Meeting Abstract
Ancestral snakes may have been fossorial and had poor vision. Furthermore, some anatomical features suggest that, after nearly being lost, the eye of snakes was re-elaborated. However, experimental data are so sparse that the variation in visual acuity and its correlates among more than 3,000 extant species of snakes are not well understood. Hence, we used the optokinetic response to test the visual acuity of a diverse sample with 9 species of Henophidia (boas, pythons & relatives) and 8 species of Caenophidia (advanced snakes). We tested the snakes with a large drum (diameter = 176 cm) that rotated in a horizontal plane at 3 and 9 deg/s. The smallest widths of the alternating black and white vertical stripes lining the drum ranged from 30.8-0.96 mm, which corresponded to visual acuities ranging from 0.25 – 8.0 cycles per degree (cpd). Our study is the first to determine the visual acuity of any henophidian. With only one exception, all henophidians had values of 0.5 cpd or less. Within the henophidians in our sample, similar sized terrestrial and arboreal species had similar visual acuities (0.5 cpd), whereas some small burrowing specialists such as sand boas had values of only 0.25 cpd. All of the caenophidians had visual acuities of 1 cpd or greater. A nocturnal, arboreal caenophidian with large eyes (brown tree snake) had a visual acuity of 2 cpd, whereas some similar sized semi-arboreal, diurnal rat snakes had values two to four times greater. Compared to rat snakes, the visual acuities of some extremely specialized diurnal arboreal species, such as vine and gliding snakes, were not noteworthy. In addition to retaining some primitive traits such as vestigial legs, perhaps henophidians have also retained inferior visual acuities compared to those found in advanced snakes.
