Dissertations, Theses, and Capstone Projects

Date of Degree


Document Type


Degree Name





Eric Delson

Subject Categories

Biological and Physical Anthropology


Evolutionary morphology, Primate evolution, Sensory ecology, Vomeronasal organ


Primates have remarkable visual adaptations compared to most other mammals, long explained as associated with a trade-off with olfaction (smell). However, as more information comes to light on the role of olfaction in primate behavior it becomes apparent that olfaction is not a trivial sense. Even humans use smell to communicate, albeit in subtle ways, and the olfactory systems of the lemurs and lorises are very well-developed. Olfaction, however, is actually comprised of two distinct systems - the main olfactory and vomeronasal systems. These two systems overlap in many functions, but the main olfactory system is considered fairly generalized while the vomeronasal system is responsible for detecting odors specifically related to reproduction and predator avoidance. The vomeronasal system is incredibly variable in primates, being well-developed in the lemurs and lorises (strepsirhines) and absent in Old World monkeys and apes (catarrhines). Such variation does imply relaxed selection pressure to maintain a functional vomeronasal system in catarrhines, perhaps in response to gains in visual specialization. The goal of this dissertation is to investigate that evolutionary scenario using a multifaceted approach.

A combined approach of comparing histology of the vomeronasal organ (the peripheral organ of the vomeronasal system) and computed tomography of the cranium is used to reveal variation in the vomeronasal organ across primates and to relate the soft-tissue organ to hard-tissue correlates. Indeed, the cartilage that surrounds the soft-tissue vomeronasal organ leaves a distinct impression on the nasal floor, which is here termed the "vomeronasal groove". To assess the utility of inferring biological function from gross dimensions of the vomeronasal organ and its groove, vomeronasal organ length is compared to the number of genes underlying vomeronasal olfaction. To test whether or not the main olfactory system is evolving in tandem with the vomeronasal system, a hard-tissue correlate of the main olfaction (area of the cribriform plate) is compared to the number of genes encoding main olfaction. Results indicate that main olfaction and vomeronasal olfaction are affected by evolution differently and that vomeronasal organ length when adjusted for body size has a strong statistical relationship with the proportion of functional vomeronasal receptor genes across mammals. To test whether or not phylogenetic history, ecology, and reproduction strategies affect the evolution of the vomeronasal organ in primates, size-adjusted vomeronasal groove length is compared across related categories. Mating categories, probably reflecting sexual selection, appear to drive variation in vomeronasal groove length in lemurs and lorises, while color vision phenotypes appear to drive variation in the tarsiers, monkeys, and apes. The acquisition of trichromatic color vision in Old World monkeys and apes is associated with vomeronasal organ loss, but trichromatic color vision does not appear to be a primary driving force of vomeronasal organ reduction in other primates. The acquisition of high visual acuity, rather, appears to affect initial reduction in length of the vomeronasal groove in crown haplorhines. Fossils representing various "stages" of primate evolution show presence of the vomeronasal groove, and the presence of this groove in the recent ancestors of Old World monkeys and apes suggest that the vomeronasal organ was not lost until crown catarrhines (the group containing Old World monkeys and apes) diverged from all other primate lineages. High visual acuity, routine trichromatic color vision, environments with increased visibility, and changes in social dynamics could have shifted the way in which socio-sexual information was perceived in some primates, increasing the priority of visual and main olfactory signals over vomeronasal signals. Thus, a strict "trade-off" may not have occurred as much as a "reallocation" of sensory information from the vomeronasal system to vision and main olfaction.