-female pairs, and sometimes small groups of either sex --- split apart in order to forage for food. These small groups recombine and split in different combinations, and sometimes all of them come together, especially when food is plentiful. This kind of social organization is called a fission-fusion society. Individuals divide into small foraging groups and come back together into the full community for social interactions.

    Study terms: 
    polygynous
    fission-fusion
    monogamous
    polyandrous
    Tags: 
    Anthropology 105
    primates
    behavior
    mating

  • Neandertal anti-defamation files, 17

    Tue, 2013-01-01 17:30 -- John Hawks

    Let no one say that I'm an uncritical voice about the many advantages of releasing preprints. They do have their downsides. Lack of editing is one.

    Here's a passage from a new preprint from Peter Waddell and Xi Tan, "New g%AIC, g%AICc, g%BIC, and Power Divergence Fit Statistics Expose Mating between Modern Humans, Neanderthals and other Archaics":

    The apparent lack of Denisovan alleles on the X chromosome suggested that some of these archaic interbreeding events were male biased, that is archaic males mating with modern females (Waddell, 2011). This was formerly dubbed the “archaic Ron Jeremy” hypothesis, after the well-known American thespian. Formerly known, because a journal editor has recently urged us to alter our manuscript, to avoid confusion with a “Ron Jeremy Event”, which they referenced to the Urban Dictionary. The new synonymy is the “lecherous archaic man” hypothesis.

    I'll return to the argument in the paper later, I just wanted to consider the question of Neandertal similarity to well-known thespians. This is a followup to another preprint from last 2011, which addressed the question of male-biased gene flow into the ancestry of Papua New Guinea from Denisovan peoples ("Homo denisova, Correspondence Spectral Analysis, Finite Sites Reticulate Hierarchical Coalescent Models and the Ron Jeremy Hypothesis"). From that preprint:

    While the origin of the unusual features of the NSYFHP pattern is just a hypothesis at this stage, it is testable and deserves a name, so we call it the “Ron Jeremy hypothesis” (after the accomplished American thespian Ron Jeremy, who is adroit at debauching modern young women, whose father’s might well call him a Neanderthal or a Denisovan, and who looks remarkably like reconstructions of these archaic humans in museums, including being very big boned).

    Big boned.

    Similarly, we may refer to the low frequency of the NSYFHP on the X chromosome as “Ron’s Grandfather hypothesis” which is the mixing of the Denisovan lineage with an even more ancient hominid lineage due to a male biased infusion.

    Obviously we badly, badly need a better system of terminology to discuss the relationships of archaic human groups, including MSA and earlier Africans, which we now understand to have been subject to recurrent gene flow. Male-biased gene flow has often happened in human groups, sometimes due to warfare or the dominance of elites, sometimes as a simple function of greater male dispersal. Male-biased gene flow also appears to characterize orangutan population history, but not chimpanzees, so it depends on species-specific aspects of population structure and dispersal strategies.

    We unfortunately have a 150-year history of looking at Neandertals, and secondarily at other archaic human groups, as strange evolutionary dead-ends. When faced with the evidence that these ancient people are among our ancestors, some scientists have turned first to the idea that mating among ancient people was exotic and strange. Hence the "Ron Jeremy" angle.

    Tags: 
    Neandertal anti-defamation files
    Neandertals
    humor
    population structure
    mating
    dispersal
    Neandertal DNA
    Denisova

  • The real "junk" DNA

    Wed, 2011-03-09 22:47 -- John Hawks

    Let me be honest: when I started doing paleoanthropology, I really did not expect I'd be talking about Neandertal penises.

    And yet, here I am. Cory McLean and colleagues [1] combine a straightforward genomic analysis of human-specific deletions with a couple of transgenic mice, and take us straight to penis spines.

    You see, most primates, and indeed many mammals, have at least some spines on their penises. "Spine" means more or less what you would expect: little projections that are covered in hard material, generally keratin, curving toward the base of the penis. These spines are sometimes called "horny papillae."

    No, I cannot make this stuff up.

    The morphology of these spines varies among primates. They overlie sensory receptors, and they intensify or enhance sensations accompanying intromission of the penis. Like a KY commercial, except they don't enhance sensations for the female. The net effect in some species is to reduce how long it takes the male to ejaculate. For example, a 1991 paper [2] by A. F. Dixson...

    No, I cannot make this stuff up.

    ...removed the penile spines of several male marmosets, finding that they took twice as long to achieve penile intromission after starting pelvic thrusts. Of course, "twice as long" in marmosets only means 15 seconds. The spineless males took 2 seconds to ejaculate, compared to only 1.73 seconds for those who had a "sham surgery" -- that is, they got the same depilatory spine-removal procedure without the active ingredient. That's some evidence in favor of the idea that losing penile spines might be related to longer coital duration.

    But penile spines don't always mean fast sex. Galagos have penises covered in long hook-like spines, which they use in virtual sex marathon sessions lasting two hours or more. Prosimians tend to have much more elaborated spines, in contrast chimpanzees' spicules are comparatively minor -- in a broad comparison across primates, Harcourt and Gardiner [3] rated chimpanzees along with humans as having insignificant penile spinosity.

    Let me just say that the comparative data don't convince me of an adaptive model for loss of penile spines in humans. Evidence from mutilated monkeys is not all that persuasive. I mean, really, how fast do you think you would manage after the "operation"? More important, the differences among hominoids run against the hypothesis -- gibbons have the spiniest penises among the apes, despite their monogamous, pair-bonded social habits.

    And I'll pause to savor the surreality: I'm here making value judgments about genital cacti.

    One thing that is definitely well-known about these penile spines is that their development depends on testosterone. Castrated monkeys do not develop the characteristic spines, and they lose them if already present. The androgen receptor (AR) locus is surrounded by promoter/enhancer sequences that are tissue-specific, capable of being flipped on or off as development proceeds within different parts of the body.

    Within this system, the genetics in humans and chimpanzees are simple: A long (60 kilobase) deletion of DNA in the human lineage has knocked out a 5 kb conserved region that enhances AR. That enhancer is specific to the follicles around the developing facial whiskers (vibrissae) and in the skin layers of the penis. This specificity was discovered in transgenic mice, in which a reporter gene is inserted with the enhancer, and embryos display expression of the reporter wherever the enhancer is active. Very straightforward, very cool science.

    One more thing: The chimpanzee version can drive expression when implanted into transgenic human foreskin fibroblasts. That indicates that the overall genetic system to make penile spines is still there lurking in our genomes. If we could turn on the gene at the right time, replacing the function of the enhancer, we can still grow penile spines.

    Just saying -- there may be a market there. Maybe the "male enhancement" companies will hit that next. I can only imagine what the wrapper on the NASCAR circuit will look like. OK, I know, don't encourage them. It's bad enough that we have labs full of foreskin tissue with chimpanzee genes floating around.

    I couldn't make this stuff up if I tried.

    Finding the deletion was straightforward genomics: They scraped the human genome for parts missing from chimpanzees and macaques, and then extracted from that set all deletions that included sequence conserved in other mammals. Others have done similar comparisons for conservation and human-specific changes; this is a clever twist on the same problem. It does fit an ongoing theme -- many essential aspects of humans may involve the loss of genes or functionality from our ape ancestors.

    Ok, so where do Neandertals fit in? They have the sequence deletion just like the rest of us do. If that deletion rules out chimpanzee-like spiky penises, then Neandertals could glide like the rest of us.

    All in all, it's a nice short paper, and very straightforward. The only questionable part to me is the social model. The genetics and expression data are solid.

    Speaking of Neandertals and the androgen receptor (AR) locus, my genome appears to have a Neandertal-derived haplotype across that gene. I'll expose this fact at greater length later, but I thought it worth sharing that the current paper is not the end of the story. Neandertals may not have had penis spines, but some functional polymorphisms in testosterone response might still have come into our population from them or other ancient people.

    UPDATE (2011-03-11): Eric Michael Johnson gives us the real dirt on this story ("Penis spines, pearly papules and Pope Benedict's balls"). He points out the relatively small extent of these features of the chimpanzee penis compared to other primates, and adds detail about the lack of association between their presence and sexual system in hominoids.

    He also reveals a shocking fact: a fairly large fraction of men still have the chimpanzee-like pearly papules.

    Scicurious also takes on the topic "Friday Weird Science: Penis Spines, what are they REALLY?", reviewing the original Osman Hill study of chimpanzee penis morphology. I think the Nature paper is very misleading in its use of galago illustrations for these spines, the chimpanzee version is comparatively minor.

    References

    1. McLean CY, Reno PL, Pollen AA, Bassan AI, Capellini TD, Guenther C, Indjeian VB, Lim X, Menke DB, Schaar BT, et al. Human-specific loss of regulatory DNA and the evolution of human-specific traits. Nature [Internet]. 2011;471:216–219. Available from: http://dx.doi.org/10.1038/nature09774
    2. Dixson AF. Penile spines affect copulatory behaviour in a primate (Callithrix jacchus). Physiology & behavior [Internet]. 1991;49:557–562. Available from: http://view.ncbi.nlm.nih.gov/pubmed/2062934
    3. Harcourt AH, Gardiner J. Sexual Selection and Genital Anatomy of Male Primates. Proceedings: Biological Sciences [Internet]. 1994;255. Available from: http://dx.doi.org/10.2307/49837
    Tags: 
    chimpanzees
    Neandertal DNA
    mating
    primates

  • Mental mismatches

    Thu, 2010-09-23 08:30 -- John Hawks

    A Primate of Modern Aspect ("The sexuality wars, featuring apes") writes about some of the reactions to the new book, Sex at Dawn: The Prehistoric Origins of Modern Sexuality. As the subtitle suggests, the book is an account of human sexuality from the viewpoint of evolutionary psychology, written by Christopher Ryan and Cacilda Jethá. Ryan blogs at Sex at Dawn, I'm a frequent reader.

    Anyway, I loved this point about comparative studies:

    [F]or some reason, the only time primate sexuality gets any attention is when we turn it into a debate about how humans should be having sex.

    We never say, “Hey, those muriquis are too promiscuous. Don’t they know that all of their close evolutionary cousins are polygynous? If they just did what came naturally to them, they’d have a lot less psychological stress.” Or, “Those gibbons are so sexually repressed. If they just gave in to their natural predilection for promiscuity, I bet those nasty gibbons would have fewer territorial disputes and gibbon society would be much more peaceful.”

    Why worry about the "echoes" of psychic distress that may linger after the mating system changes? That's a very interesting point; there are unexplored assumptions here about the nature of adaptation and the structure of genetic causation of mental states. Clearly if major aspects of human social life change, we cannot expect people's minds to be perfectly optimized to the new regime. But what is the force of selection? What are the mental "rough spots" that differential fertility will ultimately iron out? How much "mismatch" between mental and social adaptations can persist?

    Primates may not be the best non-human model for such questions. Some domesticates have undergone social changes as great as humans, with strong selection against individuals who buck their human masters. But for many wild primates we may reasonably wonder, to what extent are social dynamics constrained by mental adaptations, and how quickly can mental lives shift under selection to fit a new social system?

    Tags: 
    sexual selection
    behavior
    domestication
    mating
    primates
    brain
    evolutionary psychology
    mind

  • "Naughty Neandertals" did what?

    Thu, 2009-09-24 12:40 -- John Hawks

    Gretchen found this one:

    Naughty Neanderthals nixed monogamy

    Oh, no. Where is this going?

    Emma Nelson of the University of Liverpool and a team of researchers combed through literature on early human-like primates in search of fossils that contained hands with intact index and ring fingers (the second and fourth digits).

    Oh, no. Now I can see where this is going. It's the magical anthropometric: the 2D:4D digit ratio!

    Though highly contentious, studies indicate that men who receive high levels of androgen before birth are more likely to be stronger, faster, and more sexually competitive. Women who receive high levels of androgen may have similar traits.

    Well, it's true -- a number of studies have found 2D:4D correlated with lots of things. My favorite is the one about international athletes:

    We have found that low 2D:4D ratio is associated with a high level of attainment across a number of sports and also with high mental rotation scores. Professional football players had lower 2D:4D ratios than controls; 1st team players had lower ratios than youth team members or reserves; international players had lower 2D:4D ratios than those who have not yet represented their Country; and in a one-tailed test, 2D:4D ratio was negatively related to number of international appearances after the affect of Country was removed. We suggest that low 2D:4D ratio in men is a correlate for high ability in many sports, including football (Manning and Taylor 2001).

    The problem is that the ratios have a high variability within groups like this. So that the 2D:4D ratio may be correlated with performance, but it's a poor predictor of performance. It's not like you'd want to measure this ratio on one person and then start talking about whether he would be a good football player.

    Of course, if you're dealing with fossil hominins, then you're stuck with what you can dig up:

    Nelson's work suggests the same holds true for most primates living today, but the team wanted to see how our ancient relatives stacked up. They found two Neanderthals and one Australopithecus afarensis skeleton with the first bones of the index and ring fingers intact — enough detail to do the job.

    The digit ratio varies a lot within humans, much more than its correlation with anything. So it would take a very extreme ratio in these two Neandertals to say you had any significant evidence of anything -- we're talking, outside the human range. And since we're not talking about complete hands, but instead parts of hands, I don't think we're going to get there.

    Well, what about Australopithecus?

    A. afarensis, made famous by the popular "Lucy" skeleton, lived between 4 and 3 million years ago, long before modern humans. Its short ring finger hints that it was faithful to a single mate, but Nelson says that doesn't sit well.

    "These were small creatures that probably lived in groups and were being eaten by predators." she said. "How do you keep from mating with different members of the group?"

    I'm sure the conclusion of monogamous early hominins would make Owen Lovejoy happy, but it's really just utter sheer speculation, which a quote from Nelson clarifies later in the article. Nelson has a good website, where we can see where her research is going. The description along the same lines, but more reasonable than the "Naughty Neanderthals" article -- the idea is that we already know that the 2D:4D ratio varies within primate species and that it has several interesting social correlates within some species. For example, it correlates with social rank in female macaques. So there's some reason to think that it might vary among species in a way that reflects social systems, which (according to Nelson) they've found.

    I just want to remind people of the obvious: 2D:4D may be correlated with mating system in primates, but that doesn't mean it's a good predictor of mating system. Canine dimorphism, which has been studied for a long time in relation to sexual dimorphism and mating competition, isn't even that good a predictor of mating system. And even if the mean were a good predictor among species, doesn't mean that an individual ratio is a good predictor of the species mean.

    As fossil hominins go, I wouldn't expect the story to go any further -- there just aren't many hands, so there's never going to be a significantly predictive result.

    And as stories go, this one could have been a lot worse. After all, the digit ratio is also correlated with homosexuality in men. I'm surprised the headline wasn't "Fingers Point to Gay Neanderthals".

    References:

    Manning JT, Taylor RP. 2001. Second to fourth digit ratio and male ability in sport: Implications for sexual selection. Evol Hum Behav 22:61-69. doi:10.1016/S1090-5138(00)00063-5

    Tags: 
    mating
    primates
    development
    Neandertals
    sexual selection

  • For maximum fertility, marry more than 20 km from your birthplace

    Mon, 2008-02-18 21:56 -- John Hawks

    Labouriau and Amorim (2008) show that women have more children if they were born farther from their husbands:

    We report a positive association between marital radius (distance between mates' birthplaces) and fertility detected in a large population. Spurious association due to socioeconomic factors is discarded by a conditional analysis involving income, education, and urbanicity. Strong evidence of consanguinity's deleterious effects affecting an entire human population is provided.

    They followed a birth cohort of women in Denmark, all born in 1954.

    This result may be qualitatively similar to the third and fourth cousin marriage paper from earlier this month. In that case, Iceland couples with the maximum fertility were those between third and fourth cousins -- both closer and further consanguinous matings resulted in fewer offspring on average.

    In this study, couples born further apart had more children. The distance between the woman's and man's birthplaces is called the "marital radius." The data from the paper show that the relationship between marital radius and fertility is mostly explained by a reduced fertility for couples with marital radius less than 15 km.

    Figure 1 from Labouriau and Amorim, 2008.

    The result cannot be explained by a small proportion of high-reproducing couples, as demonstrated by the percentage of couples with more than two children, which tracks the average number of children. There is a slight negative correlation between urban residence and fertility; the authors show by partial correlation that this alone does not explain the reduced fertility with low marital radius (which is important, since a large fraction of the low-marital-radius couples may have been born in the city).

    One potentially important variable was not included: marital age. To be sure, some women who marry men at low marital radius may be swept away as teenagers and go on to have eight children. But others may be settling, marrying late in life.

    This study isn't orthogonal to the Iceland cousin study, but it adds another element. These people are about as close to the genetics of Icelanders as we can hope to get. The authors suggest that greater mobility in the last 50 years has removed a significant inbreeding depression. Yet, the inbreeding between people born within 15 km of each other is mostly at the level of third cousins or further -- the sample that had the highest fertility in Iceland. Curious.

    It would be interesting to see whether this result holds over longer distances. levels out, or even reverses.

    References:

    Labouriau R, Amorim A. 2008. Human fertility increases with marital radius. Genetics 178:601-603. doi:10.1534/genetics.107.072454

    Tags: 
    mating

  • Third and fourth cousin marriages more fertile

    Sun, 2008-02-10 16:10 -- John Hawks

    At least, in Iceland. Jeanna Bryner's story tickles the "ick" factor and does a fairly good job of explaining the study by Anna Helgason and colleagues, reported in this week's Science:

    The team found that women born between 1800 and 1824 and who partnered with a third cousin had an average of about four children and nine grandchildren, while those related to their mates as eighth cousins or more distantly had three children and seven grandchildren. A similar pattern showed up for women born between 1925 and 1949. Third cousins had an average of three children and about seven grandchildren, compared with two children and five grandchildren for eighth cousins and beyond.

    I'd like to see how these data compare with other populations -- it seems to me that French Canadians probably have extensive enough records, and maybe Swedes. Not that there's any reason to disbelieve the results, just that it is very hard to correct for the social and cultural factors leading to marriages between distant relatives.

    For one thing, people whose great-great-grandparents had lots of kids will obviously have more third cousins. To the extent that high-fertility great-great-grandparents are clumped together, it will be very hard to separate the effect of third cousin marriage from genetic heritability of fertility. As we all know, there has been lots of selection in recent human populations -- meaning that many alleles that influence fertility variance are segregating. So all things being equal, people who reproduce more are likely to be distant relatives.

    There is a possible test -- just compare people who married third cousins with people who didn't marry their third cousins, but still had just as many third cousins. OK, so that might not be so easy, but if the demographic data are good enough it should be possible.

    Another potential problem is outlined in the article: age at first birth:

    One caveat: More closely related couples may just start making babies earlier than others. Past research has revealed "strong evidence that couples who were first cousins married earlier and were less likely to use contraception, the wives had their first child earlier, and they continued child-bearing at later ages," Bittles told LiveScience.

    Yeah, that could be going on, too -- particularly if young, marriageable women are snatched up by distant relatives. Not too unlikely in a small town or farm community.

    The paper gives some details that tend to blunt such criticisms. For one thing, it makes a significant difference whether the couple is sixth or seventh cousins. At that distance -- people who share a great-great-great-great-great grandparent -- the authors argue that any effect is most likely "biological" (read genetic), because other possible differences do not discriminate at that level.

    In the end, the paper suggests that the demographic transition in modern urban societies is related to this kinship-mediated fertility effect:

    The formation of densely populated urban regions that offer a large selection of distantly related potential spouses is a new situation for humans in evolutionary terms. We note that if the relationship between kinship and fertility has a basis in human reproductive biology, then it follows that the kind of demographic transition recently experienced by the Icelandic population could directly contribute to the slowing of population growth elsewhere through the relative increase of distantly related couples.

    In other words, you thought that you were having fewer kids than your great-grandparents because you're living in a city instead of on a farm. But you're really having fewer kids because you're not married to your third cousin. Or something like that.

    References:

    Helgason A, Pálsson S, Guðbjartsson DF, Kristjánsson Þ, Stefánsson K. 2008. An association between the kinship and fertility of human couples. Science 319:813-816. doi:10.1126/science.1150232

    Tags: 
    mating

  • Seeing red

    Sat, 2007-07-07 10:11 -- John Hawks

    André Fernandez and Molly Morris have an interesting paper in American Naturalist examining the effect of color vision in primates as a bias toward the evolution of sexual signaling. Their conclusion is that the evolution of red coloration in different lineages of primates followed (rather than drove) the evolution of trichromatism. The convergent evolution of red coloration in many primate lineages can be said to result from a bias in the perceptual systems of primates with trichromatic vision.

    Which is all just to say that primates who can't see red don't usually become red. Fernandez and Morris conclude that trichromatism evolved for ecological rather than social reasons, such as the detection of edible leaves and/or fruits.

    (via henry)

    Dennis O'Neil has put up a primer on primate color vision.

    References:

    Fernandez AA, Morris MR. 2007. Sexual selection and trichromatic color vision in primates: statistical support for the preexisting-bias hypothesis. Am Nat 170:10-20.doi:10.1086/518566

    Tags: 
    mating

  • Here's to you, Mrs. Chimpinson

    Wed, 2006-11-22 10:51 -- John Hawks

    Muller and colleagues (2006) found that male chimpanzees approach older females more often for copulation (compared to younger females), more males tend to hang around with older females in estrus (compared to younger females), older females tend to mate more often with high-ranking males, and males compete more aggressively to mate with older females:

    This study demonstrates that male chimpanzees do not merely disdain young females but actively prefer older mothers to younger mothers. Our findings are consistent with evidence that a variety of mammals demonstrate male choosiness and with prior indications that, in other promiscuously breeding primates, young females are generally eschewed. They provide a stark contrast to patterns of male mate choice in our own species. Such choice has been clearly established, both by cross-cultural studies showing that women's sexual attractiveness peaks in the late teens to early twenties and declines steadily thereafter with age and parity, and experimental data showing that many of the features men find sexually attractive are paedomorphic; these features include large eyes, wide cheekbones, narrow cheeks, small noses, slight chins, slender jaws, low waist-to-hip ratios, and high voices.

    At least three human traits that do not occur in chimpanzees could contribute to the observed difference in male mate choice. Long-term pair bonds are hypothesized to promote preferences for youth because men who choose relatively young partners maximize their future reproductive opportunities with those partners. Direct paternal investment may favor men who choose young mates because young women are less likely to have offspring from previous partners, and this minimizes men's contributions to unrelated offspring. Menopause, and the associated decline in female fecundity starting in the late twenties, may further exaggerate preferences for youth by limiting women's future fertility. In contrast, males of more promiscuous species, such as the chimpanzee, are expected to focus on their immediate reproductive opportunities and discount females' longer-term reproductive value. Additionally, unlike humans, chimpanzees provide no direct paternal care to offspring and show no evidence for menopause. Further analysis of male mate choice among species in which such traits are isolated will help in discriminating their relative importance for the evolution of male preference for youth.

    The title of the post suggests the obvious: there is no single human pattern of male mating preference; there is variability around a mean, including certain strong aversions. This must be true of chimpanzees also, so the question is why the mean should be different.

    The suggestion here is not unreasonable, but I think it is incomplete. In humans, for example, there is a very strong influence of personal history and continuing interactions -- which explains why older men would choose to continue with their wives instead of going for younger women. Old-fashioned sentimentalists like me call this "love" (Hi, Gretchen!).

    Since older chimpanzee females have a much longer history with resident males than do young females, who mainly came from somewhere else, it seems just as plausible that familiarity is important to chimpanzees as well. In that context, a preference for younger females might depend on group structure, dispersal strategies, and the lack of availability of older females, not merely on ultimate lifetime investment. I mention this because it would have been very difficult for a hominid to make the jump immediately to very long consortships from a chimpanzee-like group structure, but it might have been relatively easier from other group structures.

    Another solution, which the paper mentions, is the importance of rank for older chimpanzee females, which plays a direct role in their offspring's fitness -- including their male offspring. If a male wants to maximize the chance of having high-ranking male offspring, the way to do it is to mate with a high-ranking female. In humans, this can sometimes be much less important because paternity may be recognized -- a high-ranking male may confer rank to his sons directly.

    References:

    Muller MN, Thompson ME, Wrangham RW. 2006. Male chimpanzees prefer mating with old females. Curr Biol 16:2234-2238. DOI link

    Tags: 
    chimpanzees
    mating

  • Buller on mating preferences

    Fri, 2005-10-07 20:13 -- John Hawks

    Chapter 5 of David Buller's Adapting Minds : Evolutionary Psychology and the Persistent Quest for Human Nature is mostly about the critique of studies that purport to demonstrate human mate preferences, covering males and females in turn. Buller's critique here is not that the theoretical basis for differences in male and female preferences is weak, or that there is no theoretical reason to suppose the mate preferences asserted by evolutionary psychology (males prefer young fertile females, females prefer high-status males). He does argue against both theoretical points later in the book, however, and here he reiterates his doubt that such preferences might be adaptations and part of a universal developmental program.

    But the focus here is to refute the specific evidence that is supposed to demonstrate human mate preferences. Most of the chapter therefore takes the form of a batting practice, as Buller takes pitches from many different studies and clubs them down one by one. It's therefore not the most interesting piece of writing, but it does carry a sort of emotional satisfaction -- sort of like a long game of Whack-a-mole.

    Now, if the evidence for the claim that females prefer high-status males is as weak as I've made out, why is the claim so widely accepted? I think the reason is that we are captivated by a particular picture of the relation between sex and status among our primate relatives, and this picture affects our perception of human mating. It is widely accepted that among non-human primates high-status males have greater mating success than males lower in the status hierarchy. This belief is due partly to the popularity of the engaging work of the primatologist Frans de Waal, who has been one of the main purveyors of this idea. Once we're convinced of the strength of the correlation between status and mating success among our primate relatives, the standards of evidence that are required to convince us of a correlation in humans get lowered considerably. As de Waal says: "In monkeys and apes there is a clear link between power and sex. High-ranking males enjoy sexual privileges, and are more attractive to the opposite sex. We need only look at recent events in the White House (and at a television spectacular like 'Who Wants to Marry a Multimillionaire') to see how much the link exists in us too." (Buller 2005:250, citing de Waal's 2000 New York Times book review of Thornhill and Palmer's Natural History of Rape)

    Buller lists two problems with this viewpoint. First, of course anecdotal evidence is not enough to demonstrate mating preferences in humans generally. Beyond anecdote, there is precious little evidence that female humans actually do prefer high-status mates.

    This may sound surprising to anyone cognizant of the literature on mating preferences in humans. As Buller describes, a major confounding factor is that people tend to mate assortitively with others of like backgrounds, interests, and prospects. These include financial prospects. They also include attractiveness, which is correlated with financial prospects in industrialized societies. This means that saying anything generally about female mate preferences must both make sure that the samples are representative (i.e. that they don't include one socioeconomic class to the exclusion of others) and that the confounding factors must be controlled (i.e. that an apparent preference for high status is not actually explained equally well by the preference for attractiveness).

    As it turns out, the majority of work attempting to determine female mate preferences in humans has been done by surveying female undergraduate students at universities. Some of this work has been done by surveying women at their sororities! Now, of course female undergraduates including sorority members have mating preferences, and these preferences carry information about the mating preferences of females in society. But the sample of females who attend universities (especially private ones) and who belong to sororities (especially at private universities) is not characteristic of the population at large -- it is a sample biased toward women who expect to achieve high education status, who may have expectations of high income levels, and who disproportionately come from upper middle class backgrounds. That these women may prefer high-status men might be explained by a general preference for high-status men. Or it may be explained by a preference for men with similar interests, education, and prospects to their own.

    Buller reviews studies that don't follow this bias and concludes that the evidence for a female mating preference for high-status men is weak or nonexistent. Indeed, reading this section is a bit like reading a slasher-movie as the hypothesis raises up again and again with each new study, and Buller strikes it down once more.

    For me reading the book, the message is that data on human psychological preferences (at least for long-term life choices) really are not available in studies of human self-reported preferences or real-world behavior. There are just too many ways that preferences can vary (where studies generally ignore variation in preferences and focus on the averages only) and too many compromises that people must make in their behavior (when they can't get what they might prefer). This is the case in spades when Buller considers male mating preferences -- and the question of whether males prefer young women with waist-to-hip ratios of 0.70. He finds, again, that there is strikingly little evidence in favor of this preference, and that most of the evidence is flawed.

    A sophisticated reader might point out that Buller hasn't really refuted the EP interpretations; he has merely provided alternatives that explain the observed data equally well. Buller is able to make these arguments effectively because of the large possible set of confounding factors, and because of the evolutionary psychology focus on averages instead of variation. When more information is available, Buller shows that it fails to support the EP interpretation. When more information is not available, Buller argues that the missing information is necessary to test the EP interpretation. In both cases, he is convincing that the evidence is weak or nonexistent. But Buller actually does note that some studies actively refute the EP expectations -- for example, the study of Kinsey's sexual behavior data that shows greater sexual activity by lower status males.

    The other objection to the analogy from primates is that primate studies don't actually show a strong female preference for high-status males. Certainly some studies (on some species) do show this, but others show either no apparent female preference, a slight preference for low-status males, or multiple strategies where some females prefer low-status males at least some of the time. And many of the studies that show a correlation of high-status with male mating success are not demonstrating anything about female preference, but instead about male control of mating access. The idea that primate females generally prefer high-status males is a non-question: some species may show such preferences, others do not, and within species there may be substantial variation in mate preferences among females.

    Buller leaves the chapter with one final point: selection might not be able to make a mate preference adaptation like the one proposed by EP anyway. Consider the way such a preference should work: people prefer as good a mate as possible, but since these are in high demand, they may have to settle for less than the prefer. Buller puts people on a scale from 1 to 10 -- the preference hypothesis supposes that everyone wants a 10, but they will tend to be able to mate with 10's themselves, which leaves 9's mating with 9's and 6's mating with 6's. But. says Buller, there's no reason to suppose that 6's have less offspring or lower long-term reproductive success than 10's. In genetic terms, if there is no reproductive benefit to a 6 in mating with a 10, then a 10 preference does not have a selective advantage over a 6-preference, at least as far as 6's are concerned. We might even think that 6's would be better off assessing their mating prospects early on and choosing 6's deliberately, instead of wasting a lot of time trying to attract a 10.

    Noting that there may be a limit beyond which settling for a lower-quality mate may negatively impact reproduction, Buller notes:

    The real question, then, is whether male status and female youth are characteristics that females and males respectively can "trade down" while still achieving comparable reproductive success. My skeptical argument presupposes that, within limits, they are. Evolutionary Psychology's view of human mate preferences presupposes, in contrast, that male status and female youth are characteristics that couldn't have been traded down by our ancestors without a corresponding decline in reproductive success (Buller 2005:256).

    Buller points out that the most EP has to support this hypothesis is hypothetical arguments about the EEA. Indeed, the sort of thing one would have to know is the long-term success over many generations of different mate preferences, the heritability of such preferences within a hypothetical ancient population where they were polymorphic, and the differences between the modern environment where these features were observed and the hypothetical ancient environment where humans evolved. It's all exceedingly tenuous.

    More on Adapting Minds

    References:

    Buller DJ. 2005. Adapting Minds : Evolutionary Psychology and the Persistent Quest for Human Nature. Bradford Books, New York. Amazon

    Tags: 
    behavior
    evolutionary psychology
    mating
    minds
    David Buller
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