Visual acuity in a cathemeral lemur (Eulemur macaco flavifrons) : implications for strepsirrhine visual ecology

For many nocturnal, cathemeral, and diurnal strepsirrhines, acute vision is important in foraging for food, detecting predators, and communicating with conspecifics. However, comparable visual acuity measurements are currently only available from four strepsirrhines: Lemur catta, Galago senegalensis, Otolemur crassicaudatus, and Microcebus murinus. In light of the role of increased visual acuity in early primate evolution and the controversies surrounding the selective pressures influencing primate origins, it is important to understand how visual acuity varies with ecological variables such as diet or activity pattern. In this study, the first behavioral assessment of visual acuity in a cathemeral strepsirrhine was conducted. Acuity in two blue-eyed black lemurs (Eulemur macaco flavifrons), a 3-year old male and a 16-year old female, was measured using a two-alternative forced choice discrimination task. Visual stimuli consisted of high contrast square wave gratings of seven spatial frequencies. Acuity threshold was determined using a 70% correct response criterion. Results indicate a maximum visual acuity of 5.1 cycles per degree (c/deg) for the female (1718 trials) and 3.8 c/deg for the male (718 trials). Acuity was also assessed anatomically from retinal ganglion cell densities for Microcebus murinus (2.8 c/deg) and Tarsius syrichta (8.9 c/deg) reported in the literature. Preliminary analyses of ecological effects on prosimian visual acuity were conducted. Although the results are not statistically significant, acuity appears to vary with activity pattern, diet, and prey mobility. In particular, functional, behavioral, and anatomical evidence suggest that cathemeral strepsirrhines are balancing the visual acuity necessary for diurnal activity with the visual sensitivity necessary for nocturnal activity. Additionally, visual predation may be an important selective factor favoring high acuity.