Varying degrees of fear : how do large herbivores adjust their anti-predator behaviour in response to different predators?

Abstract

Large predators are returning to areas where they have previously been absent through both predator reintroductions and natural range expansion. As a result, these re/introductions are changing the nature of predator-prey systems, with potential shifts from single to multi-predator environments. In response, prey species must make spatial (i.e. patch use) and behavioural (i.e. vigilance, feeding) adjustments to minimize risk from predators with varying hunting strategies, prey preferences and activity patterns. With this in mind, the broad aim of my study was to determine how perceived predation risk from different large predators affected the spatial and behavioural decisions made by individuals in a community of mammalian herbivore species. To achieve this, I first investigated, how kudu, sable and warthog responded to the introduction of wild dogs into an area with cheetah. In response to this shift from single to multiple predators, all three-herbivores decreased their feeding effort. However, sable (not hunted by cheetah) decreased their feeding intensity more than kudu (preyed upon by cheetah). In contrast, warthog (avoided by cheetah) showed a different feeding pattern, but then displayed a dramatic response to the wild dog introduction in that they disappeared from the study site. These patterns suggest that the magnitude for increasing anti-predator responses to predator introductions may be greater for prey (e.g. sable, warthog) living in initially low risk environments compared to prey species (e.g. kudu) that already have high perceived predation risk from the resident predator (e.g. cheetah). Despite decreasing their feeding effort, all three herbivore species preferred patches located in open grasslands compared to denser vegetation, before and after the wild dog introduction, possibly due to increased predator detection in open areas. In my second experiment, I explored how prey species employed a range of anti-predator behaviours (i.e. vigilance, grouping and temporal shifts in activity) of various combinations to minimize risk from predators with different hunting strategies (ambush vs cursorial) and prey preferences. Overall, herbivores had higher vigilance, moved in larger groups and avoided waterholes at night when exposed to predation risk from lions (ambush predator). In contrast, in response to cheetah and/or wild dogs (cursorial predators), only prey species within the accessible weight range of both these predators (warthog, red hartebeest) moved in larger groups compared to conspecifics in the lion section. They did not however, shift their waterhole use to night to avoid the largely diurnal cheetah and wild dogs. These results suggest that the potential threat of ambushing lions was greater than that of the cursorial cheetah and wild dogs. I then expanded on this experiment by investigating, how prey adjusted their anti-predator behaviour (i.e. vigilance) in response to the alarm calls of con/heterospecifics (i.e. zebra, wildebeest) or to non-hunting predator calls (i.e. lion roars). Overall, red hartebeest and wildebeest living with lions tended to show greater vigilance in response to the lion roars compared to the alarm calls. This suggests that these species perceived the direct cue of lions as a better indicator of risk than the alarm calls. I then compared these responses for prey species living with and without lions. I found that herbivores living with lions had higher vigilance than conspecifics living without lions. Despite a greater overall response in the lion section, herbivores in the lion-free section still significantly increased their vigilance in response to the lion roar. Yet, it was two of the lions’ preferred prey species (i.e. zebra, wildebeest) that showed the greatest response. This suggests that species under the greatest threat may maintain innate anti-predator responses to a dangerous but absent predator longer than less preferred prey. Ultimately, my findings indicate that simple cues from dangerous predators can have a greater effect on anti-predator behaviours of prey species than alarm calls. Overall, the results from my PhD highlight that prey respond to changing predation risk after a predator introduction by modifying their fine scale patch use and foraging behaviour. Moreover, prey responses to different predators are not uniform, but reflect differing degrees of danger posed by the predators, resulting in prey using varied combinations of anti-predator behaviours to minimize risk around risky areas. Lastly, non-hunting cues from dangerous predators can trigger strong anti-predator responses from prey living with and without the predator. Ultimately, the results of my three experiments highlight the behavioural plasticity of prey species to reduce predation risk across their fluctuating landscape of fear.