Behavioral Responses of Unconfined Uinta Ground Squirrels to Trapping

Abstract

Capture and recapture data are frequently used by wildlife scientists to estimate characteristics of animal populations. A common assumption is that each animal encountering a trap has the same chance of being captured and recaptured. To test this and related assumptions, this study was made of directly observed responses of unconfined Uinta ground squirrels (Citellus armatus) to various live-trapping experiences. Initially, the trap was the primary attractant. Bait increased this attraction but held some squirrels away from the trap entrance. The effect produced equal probability of capture on first encounter with baited and unbaited traps (n = 51). Squirrels captured on their first encounter (n =18) generally moved about the trap in one direction. Those not captured (n = 23) either did not chance upon the door, showed little interest in the trap, or became fixed at the rear near the bait. Experiments on 58 squirrels indicated capture was punishing and eating bait was rewarding. Simultaneous punishment and reward produced conflict between the tendencies to approach and to avoid the trap. The relative strengths of the tendencies determined whether or not squirrels were recaptured. Prebaiting reinforced entering the trap for bait but decreased the number of squirrels (n = 10) responding to the trap. Squirrels (n = 19) were more hesitant in approaching a trap at the site of capture than at a different location. Their response to the site was consistent with their experience at the site. Biologists often trap animals to obtain information on population parameters. Researchers generally assume that each animal encountering a trap has an equal and independent chance of being captured and recaptured. Some capture data suggest that the above assumption may be invalid owing to behavioral factors (Geis 1955, Tanaka 1956, Crowcroft and Jeffers 1961, and others). However, since researchers seldom observe animals responding to traps, information concerning the relationship between the behavior of animals and a trapping program is lacking. The present study is an attempt to supply such information. The study concerned the behavioral responses of adult Uinta ground squirrels to livetrapping. Results were drawn from direct observation of known individuals in an unconfined population. The objectives were to determine through a series of experiments how squirrels responded to a trap initially, to capture and recapture, to prebaiting, and to the site of capture. 1 Investigation supported in part by U. S. Public Health Service Fellowship MPM-18,460 and Division of University Research, Utah State University.