Sexual Selection and Adaptive Coloration in Primates

Photo credit: Will Allen

Dr. James Higham (NYU) describes his research group's work on the selection pressures, especially sexual selection, underlying facial color variation in Old World Monkeys.

Photo credit: Constance Dubuc

The aim of our research program in the Primate Reproductive Ecology and Evolution group at NYU is to understand how evolution, especially the process of sexual selection, has produced the extraordinary diversity of morphological and behavioral variation seen among the primate order. To study these questions we employ techniques from multiple fields, including behavioral ecology and evolutionary biology, endocrinology and immunology, computer vision and machine learning, experimental and comparative psychology, and quantitative and functional genetics.

We work on a number of systems, though many of our recent publications have been on two major ones. Firstly, we are involved in longterm studies of the rhesus macaques of Cayo Santiago. Our work there is diverse in scope, but one long-running question has been the function of the red coloration exhibited in rhesus macaque faces. Recent work has focused especially on the function of coloration in males. We have recently shown that, unlike in many primate species, male coloration is not linked to dominance rank. Instead, it appears to be an ornament that is attractive to females. Male coloration is heritable, and appears to be under selection, with those males simultaneously combining high dominance rank with dark red face coloration having the highest reproductive output over time. These results combined suggest that male rhesus macaque coloration is an unusual mammalian trait – a color ornament selected through female mate choice and inter-sexual selection rather than male-male competition and intra-sexual selection. This makes it more similar to the ornaments of groups like birds and fish, than to the armaments (weapons) exhibited by many mammal species.

Another of our current study systems is that of the diverse facial patterns of the guenon monkeys. These monkeys show extremely diverse facial markings, which have long been hypothesized to be involved in species recognition. Multiple guenon species are sympatric (geographically overlap), and where they do so they tend to live, travel and feed together, leading to the potential for making mating mistakes, leading to hybrid individuals of two species with lower fitness. Using techniques developed in the field of computer vision for human facial recognition, we showed that guenon face patterns have evolved to be more distinct specifically from those of the other guenon species that each species geographically overlaps with. This pattern of character displacement suggests that guenon faces may have had a key role to play in preventing costly mismatings between species. Further work utilizing machine learning has shown that guenon face patterns provide highly reliable information for species classification, as well as for identifying individual animals. However, guenon faces are not sexually dimorphic, nor do they change with age. These results further reveal the likelihood that guenon face markings have been involved in species recognition mechanisms, and in creating reproductive isolation between different species. Primate evolution is characterized by a number of rapid radiations in which many species have been created, often without much understanding of the mechanisms by which many closely-related species have been maintained even in sympatry. Our recent work on this system seeks to understand such processes.

A synthesis of our work on adaptive coloration, with links to several of our papers (in bolded text), is available on BBC Earth.

Some coverage of recent guenon studies can be found here on National GeographicBBC Earth, and Wired.