Understanding the behavioural trade-offs made by blue wildebeest(Connochaetes taurinus): the importance of resources, predation and the landscape

Abstract

Prey individuals must constantly make decisions regarding safety and resource acquisition to ensure that they acquire enough resources without being predated upon. These decisions result in a trade-off between resource acquisition behaviours (such as foraging and drinking) and safety behaviours (such as grouping and vigilance). This trade-off is likely to be affected by the social and environmental factors that an individual experiences, including the individual’s location in the landscape. The overall objective of my PhD was to understand the decisions a migratory ungulate makes in order to acquire enough resources, while not becoming prey, and to understand how these decisions are influenced by social and environmental factors. In order to do this, I studied the behaviour of blue wildebeest (Connochaetes taurinus) in Etosha National Park, Namibia, between 2013 and 2015. I studied wildebeests’ behaviour while they acquired food and water and moved within the landscape. Along with observational studies, I also used lion (Panthera leo) roar playbacks to experimentally manipulate perceived predator presence to test wildebeests’ responses to immediate predation risk. For Chapter 2 I studied the foraging-vigilance trade-off of wildebeest to determine how social and environmental factors, including the location within the landscape, were correlated with wildebeests’ time spent foraging and vigilant as well as their bite rate. I found that environmental variables were more important than social variables in predicting the foraging-vigilance trade-off of adult wildebeest. Food availability was the most important factor, with wildebeest increasing time spent foraging, at a cost to vigilance time, when food was more readily available. This result suggests that wildebeests’ behavioural decisions were more affected by forage availability than predation risk. The objective of Chapter 3 was to study the effects of individual characteristics, social variables and environmental variables on wildebeests’ behaviour during their approach to, time at, and retreat from, waterholes, to understand the water acquisition-safety trade-off. I looked at time spent vigilant, moving and drinking as well as step rate. Wildebeests’ behaviour was associated with individual, social and environmental factors. Wildebeest approached with less caution when in larger groups and when more heterospecifics and non-focal group wildebeest were present. Wildebeest spent more time during the at-water and retreat phases vigilant in 2013 than in 2014 and spent more time during the at-water phase vigilant earlier in the year. These results suggest that prey species may consider different factors when deciding how cautiously they should approach waterholes, how vigilant they should be at waterholes, and how quickly they should retreat from waterholes. For Chapter 4 I used lion roar playbacks to test the effects of immediate predation risk on the foraging-safety trade-off of wildebeest. I assessed the changes in times spent vigilant and foraging as well as the change in within-group density after wildebeest heard lion roars. I also looked at vigilance strategy by analysing the change in the type (social and antipredator) and intensity (exclusive and chewing) of vigilance used. After hearing lion roars, time spent vigilant (including excusive and antipredator vigilance) and within-group density increased, while time spent foraging decreased. The foraging-vigilance trade-off was not affected by any variable except playback type (lion or control). These findings suggest that wildebeests’ reactions to predator presence were not affected by the social or environmental variables that affected their feeding-safety trade-off when they are not under immediate predation risk. The objective of my final data chapter (Chapter 5) was to assess what factors affected wildebeests’ landscape use and to determine whether they altered their within-group density in response to social or environmental factors, including location within the landscape. The results suggest that when choosing their location in the landscape, wildebeest considered both landscape-scale predation risk factors (the landscape of fear) and forage competition factors. Wildebeests’ within-group density was associated with forage competition more than the landscape of fear. These findings suggest that wildebeests’ landscape-use and within-group density were altered in response to predator avoidance and resource acquisition. My thesis provides insights into how prey species trade-off between resource acquisition and safety. By assessing the social, environmental and landscape variables that affect the trade-off, this study showed that prey adapt their behaviour according to group dynamics, forage availability and predation risk and suggests that individuals may consider different aspects of their environmental and social surroundings when accessing different resources. Therefore, future studies should examine social, environmental and landscape variables together, rather than only considering a subset of these variables. My thesis also showed that different intensities and types of vigilance are used under different circumstances, suggesting the importance of considering both vigilance type and intensity in future studies to fully understand the behaviour and potential cost.