COLLECTIVE VIGILANCE AND RISK MANAGEMENT: ANTI-PREDATORY VIGILANCE BEHAVIOR IN THOMSON’S GAZELLE (EUDORCAS THOMSONII)

Abstract

As a result of selective pressure from predation, Thomson’s gazelles (Eudorcas thomsonii) have evolved anti-predatory behaviors. Vigilance behavior consists of periodically surveying the surrounding environment for potential threats. When Thomson’s gazelles aggregate, personal vigilance decreases as a function of increasing group size. In order to identify the ecological factors that drive the group-size effect and quantify the relationship between group size and vigilance, observations on Thompson’s gazelles were conducted as a series of scan samples at Ol Pejeta Conservancy. Based on the scan samples, individual and collective vigilance were calculated to determine the risk of predation. The purpose of examining anti-predatory vigilance is to gain a better understanding of how organisms manage tradeoffs associated with foraging, risk management, reproduction, and fitness. A simple model was created to determine if group members could coordinate vigilance antiphonally. The observed vigilance in response to group size was significantly different from the model, indicating that group members engage in vigilance behavior independent of one another. Solitary males were significantly less vigilant than solitary females. However, when the scope is expanded from singletons to groups, no significant difference exists among the individual vigilance of female groups, male groups, and mixed gender groups. Multiple regression analysis of collective vigilance by group size and inactivity shows that groups with higher proportions of individuals lying and ruminating have significantly lower levels of collective vigilance.