Exploring adaptive functions of geophagy across non-human primates : new evidence for sexual selection

Abstract

Evolutionary theory predicts that non-immunological defenses against pathogens and toxins evolved as counter-measures to protect non-human primates during sensitive periods of development. The current study focuses on one type of non-immunological defense, geophagy, the consumption of soil. According to life history theory, pregnant females and juvenile nonhuman primates might be at heightened risk for the effects of specific infections and toxins due to immunological constraints. This study therefore predicted that geophagy would be observed more in pregnant and juvenile non-human primates. Data were collected by scanning databases: Agricola, Google Scholar, Web of Science, PubMed, and JSTOR. The search was initiated by searching for the keyword “geophagy” and scanning for entries related directly to that defense. I coded for age, sex, and reported function of geophagy. Findings suggest that the primary purpose for geophagy is detoxification and digestion. The secondary purpose being nutrition, and the third reported purpose being sexual selection. Contrary to the hypothesis and current findings in humans, adult males were reported to have similar observations of geophagy with adult females. However, the sex difference may be biased given the difficult nature of determining sex and age through scan sampling. My findings provide support for mainstream hypotheses of geophagy, with evidence towards both the supplementation and protection hypotheses. A newly reported function of geophagy points to sexual selection where researchers suggest that if adult males are consuming more soil, sexual selection might play a role, such that male-male competition may lead injured adult males to engage in geophagy as a method to boost iron levels or ward off parasitic infections from wounds. Alternatively, adult males could be using geophagy to signal good health; thus increasing the potential for mates. Future research should consider this understudied role of geophagy and other non-immunological defenses.