Review of “Evolution in the Social Brain” (Dunbar & Schultz, 2007)

by Jake Aronoff

Dunbar & Schultz take on the ambitious and contested explanation for why primates have unusually large brains. They posit that this was driven by group size and complexity. As they reason to this conclusion, they highlight how previous literature on the subject of evolutionary brain growth has unfortunately only focused on restraints to brain growth rather than selective pressures acting on this. Their analysis examines both qualitative and quantitative social bonds, taking them from highlighting pair-bonding as a driver of brain growth in vertebrates (qualitative) to the evidently unique association of group size and complexity with brain size found in primates (quantitative). They reason that in vertebrates, pair-bonding acted as a selective pressure on brain growth. However, in primates, this level of bonding became present between non-potential mates. Thus, as the type of bonds found among mates of a pair-bonded species emerged in primates among non-mating members, group size and complexity became the selective pressure on primate brain size instead of pair-bonding, as observed in other vertebrates.
 
A strength I found with this article was its aim to move away from constraint explanations for brain growth, rightly highlighting the fact that just because an organism can evolve a larger brain does not mean this will be the case. A good example of this problem is in the “Expensive Tissue Hypothesis” by Aiello & Wheeler (1995). They reason that a reduction in the human gut allowed for an increase in brain size, and they create ratios of expected versus observed energy demands per major organ in different primate species. This led them to find that while other major organs in humans were observed similarly to their expected energy demands, the human gut required much less energy than expected while the brain required much more than expected. They concluded that there was a pretty clean “swap” of energy demands from the gut to the brain. However, while this is interesting, the authors were criticized for the very same thing Dunbar & Schultz mention; they explained lifting constraints, but there was no reason for why this energy would be going towards a larger brain. To be fair, the authors responded by saying they were only suggesting that this created a “prime allower” for brain growth, and that they were not suggesting a “prime mover” for brain growth. I think this highlights why constraint research can be valuable as a complement to research on primate (and specifically human in this case) brain growth (side note; there is still controversy over the cause for human gut size reduction, while Aiello & Wheeler posit meat eating, Wrangham posits cooking).
 
Another point certainly worthy of mention in this article is its call for niche construction research, though this is only very briefly mentioned. They claim that, “In effect, we are dealing explicitly with multi-level selection and the long-overlooked topic of niche construction” (1346). Mackinnon & Fuentes (2012) carry this torch much further. They define niche construction as “modification of the functional relationship between organisms and their environment by actively changing one of the factors in that environment” (73). They explain the mind-numbingly complex continuous interactions between primates and their social and ecological environments, in which they can strategically alter their environment, which then at the same time acts upon their evolutionary fitness. Applying this to what Dunbar & Schultz are claiming, primates form groups not in the interest of immediate personal fitness, but in the interest of long-term payoffs in fitness (reduced predation is one example provided) that lead to overall net fitness gains for the individuals over the course of their lifetime, which is made possible by group fitness (the authors mention a “trickle-down” effect going from group fitness to individual fitness).
 
After making vast species comparisons of pair-bonding and brain size, Dunbar & Schultz find that primates are unique, in that there is not a significant difference between pair-bonding species and non-pair-bonding species in relation to brain size. This leads them to the conclusion that primates must utilize the type of bonds seen in pair-bonds of mates of other vertebrate species with non-mates. However, one problem with this (I am hesitant to call this a weakness, as the authors bring it up themselves) is the difficulty in understanding and studying “bonded-ness”. In understanding bonded-ness in humans, the authors warn against what they have observed, in which researchers attempt to put forth a single hormone or single gene to explain bonded-ness. Rather, they note the likely complexity and many contributions influencing the unique bonds among and between primates.
 
In the interest of studying bonded-ness from a neuroanthropological perspective, it may prove insightful to utilize the combination of ethnographic work (attempting to capture the meanings crafted by individuals regarding their bonds) along with research tools provided by neuroscience such as fMRI’s. If this produces any noteworthy findings, then studies of the brains of other primates could add another element through comparisons with humans. For example, Dunbar & Schultz note the limits of ethology, as what is going on inside the animal is left much to question. However, if studies of human understanding and meaning making regarding their bonds (both with mates and non-mates), combined with neuroscience methodology (such as brain scanning), these findings could potentially be applied to other primate species in order to (very roughly) approximate understanding and meaning making in other primates, thus capturing what has been elusive to ethologists in understanding what is going on inside the animal.

 
Sources:
 
Aiello, L. C., & Wheeler, P. (1995). The expensive-tissue hypothesis: the brain and the digestive system in human and primate evolution. Current Anthropology, 199-221.
 
Dunbar, R. I., & Shultz, S. (2007). Evolution in the social brain. Science, 317(5843), 1344-1347.
 
MacKinnon, K. C., & Fuentes, A. (2012). Primate social cognition, human evolution, and niche construction: a core context for neuroanthropology. The encultured brain, ed. Daniel H. Lende and Greg Downey, 67-102.
 
Wrangham, R. (2009). Catching fire: How cooking made us human. Basic Books.