Quantifying visual acuity of Terrapene carolina triunguis utilizing optokinetic response.

Abstract

OBJECTIVE: Quantify visual acuity of a semi-aquatic turtle species, Terrapene carolina triunguis (three-toed box turtles), utilizing optokinetic response/reflex/nystagmus. METHODS: Ten turtles were placed on a custom platform and an image of white and black gratings of varying sized widths was projected onto a white screen in front of the turtles. Optokinetic response (OKR) was observed through a web camera as well as physically by an observer. The grating sizes were decreased until OKR could not be elicited. The size of grating was recorded and was used to calculate visual acuity. RESULTS: The average visual acuity was 0.26 cycle per degree (cpd) with the highest recorded value being 0.43 cpd and lowest being 0.15 cpd. Neither sex nor weight were significantly associated with visual acuity (p = .24 and .98, respectively). CONCLUSIONS: Visual acuity of these semi-aquatic turtles was much lower than previously reported visual acuities of aquatic turtles. This could be due to differences in refractive indices between water and air, behavioral adaptations for the different species and/or method of obtaining visual acuity.