Vigilance in Columbian Ground Squirrels: The Effects of Kinship and Mechanisms of the Group-size Effect

Abstract

Behavioral observations of Columbian ground squirrels (Spermophilus columbianus) were made to investigate two factors that can affect vigilance: the group-size effect and the presence of kin. Vigilance was timed in over 700 observations of 230 individuals on 14 meadows in the summers of 2004 and 2005. One of the goals of this study was distinguish which one of two mechanisms has a greater influence on the group-size effect in Columbian ground squirrels, dilution or detection. To distinguish between the two, equations of the models were fitted to the data and hypotheses were tested by making a number of predictions. If dilution is the main factor in the group-size effect, it was predicted that nearest neighbor distance, and the distance to the edge of the meadow would influence vigilance and that alarm calls and factors that effect hearing alarm calls (wind speed) would not. The opposite predictions were made for the detection effect. The results of both the model fitting and the hypothesis testing supported detection as the main factor causing the group-size effect in these ground squirrels: nearest neighbor distance and distance to the edge of the meadow had no significant effect on vigilance, whereas alarm calls significantly affected vigilance, and vigilance increased with wind speed. A comparison of our results with those of other studies of dilution and detection suggest that group type as well as means of information transfer about predators may indicate whether dilution or detection is the greater influence producing the group-size effect in a species. I also examined whether vigilance was affected by the presence of kin in a population. I found that the only group with differing vigilance was females with adult offspring. These females had significantly lower vigilance than other groups (Rank-sum test, 2004 and 2005 pooled: Z=-2.62, P=0.01). Several possible confounding factors, such as group size and current reproduction, were examined and rejected. Mothers’ decreased vigilance may be an example of social parasitism, in that mothers may be taking advantage of their adult offspring’s vigilance in order to decrease the cost incurred by being vigilant.