9701137 Leger Studies of the decision-making capabilities of animals while they are under the risk of being eaten by predators can reveal the complex process by which animals evaluate optimal trade-offs. A simple example of an optimal trade-off is where an animal is faced with two conflicting choices, e.g., foraging versus scanning for predators (antipredator vigilance). Foraging reduces starvation risk while antipredator vigilance reduces predation risk. The animal is unable to maximize the benefit from one without incurring serious risk from the other. An optimal trade-off is where the animal's choice is the joint maximum of both feeding and antipredator vigilance. Decision-making is the framework within which this research is based. A very common observation is that a group size increases, prey animal vigilance decreases. This decrease in vigilance could result in the animal having more time to feed and could be a significant force in the evolution of sociality. Although visual obstruction is a common contextual variable in many prey animals' environment, this variable has not been given much empirical attention. Thirteen-lined ground squirrels are burrowing rodents that occur across the Great Plains. They are preyed upon by birds, terrestrial mammals, and reptiles such as snakes. It has been shown that thirteen-lined ground squirrels increase their antipredator vigilance only when their long-range (greater than 20m), lateral field of view is obstructed. Considering how many animals prey upon these ground squirrels, it is quite surprising that they do not react with increased vigilance when their 'overhead' view is obstructed. To test hypotheses about antipredator vigilance and visual obstruction, thirteen-lined ground squirrels are allowed to feed inside plexiglass boxes that differentially obstruct the squirrels' filled of view, and detect the approaching threat. This is an important test since ground squirrel antipredator vigilance behavior indicates that they do not co nsider certain positions of visual obstruction to be important. Therefore, even without increasing their vigilance, ground squirrels should be able to detect predator attacks with 'overhead' visual obstruction as when there is no visual obstruction.
