by Jennifer Fortunato
In Dunbar and Schultz, they discuss the topic of how and why the brain has evolved to be larger, compared to body size, in mammals. They initially discuss ecologically and ontogenetically important reasons for why this may have occurred, for example foraging and efficient energy use respectively. They then go deeper into the social aspect of why the brain has gotten larger in mammals, and a few other species including birds. They consider social complexity, pair bonds, and the microbiology of the brain to try to explain why the brain has gotten larger in these species than needed to maintain the body.
Dunbar and Shultz’s analysis of social complexity involves discussion of the importance of living in a social group and the challenges that come with it. For example, the authors examine how group cohesion and brain size relate to one another. Group cohesion mechanisms such as technical innovation and food acquisition via social learning comes with survival and increased effectiveness in reproduction. This group cohesion correlates with a larger brain size. Another beneficial aspect of social groups is minimizing predation risk. They argue that the selection pressure to minimize predation risk by being social will increase brain size due to those who have larger brain having higher fitness.
The authors also extend their analysis beyond primates to other related species such as birds, bats, and ungulates (hoofed animals). They recognized that there are some mixed results in attempts to apply sociality as a reason for increased brain size in these other animal species. Dunbar and Shultz briefly discuss the hypothesis that sexual selection is a driver of larger brain sizes but conclude that there is no evidence for this. They then go on to discuss pairbonds as an important factor in brain size evolution. Anthropoid, or human like, primates have, according to multiple data sources, a positive correlation between brain and social group size. An issue that the authors came across was what bondedness actually entails as they determined it is an emotional state rather than one that can be quantified. Dunbar and Shultz argue for the idea that higher vertebrates, such as primates, have a more complex version of social bonding than other species. They do so by analyzing how social relationships in primates differ from those of other species because the social interaction to benefit the group will benefit the individual’s fitness down the line.
Microneurobiology is the last point that Dunbar and Shultz discuss. They shortly discuss the function of hormones and genes in the role of brain growth. The authors dismiss these two aspects as an important role in brain growth and sociality. They relate these mechanistic approaches to developmental approaches to analyze brain size.
Though an informative article in articulating possible solutions for the problem of brain size and brain growth, I do have a few criticisms. Dunbar and Shultz focus on entirely on sociality being a fundamental driver for the increase in brain size. However, sociality could be a by-product of ecological and life history constraints (Brooks et al. 2017, Armitage 1981). This would indicate that it is these constraints that actually influence brain size rather than sociality itself. Also, in direct contradiction is a later article by DeCasien et al. (2017) that argues that primate brain size is predicted by diet and not sociality. This furthers the argument for increased brain size being more related to ecological factors than sociality.
Dunbar and Shultz also discuss the importance of having a large brain in comparison to body size as an important feature of mammals and birds and how that relates to sociality. However, they fail to take into account both invertebrates, such as ants, and very large mammals, such as whales, whose brain sizes are considerably larger or smaller respectively. Both ants and whales have complex social structures but have completely different brain size to body ratio with ants having 14% and whales having less than 1% of their body weight being their brain (Seid et al 2011, Koch 2016). This seems to go against Dunbar and Shultz’s argument that having a larger brain is a strong correlate with sociality.
Overall, this article brings up good points about the evolution of social brain in primates, though it does leave out an entire group of social insects. Dunbar and Shultz have brought up a multitude of avenues of study for neuroanthropologists. For example, analyzing how bondedness can be quantified in non-human primates. Another line of study could be the integration of molecular, behavioral, and phylogenetic data of how the social brain has come to evolve both in size and other aspects. As neuroanthropology is an integrative field, the integration of multiple techniques from psychology, biology, and anthropology can be utilized to understand the evolution of the size of brain in a social context.