Black and White and Spread All Over: The Role of Environmental Variation on Patterns of Sociality in Plain's and Grevy's Zebras

Abstract

It is known that the environment shapes the social system of any species, and mating systems, group size, and spatial dispersion will be influenced as a result of environmental changes. More specifically, bottom-up factors such as resource availability and top-down factors such as predation are known to impact these social systems, but quantitative measurements of change in sociality are necessary to fully comprehend how ecological conditions impact social systems. While plains (Equus quagga) and Grevy’s zebras (Equus grevyi) are two closely related species, they have evolved to exhibit contrasting social structures. The sympatric plains and Grevy’s zebras of central Kenya can be used as a case study to attempt to understand and quantify how changes in environmental condition, such as decreased rainfall, vegetation, predation, etc., may drive the changes in a social network. Using past data collected from the summer of 2013 on the same sample of plains and Grevy’s zebras, and utilizing social network theory, with its corresponding network metrics, we aimed to quantitatively compare how social networks change as a result of differences in rainfall and resource abundance. Network metrics revealed that in wetter environments, where there is greater vegetation abundance, these two species of zebras are generally more likely to have greater cohesion and connectivity with one another as compared to the networks in a drier environment. In a more arid environment (summer of 2016), plains zebra stallion males are less likely to form herds with other stallions, and will instead break off into their respective harems in search of better vegetation. However, we found that bachelor males will remain preferentially associated to other bachelor males in both years. Territorial Grevy’s males in 2016 appeared to occupy the same general regions that were previously observed in 2013, however, the territory boundaries were much less strict than before. It would be expected that because of the pronounced decrease in social connectivity within both species’ networks, the rate of disease transmission is lower during drier periods and higher during wetter ones. This has important implications for future studies on the relationship between social networks, environmental change or seasonality, and disease transmission.