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I just started Robert Sapolsky lectures series on Youtube on Human behavior biology, and on the second lecture, at this point, he is explaining how evolutionary biology can be used to predict lots of animal behaviors just through the differences on the skull size of adult males and females.

He considers two groups of primates, one in which the skull of the adult male is quite bigger than the female and the other where they are about the same size. He then asks the student: "which group would display higher male aggression?" I am lost at this point. How can the size difference alone be used to answer that question? The students answer the question: "the first group (in which the males are bigger)", and the reason provided is that their body is built for it. Why and how?

Following up, the next question is: which of the species has more variability among males in reproductive success? The answer provided: "obviously, it is the first one because the males are aggressive for a reason, and that is they are competing for reproductive access." Couldn't it be that the second group had a biological variation in sperm productivity, that greatly outmatched the fight for reproductive access of the first group? I mean, how can it be this easy to conclude such a complicated process? Shouldn't you carefully consider millions of other things before concluding that one group will have higher reproductive variation simply because of differences in the size of the skulls?

Thanks!

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    $\begingroup$ it's an "all others things being equal" kind of statement. If I were writing an exam question based on this idea, I would certainly phrase it as "which group would we expect to display higher male aggression"? $\endgroup$
    – Ben Bolker
    Dec 31, 2018 at 16:53

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Having a large difference between the sizes of the skulls between males and females is strong sign of sexual dimorphism, particularly of males that are larger in general. Knowing which form of sexual dimorphism there is allows you to predict several other features. In particular large males is seen almost exclusively in animals in which the males fight each other for control of a harem AKA contest competition. This means high male aggression, since they are driving away other males. Since they drive away other males they have little to no sperm competition so they also don't need competitive sperm. These species also show drastic difference in male reproductive success, males with a harem produce many offspring males without one produce few to none.

https://www.intechopen.com/books/sexual-dimorphism/the-evolution-of-sexual-dimorphism-understanding-mechanisms-of-sexual-shape-differences

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  • $\begingroup$ Thank you for the answer. So, the reason we can say that the species with the larger male skull will show greater male aggression is because "large males is seen almost exclusively in animals in which the males fight each other for control of a harem". Or basically, "It is so, because it is what we see in nature." I thought, there would be some evolutionary explanation? I mean, equal size males could also, in theory, be aggressive and dangerous. I come from engineering, and I might be missing some fundamentals here, so, please bear with me. $\endgroup$
    – rajendra
    Dec 31, 2018 at 23:08
  • $\begingroup$ absolutely equal sized males could be, the question is asking "given X we can extrapolate Y" not "What all conditions can cause Y". This actually is an evolutionary explanations, sexual dimorphism needs a reason to evolve, and for large males that situation is basically only direct male to male competition favoring large males who more reliable win said competitions. There are no other reliable reasons, evolutionarily speaking, to have males larger than females. Given X we can extrapolate Y, it is not a guarantee but it is a fairly robust assumption. $\endgroup$
    – John
    Jan 1, 2019 at 6:51
  • $\begingroup$ I see. But it could also be that the bigger male could be a sexual selection (that females are just attracted to big males)? I guess, it could be, in theory, but that is not what we see in nature? So, looks like, like you said, X doesn't imply Y, but it is what we regularly see in nature. If it is so, I find it quite 'wrong' for Dr. Sapolsky to expect new-coming student to be able to extrapolate Y from X (because there is no definite logical route from X to Y; only one of the many plausible route); that gave me the impression that X implied Y. $\endgroup$
    – rajendra
    Jan 1, 2019 at 17:02
  • $\begingroup$ Since I don't know what support material or prerequisites the videos have I can't say how fair it is. But it would not be out of place if there is an expectation of competence in evolutionary concepts or primate in general. Contest competition is a pretty standard concept used when discussing sexual dimorphism and reproductive strategies. It is actually well studied in primates, and correlates with things like tesitical size, sperm production, and reproductive strategies. $\endgroup$
    – John
    Jan 2, 2019 at 0:03
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I suspect the answer by @John is exactly the answer Dr. Sapolsky would be looking for. However, without supporting data from behavioral observation and actual genetics to rule out say, sexual selection, I'd remain skeptical.

The scenario posited by Dr. Sapolsky strikes me as the sort of ad hoc story telling that shows up so often in evolutionary psychology. It might be a good starting point for investigation, but the story by itself is not dispositive. We all "knew" for over a century that the longs necks of giraffes were an adaptation to grazing the upper levels of trees. It's an obvious and plausible story, but apparently the genetic evidence is more complicated and it may be an example of sexual selection (female giraffes acquired a fairly random preference for long necks), or the trend started as an adaptation for grazing higher vegetation, but was then hijacked as a tool for sexual competition (male giraffes fight by whacking each other with their necks).

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  • $\begingroup$ This is really well studied in primates, it is not ad hoc. $\endgroup$
    – John
    Jan 2, 2019 at 0:07
  • $\begingroup$ @John it may be, and I'd trust those studies a lot more. I've got no problem with field studies and actual genetics. What I object to is Sapolsky encouraging a popular audience to believe that it can be derived from hand-wavy principles of evolutionary biology. It's a plausible story, but a story is not dispositive. $\endgroup$
    – Charles E. Grant
    Jan 2, 2019 at 0:33

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