Humans are obsessed with the things that make us different from other animals. And thus we are obsessed with trying to figure out what makes large brains.
Ideas for what drives selection for large brains include the idea that predators have bigger brains than prey, generalist feeders have bigger brains than specialist feeders, and that organisms living in complex social groups have bigger brains than those that do not. In this paper, author Michael Schillaci focuses on the latter possibility, and in particular, mating systems. The paper does a fairly complex set of comparisons, but I want to focus in on brain size, since I am a neurobiologist.
The approach is simple. Measure a bunch of brains, bodies, and testes, and see if there are any correlations between brain size and various other measures of social systems and mating. There are no experiments here, only analyses.
This sort of analysis can only be as good as the initial data, so it’s worth looking at where these are drawn from. The author did not collect any of his own data, but reanalyzed data compiled in a book chapter of about 31 primate species, including humans.
Of all the primate species, I think it’s fair to say that we understand humans better than any others. And the data for humans here say that the average mass for humans is... 40 kilograms? Less than 90 pounds for the average human?
Similarly, the human mating system is listed as... monogamous? It seems to me that there is anthropological data for a very wide range of human mating systems (although the mode may well be monogamy).
When two of your data points for the best known species are... arguable... you burn a fair amount of credibility in the other data included in the analysis. Unfortunately, since these data are all published elsewhere, in a book chapter, if you can’t track down that original book chapter, you’re sort of stuck in terms of understanding the argument and logic behind those choices.
Schillaci finds a correlation that monogamous species tend to have larger brains. This is perhaps surprising, given that the hypothesis has usually been that large brains are positively correlated with complex social systems, and monogamy is usually viewed as a simpler social system.
From here, Schillaci advances an hypothesis: If you’re monogamous, you have a large brain to have extra-pair copulations. That is, to cheat on your partner. This is a modification of the “social complexity” hypothesis: the complexity is not in the number of individuals you partner with, but in deception necessary to find mating opportunities outside of monogamy.
This is an interesting idea, but it is way more speculative than the last sentence in the abstract suggests. For one thing, only four of the sampled species are monogamous (one of which, Aotus trivergatus, is pictured), even when humans are included. It seems to be a small sample from which to draw conclusions.
The good news is that testing this hypothesis is conceptually straightforward with DNA fingerprinting. You could measure the rate of extra-pair paternity of offspring. The species with the highest rates would be predicted to have larger brains, according to this hypothesis.
Schillaci, M. (2006). Sexual Selection and the Evolution of Brain Size in Primates PLoS ONE, 1 (1) DOI: 10.1371/journal.pone.0000062