The relative importance of phylogeny and habitat in determining the presence and prominence of a granula iridica in hooved mammals.

Abstract

To investigate the relationship between phylogeny and amount of shade in a species' habitat regarding the presence or absence of an iridal granula iridica (GI) in a large sample of Artiodactyl and Perissodactyl clades and using online resources. The Comparative Ocular Pathology Laboratory of Wisconsin (COPLOW) archives were searched for glass slide material from Artiodactyl (even-toed) and Perissodactyl (odd-toed) ungulates. The slides were examined, and the presence or absence of the GI was noted. The phylogenetic tree of the ungulate species was inferred using TimeTree (http://www.timetree.org), and the habitat data are derived from Animal Diversity Web (https://animaldiversity.org/). We assessed the probability of the presence of GI occurring given the amount of shade in a species' environment using phylogenetic logistic regression. Forty-eight artiodactyl species were able to be evaluated and tabulated. Nine perissodactyl species were able to be evaluated. The phylogenetic logistic regression showed that the probability of GI presence was lower in artiodactyl species that inhabited shaded environments (βshaded =-1.774). Arctiodacyl species inhabiting a nonshaded environment were slightly more probable to have the GI present (βnonshaded =0.023), with species inhabitating ambiguously shaded environments having a high probability of GI presence (βambiguous =2.214). Our results suggest that the GI may be a common morphological feature to shade the pupil in nonshaded environments, and, in its absence, increase the amount of light reaching the retina to improve vision in shaded environments for hooved mammals. Further research on the functional optics of the GI and studies that include additional ungulate species would further elucidate phylogenetic and ecological factors influencing the occurrence of GI in hooved mammals.