The coevolution of play and the cortico-cerebellar system in primates.

Primates are some of the most playful animals in the natural world, yet the reason for this remains unclear. One hypothesis posits that primates are so playful because playful activity functions to help develop the sophisticated cognitive and behavioural abilities that they are also renowned for. If this hypothesis were true, then play might be expected to have coevolved with the neural substrates underlying these abilities in primates. Here, we tested this prediction by conducting phylogenetic comparative analyses to determine whether play has coevolved with the cortico-cerebellar system, a neural system known to be involved in complex cognition and the production of complex behaviour. We used phylogenetic generalised least squares analyses to compare the relative volume of the largest constituent parts of the primate cortico-cerebellar system (prefrontal cortex, non-prefrontal heteromodal cortical association areas, and posterior cerebellar hemispheres) to the mean percentage of time budget spent in play by a sample of primate species. Using a second categorical data set on play, we also used phylogenetic analysis of covariance to test for significant differences in the volume of the components of the cortico-cerebellar system among primate species exhibiting one of three different levels of adult-adult social play. Our results suggest that, in general, a positive association exists between the amount of play exhibited and the relative size of the main components of the cortico-cerebellar system in our sample of primate species. Although the explanatory power of this study is limited by the correlational nature of its analyses and by the quantity and quality of the data currently available, this finding nevertheless lends support to the hypothesis that play functions to aid the development of cognitive and behavioural abilities in primates

Rapid evolution of the primate larynx?

Tissue vibrations in the larynx produce most sounds that comprise vocal communication in mammals. Larynx morphology is thus predicted to be a key target for selection, particularly in species with highly developed vocal communication systems. Here, we present a novel database of digitally modeled scanned larynges from 55 different mammalian species, representing a wide range of body sizes in the primate and carnivoran orders. Using phylogenetic comparative methods, we demonstrate that the primate larynx has evolved more rapidly than the carnivoran larynx, resulting in a pattern of larger size and increased deviation from expected allometry with body size. These results imply fundamental differences between primates and carnivorans in the balance of selective forces that constrain larynx size and highlight an evolutionary flexibility in primates that may help explain why we have developed complex and diverse uses of the vocal organ for communication

The evolution of mammalian brain size

Relative brain size has long been considered a reflection of cognitive capacities and has played a fundamental role in developing core theories in the life sciences. Yet, the notion that relative brain size validly represents selection on brain size relies on the untested assumptions that brain-body allometry is restrained to a stable scaling relationship across species and that any deviation from this slope is due to selection on brain size. Using the largest fossil and extant dataset yet assembled, we find that shifts in allometric slope underpin major transitions in mammalian evolution and are often primarily characterized by marked changes in body size. Our results reveal that the largest-brained mammals achieved large relative brain sizes by highly divergent paths. These findings prompt a reevaluation of the traditional paradigm of relative brain size and open new opportunities to improve our understanding of the genetic and developmental mechanisms that influence brain size

Understanding the Complex Causes of Primate Brain Evolution

Understanding the system of selection pressures and constraints that caused primates to evolve their distinctive brains is key to understanding primates as an order and ourselves as a species. As such, it has been a major goal in Biological Anthropology. However, I argue that the research approach that has traditionally been employed in the field is not capable of producing a realistic understanding of primate brain evolution because it cannot effectively model the complex causal systems that likely underlie it. In this thesis, I describe a new approach to understanding the complex causes of primate brain evolution that I have developed and tested. I demonstrate the effectiveness of this new approach by showing that it has been able to produce the first realistically complex models of the causes of primate brain evolution. These models both challenge our current understanding of primate brain evolution — including the social brain and frugivorous foraging hypotheses — and reveal previously unrecognised patterns, such as a core system of niche variables underlying primate brain evolution and unique systems of selection pressures and constraints operating in different primate clades

Supplemental Material - Understanding the Complex Causes of Primate Brain Evolution

This is the supplemental material to the thesis "Understanding the Complex Causes of Primate Brain Evolution" by Max Kerney (2021). The main thesis can be found at https://arro.anglia.ac.uk/id/eprint/707381/.This .zip file contains a complete offline version of the website that hosted the tables and figures for the thesis. It can be accessed locally on a computer (if R is installed) by downloading the .zip file and running the script “LAUNCH VIEWER.r”. <br