THE EFFECTS OF SUBSTRATE AND VERTEBRAL NUMBER ON LOCOMOTION IN THE GARTER SNAKE THAMNOPHIS ELEGANS

1. Locomotor performance of limbless vertebrates depends on the substr ate through which individuals move and may result in selection on vert ebral number in different habitats. To evaluate the effect of push-poi nt density on snake locomotion, the density of vegetation and other po tential push-points was quantified at two sites in California (coastal and inland), where conspecific snakes differed greatly in vertebral n umber (230 and 256 average total vertebrae, respectively; Arnold 1988) . The coastal site had significantly higher push-point densities than the inland site. 2. Five experimental push-point densities that fell w ithin the natural range of push-point densities were employed in labor atory trials of juvenile snake locomotion. Density of push-points sign ificantly affected both crawling speed and head-to-tail distance (HTD) , an indirect measure of lateral bending. The fastest speed was achiev ed at an intermediate push-point density. The shortest HTD occurred wh en snakes moved through the lowest push-point density. 3. Sex, total n umber of vertebrae and total length significantly affected HTD, regard less of push-point density. Snakes with relatively more vertebrae had a shorter HTD, suggesting they were able to achieve greater lateral be nding than snakes with fewer vertebrae. Coastal and inland populations did not differ in HTD during locomotion. 4. Numbers of body and tail vertebrae significantly influenced speed at different push-point densi ties. In general, snakes with more: body vertebrae were slower than th ose with fewer, while snakes with more tail vertebrae were faster than those with fewer. Snakes of greater total length were faster at all d ensities. Coastal snakes crawled faster than inland snakes at all push -point densities.