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Skepticism is good, but…
October 17, 2009Posted by on
Well-informed skepticism is the best!
Earlier this week, Eric Michael Johnson drew my attention to a post by psychologist Christopher Ryan at his blog Sex At Dawn. Ryan attacks Lovejoy’s monogamous humans model by citing many different lines of evidence.
I became so distracted by the reported testes:body mass ratio of 1/160 in humans that I couldn’t stop until I had some answers. I am a female human, but even I thought that 1 kg of testicles would be an awful lot to lug around. So I got out my books and my calculator and did some math, wrote in, and it was fixed. Peer review in action!
But then I clicked through to the actual post, and realized that it didn’t get much better from there. Apart from characterizing Ardi as mere “bits of bone,” Ryan displays many instances of ignorance.
First, a few things that Ryan gets right:
- Yes, reduced male-male competition can happen in promiscuous mating systems (but I do think that characterizing the Chimpanzee as having reduced competition is dubious).
- Humans are probably able to produce more sperm per ejaculate than the number that Lovejoy cites.
- Reproductive anatomy and physiology is particularly labile, so postulating about reproductive physiology 4.4 million years ago is a risky undertaking.
Lovejoy writes that “Humans have the least complex penis morphology of any primate.” Unfortunately, he never defines what he means by “complex;” nor does he discuss the fact that the human penis is, by most measures, the longest, thickest, most prominently displayed penis among primates. No mention of the unusual flared head or the external scrotum—both strong indications of sperm competition in our species.
Lovejoy clearly states that humans lack “keratinous penile surface mechanoreceptors” (known to you and me as penis spines and ridges) and an os baculum. So there’s your complexity.
Lovejoy also states in the footnotes:
Flaccid human penis length (13 cm) is unusually great for a hominoid. Length is ~4 cm in Pongo and 3 cm in Gorilla. Its erect size is greater in the multimale Pan (8 cm), but this reflects specialized adaptation to penetrate seminal plugs. Short notes that “(e)ven the pubic hair in the male [human] seems designed to draw attention to the genitalia, rather than to conceal them as in the orangutan and gorilla.”
Lovejoy states these things in support of a rather weird argument against hand-to-hand combat in human ancestors, but why not in the discussion of sperm competition? Because a huge penis does nothing if you don’t have the testes to back it up! Clearly a large, pendulous, prominently-displayed penis is something special in humans, but it is not about sperm competition. The external scrotum- by itself- may be indicative of sperm competition, but combined with the fact that human testes are smaller and have fewer seminiferous tubules, that the human sperm midpiece volume is low, and that we have lost the physiology for creating copulatory plugs indicates that there may be something else at work here.
Perhaps the most glaring mistake lies in Ryan’s argument about canine teeth:
For example, much of his thesis hinges on the absence of pronounced canines teeth (fangs) in the fossils found. He writes that we can assume that both males and females lacked these canines (even if the teeth were from a female) because “The SCC [sectorial canine complex] is not male-limited; that is, it is always expressed in both sexes of all anthropoids….” But this is wrong. Male bonobos have long canines, while females don’t (2, 3). Lovejoy also claims an association between reduced canines and pair-bonding, but as this photo of the skull of a monogamous gibbon demonstrates, even this claim is suspect.
When anthropologists refer to the canine complex, they are not merely referring to canine length. The Sectorial Canine Complex, or the CP3 complex, or the “honing” complex all refer to the way that the top canine tooth fits in with the lower canine and premolar. It fits in this way in order to sharpen the canine tooth so that it acts as a blade. In contrast to what Ryan states, both male and female anthropoids (including the bonobo) WILL have this trait if it is present at all. Yes, the tooth will be longer in males (in many cases, much longer), but the sexual dimorphism here is in size, not in the actual way that these teeth occlude.
Teeth from more than 35 individuals have been found, and not one of them exhibits this Sectorial Canine Complex, so I think it’s safe to say that Ardi and her pals didn’t have it.
And then there’s that beautiful gibbon skull. Unfortunately, the canine monomorphism in gibbons actually supports Lovejoy’s whole thesis: In monogamous species, the canine teeth are no longer under strict sexual selection, and natural selection can take over and do its thing. In gibbons, this has meant that not only do males have large canines, but females have them as well. Both males and females defend their territory, and so both have enlarged canines. In humans, the argument is that the energy that goes into maintaining large canine teeth and getting into the fights that result when you have them could be better spent by gathering food for your partner and offspring. As a general rule for primates, the canines don’t have to be reduced to indicate pair-bonding, but if they are the same in both sexes, something is up.
Lovejoy’s hypothesis is controversial. It’s a Theory of Everything, and that should raise flags in everyone’s head. But at the end of the day, it’s really stinkin’ weird that human males abandoned their canine teeth, and it’s really stinkin’ weird that human females abandoned their ovulatory advertisement. I think monogamous pair-bonding goes further than any other hypothesis in explaining those two weirdnesses of the human. However, my little red flag pops up when we connect those things to bipedalism.
Zingeser, M. (1969). Cercopithecoid canine tooth honing mechanisms American Journal of Physical Anthropology, 31 (2), 205-213 DOI: 10.1002/ajpa.1330310210
Lovejoy, C. (2009). Reexamining Human Origins in Light of Ardipithecus ramidus Science, 326 (5949), 74-74 DOI: 10.1126/science.1175834
Frisch, J. (1963). Sex-differences in the canines of the gibbon (Hylobates lar) Primates, 4 (2), 1-10 DOI: 10.1007/BF01659148
Leutenegger, W., & Kelly, J. (1977). Relationship of sexual dimorphism in canine size and body size to social, behavioral, and ecological correlates in anthropoid primates Primates, 18 (1), 117-136 DOI: 10.1007/BF02382954