Effects of Prey Type on the Prey-Handling Behavior of the Bullsnake, Pituophis melanoleucus

Abstract

Several workers have suggested that prey type has an effect on the predatory behavior of snakes. Anecdotal references in Klauber (1956) and Fitch (1960) suggest that, after striking, pit vipers hold on to adult birds, but release adult mammals. They also report cases of pit vipers eating newborn mammals and nestling birds without envenomating them. More recent studies have described differences in predatory behavior for racers (Coluber constrictor mormon-Herzog and Burghardt, 1974) and vipers (Vipera spp.-Saint Girons, 1980) attacking live versus dead prey. The effect of prey type on the constricting behavior of a snake has never been studied. Loop and Bailey (1972) showed that the probability of constriction increases with increasing prey size in several species of boids and colubrids. Since the prey they used included both immobile newborn and active adult mice, at least part of the difference in probability of constriction could have been due to this ontogenetic difference in prey type. The present study is designed to test the hypothesis that helplessness of the prey, independent of its size, has an effect on prey-handling behavior. In particular, it is hypothesized that relatively helpless newborn rodents will not be constricted as often as adult rodents of equal size. The colubrid constrictor Pituophis melanoleucus was chosen as a test subject because it is known to prey substantially on nestling, as well as adult, mammals in the wild (Klauber, 1947; Fitch, 1949; de Queiroz, 1982). Two wild-caught bullsnakes (Pituophis melanoleucus sayi) were fed active adult laboratory mice and helpless two-week old lab rats of approximately equal mass from August 3 to September 29, 1982. The 12 mice used in the experiment had a mean mass of 14.9 ? 3.4 g; the 11 rats had a mean mass of 18.9 + 2.2 g. All mice had their eyes fully open and were capable of coordinated running. The rats were all blind and could only crawl slowly. The snakes had masses of 851.1 and 1098.4 g. Both were caught in Texas and had been maintained in captivity for 9 years on a diet of live lab mice. The snakes were housed individually in glass and plastic terraria in a room in the Museum of Vertebrate Zoology, Berkeley, California. Temperature was kept fairly constant (21-27?C); however, no regular light/dark regime was imposed. Groups of from 2 to 4 prey were fed to each snake on each trial day. Each feeding consisted of either all rats or all mice; difficulty in obtaining rodents precluded random presentation of the two kinds of prey within groups. Instead the two food types were presented alternately, that is, mice were followed by rats on the subsequent feeding day, and vice versa. The smaller snake was offered 6