Hyena-Primate Convergence: The Social Brain Hypothesis

Just wanted to point you towards a must-read for today: a NYT article by Carl Zimmer, profiling the work of Kay Holekamp on spotted hyenas (Crocuta crocuta). The NYT piece is a nice way to get a summary of Holekamp's fascinating paper that appeared in the Journal of Mammalogy last summer.

I have actually been intending to blog about that paper ever since I first read it over six months ago, because the hypothesis is extremely intriguing. This is a crazy week for me and I wish I had time to cover it in more detail, but I did want to mention the study since it is hitting the news, mainly to bring attention to the journal article behind the recent headlines. Holekamp et al (2007) examine the "social brain hypothesis" approach to the evolution of large brains and high intelligence. They suggest that the intricate social dynamics of spotted hyena society has led to selection pressures on cognition which are analogous to primate brain evolution. Their study evaluates numerous points of possible convergence between C. crocuta and cercopithecine primates (the Old World monkeys), focusing on the cognitive and neurological adaptations needed to perform complex social behaviors.

The social systems of the cercopithecines and C. crocuta show many striking similarities. Both groups have small litters (one or two offspring), long lives, male dispersal, female-structured kin groups with maternal rank inheritance, cooperative hunting, recognition of both maternal and paternal kin, the list goes on and on. Both monkeys and hyenas appear to be able to evaluate the social rank of other group members, and pick their "friends" and mates accordingly, preferring to associate with higher status individuals in order to increase their own fitness. C. crocuta has been shown to form social coalitions, and to exhibit reconciliation after aggressive interactions, which have been studied extensively in primates but are not well-documented in other mammals.

Expression of any behavior is a manifestation of the neurological organization required to process and integrate different types of information. This means that having social systems and behaviors that are similar in so many important details could be a clue to convergent features in actual brain structure between these specific groups of monkeys and carnivores. Holekamp's data suggests that the hyena has more frontal neocortex volume devoted to areas that are "exclusive of motor control" than other carnivores, which indicates that their neuroanatomy has become more specialized for analyzing and processing social interactions. The authors hint at future work comparing the different genera within Hyaenidae, which should be fascinating to follow as the results are published.

Hyenas are really fascinating animals from many standpoints, including behavior, ecology, anatomy (if someone gave me a "blank check" for graduate work, there are tons of questions I'd love to explore about the less-studied species in the family). They are terrific models for studying sociality, because the family Hyaenidae consists of four species that exhibit continuum of social behaviors, which I covered in depth here a few months ago. In a nutshell, the brown and striped hyenas, in addition to the aardwolf, are all less social than the spotted hyenas, so studying their cognitive adaptations will be a fascinating area of future research that could further support Holekamp's hypothesis about sociality leading to convergent features between primates and carnivores.

Anyway, check out Zimmer's article for an overview of Holekamp's work, and see the paper below for more details!

Holekamp, K.E., Sakai, S.T., Lundrigan, B.L. (2007). THE SPOTTED HYENA (CROCUTA CROCUTA) AS A MODEL SYSTEM FOR STUDY OF THE EVOLUTION OF INTELLIGENCE. Journal of Mammalogy, 88(3), 545. DOI: 10.1644/06-MAMM-S-361R1.1

(Image credit for hyenas and brain)