COS 164-5 - To come together or disperse? Investigating the evolutionary basis for the two equid social types

Background/Question/Methods

Equids can be divided into two social types: the mesic-adapted species form stable groups that sometimes combine into herds whereas the arid-adapted species have loose bonds and highly variable fission-fusion groups. The evolutionary basis for this divergence in equid societies remains elusive. We designed a natural experiment to compare how the mesic-adapted plains zebra (Equus quagga) and the arid-adapted Grevy’s zebra (Equus grevyi) respond to the four selection forces thought to be the primary drivers of group size. We drove monthly census loops where both species occur in Kenya, at three sites representing a progressive increase in lion density (spanning a four-fold increase) for three years. We noted group sizes and photographed all zebras encountered to identify individuals using a stripe recognition program. We tested the effects of season, site, male/female ratio, and individual ID on group size. Zebra groups should disaggregate in the dry season if food scarcity acts as a primary selection force on group size, whereas they should disaggregate in the wet season if disease transmission is a stronger force. High-predation sites and higher male/female ratio at the time of sampling should be associated with larger groups if predation and bachelor harassment, respectively, are strong influences.

Results/Conclusions

Grevy’s zebras aggregated in the wet season, but plains zebras aggregated more in the dry season. Lion density and bachelor pressure had no effect when all four selection forces were accounted for. However, at one site where we collected more detailed data, we found that higher bachelor male-breeding male ratios were associated with aggregation in groups that contained adult females. Although the relative importance of selection forces varies as conditions change, our results indicate that equids of each social type respond differently when faced with the same set of challenges. Arid-adapted equids may respond more to food scarcity because of the greater limits it typically imposes on their fitness, while mesic-adapted equids may be more sensitive to disease risk and bachelor harassment because their greater water dependence forces them to range in areas with higher zebra density. Predation risk may be an impetus to group with at least one other zebra, but no direct effect of lion density on variation in zebra group size was detected. Our study represents the most comprehensive natural experiment to date investigating the relative effects of multiple selection pressures on group size in wild animals.