Vigilance behaviour and its endocrine correlates in Plains zebra (Equus burchelli) living in a predator-free landscape

Abstract

Group size affects individual and collective vigilance levels of prey species. As individual vigilance decreases with increasing group size, the indirect risk of predation to each individual and the group as a whole will also decrease (dilution and many-eyes effect) which may have a decreasing effect on stress responses on group level as well. Where predation risk is low, other factors like group size might influence stress-related glucocorticoid output in prey species. I test the relationship between group size and individual and collective vigilance levels in a plains zebra (Equus burchelli) population living in an environment with low adult predation risk due to the absence of lions (Panthera leo) in the Dinokeng Game Reserve (DGR). I also test for an effect of season (wet vs. dry) on these levels. Vigilance levels are expected to be higher in summer than winter, due to a seasonal influence on susceptibility to predation or the ease of predator detection. Finally, I test if the presence of foals in a band influences individual and collective vigilance levels. Mothers are expected to increase their vigilance levels since foals are most susceptible to predation by smaller predators in the reserve. I also aimed to specifically investigate how group size affects adrenocortical endocrine activity in male and female individuals of plains zebra bands living in an environment with low predation risk. With an increase in group size, and individual vigilance expecting to decrease, the indirect risk of predation to each individual and the group as a whole will also decrease (due to both dilution and many-eyes effect) which may decrease stress on group level as well. Affiliative social interactions between females could also have the effect of improving fitness and reducing stress levels in this species. We therefore evaluated the reliability of different enzyme-immunoassays (EIA) for monitoring glucocorticoid metabolite levels in plains zebra faeces by conducting an adrenocorticotropic hormone (ACTH) challenge test and associated translocation event. Individual vigilance decreased with group size, confirming a classic group size effect; while collective vigilance remained constant meaning individuals could reduce their own scanning and rely on scanning by other group members. Individual vigilance differs between seasons, with higher levels recorded in summer, but no seasonal effect is found on rate of scanning and collective vigilance. Interestingly, bands with foals have a lower level of individual vigilance than bands without foals, which may be due to added nutritional stress on mothers during lactation. A constant collective vigilance level means mothers may be able to increase foraging time without increasing predation risk. I successfully identified two group-specific EIAs as suitable for assessing adrenocortical endocrine activity in male and female plains zebra, but gender-specific differences in response to ACTH administration and translocation were detected. In winter, bands of free-ranging plains zebra differ significantly in terms of faecal glucocorticoid metabolite (FGM) output, but due to the fact that the bands tested also differed in size as well as composition, the potential band size effect were analysed for males and females separately. Males in bands of greater size exhibit higher FGM levels than males in small bands; explained by the females’ need for protection, resource competition and reproduction pressure. FGM levels in females did not differ between bands of different sizes, indicating that potential changes in vigilance behaviour per se might not alter FGM output in a low-predation risk environment. This study confirms firstly, the classic group size effect on vigilance in plains zebra in this low predation risk environment and secondly, that group size and sex should be considered when looking at FGM levels in this social species.