BEHAVIORAL RESPONSES AND TIME ALLOCATION DIFFERENCES IN DESERT TORTOISES EXPOSED TO ENVIRONMENTAL STRESS IN SEMI-NATURAL ENCLOSURES

We tested the effect of water stress on behavior of desert tortoises in semi-natural enclosures. Populations of adult and immature desert tortoises were established at relatively high density in 4.1 ha pens at the Desert Tortoise Conservation Center near Las Vegas, Nevada. Pairs of pens received different levels of food (sod and forb patches) and water supplementation. We postulated that the relatively high densities of tortoises and the different levels of resources created different levels of environmental stress on the populations. Behavior was systematically observed during June-August 1991 and April-September 1992 for differences due to treatment (food and water supplementation versus no food and water supplementation). Behavioral observations were collected during 30 minute focal periods and periodic scan censuses of pens. Animals in the unsupplemented treatments, particularly males, moved further during focal periods than animals with supplemented diets. Movement varied significantly among months. There were no differences in feeding rates among treatments, although monthly effects were important in both treatments. Some differences between years in feeding rates appear related to rainfall patterns and presumably changes in resource availability. Interactions among animals were highest in May and September and were more frequent in unsupplemented pens. Length of morning activity period was shorter in unsupplemented pens for all months. Within treatments, supplemented females were more visible and were seen for longer periods of time during a morning activity period. Home ranges were not significantly different between treatments but showed significant sex effects. Males have larger home ranges than females, and both have larger home ranges than immature animals. When stressed by lack of water and food resources, tortoises reduced the length of above-ground activity time but attempted to compensate for this decreased time by increasing amounts of movement and feeding while active.