I really like brains and neuroscience. So when we each had to come up with an area of focus for this course, I knew I wanted to do something with sexual selection of the brain and human intelligence.
The first article I shared with the group argued that contrary to what is typically assumed, human intelligence is not adaptive. Rather, intelligence is an indicator of genetic resistance against infections and signals a healthy, pathogen-free brain. Thus people prefer intelligent mates. Rosza asserts that her hypothesis of intelligence as an accidental side-effect explains why we “enjoy wasting most of [our] intellectual capabilities for totally useless purposes.”
I also brought in a paper that analyzed the degree of sexual dimorphism in the brains of several primate species by comparing gene expression in the occipital cortex. From their results, the authors concluded that sexual gene expression dimorphism is conserved in primate cortexes.
Riffing off this article I shared about primate brain dimorphism, I recently brought in another one where the author, Michael Schillaci tried to establish relationships between primate brain size and two factors of primate sexual selection: sperm competition and male competition for mates. With bats, there seems to be a trade-off between sperm and brains, since both are metabolically costly. The researchers found that whereas low sperm competition isn’t correlated with larger brains in primates, decreased male competition for access to fertile females is associated with bigger brains. The present study seems to reject the expensive tissue hypothesis as an explanation for primate brain sizes. Schillaci remarks that this is to be expected since testicular tissue in primates takes up comparatively little real estate.
Let me express some reservations I had with the way these findings were presented. Both of the independent variables were measured indirectly. Sperm competition was measured by relative testes size, and male intrasexual competition was estimated from sexual mass dimorphism. Sperm competition and male competition for mates are strong components in sexual selection, but are also extremely hard to quantitatively measure. So I understand the need to indirectly measure them. Yet testes size and mass dimorphism seem like a pretty far stretch. (Granted, I myself have no clue how to get data on sperm competition and male-male competition. Dear colleagues and readers, any clever ideas?) The author of the paper could have just framed his study as investigating the correlation between brain size and testes size and mass dimorphism. This would be a more honest and direct summary of their study. But no, they forcibly equated testes size and mass dimorphism with sperm and male-male competition so that they could make more dramatic points about evolution in the discussion section. (More about forcing bio theory on data later…) Another thing about questionable use of terminology—instead of simply writing primate brain size, the author repeatedly refers to his dependent variable as “primate brain size evolution.” (I know I’m being a tad hypocritical, since dependent and independent variables don’t really exist in observational studies, but I don’t know what else to call them.) Sorry dude, but sticking the word “evolution” where it doesn’t belong does not make your findings more profound.
The author also mentions the finding that large brains among primates are associated with monogamous mating systems. Schillaci discusses this trend at great length in the context of the social brain theory. The social brain hypothesis posits that primates evolved their relatively large brains to process complex social systems. Thus, the theory goes, the larger the noggin, the more complex the social system. Schillaci writes that his results seem to contradict the social complexity theory, if you assume polygamous mating systems are more complex than monogamous ones. To my knowledge, no one associates polygamy with social complexity; it’s almost as if Schillaci is creating nonexistent controversy to make his results seem more groundbreaking than they actually are. The author then argues, “if, however, monogamous mating systems require greater social acuity and abilities for deception and…manipulation, then monogamy would select for larger and potentially more complex brains.” By thus claiming that monogamy is in fact indicative of social complexity, Schillaci makes his results fit neatly with an established theory. But how the hell would one confirm the link between monogamy and social complexity? Are there any sociologists out there with ideas?
The primate brain dimorphism article sparked conversation about cognitive and behavioral differences between men and women. In hopes of continuing this discussion with actual studies, I shared Doreen Kimura’s review article “Sex, sexual orientation and sex hormones influence human cognitive function.” Numerous studies have confirmed typical differences between men and women: men perform better on mathematical reasoning, mental rotation, and targeting accuracy, whereas women have more extensive color vocabularies, better verbal memory and finger dexterity. The most plausible biological reason for these functional brain differences is that the brain hemispheres of men are usually less symmetric than those of women. The interesting thing about spatial rotation tasks is that the top performers tend to be women with higher testosterone levels and men with lower testosterone levels. This suggests that there’s an optimal level of testosterone for spatial functioning, which happens to be in the low male range.
A quick Google search reveals that Kimura has done a lot of research on sex and cognition and is a big proponent of academic freedom. I give her major props for continuing in her line of research despite its controversial nature. The stigma surrounding human sexual selection has been a huge point of frustration and discussion in our class, and I strongly think this taboo needs to be broken because understanding sexual selection is essential to fully understanding our history as a species.
