To increase their survival, reproduction and therefore fitness, prey species foraging under a risk of predation have to maximize both their energy intake and safety. Because food acquisition and antipredator behaviour are often considered as mutually exclusive, prey have to trade off between these activities. One of the classic ways to investigate this trade-off at a fine scale is to study the trade-off between vigilance and foraging because vigilance is time consuming and therefore considered as costly in terms of food acquisition. In this context, herbivores are particularly interesting to study as they are able to manage the foraging cost of vigilance to some extent by chewing their food while raising their head up. The decisions that herbivores have to take to adjust their trade-off are complex to study because they are affected by numerous environmental, social, and individual parameters which can also vary seasonally. In addition, animal personality could be a key mechanism for understanding within-population behavioural variation in this trade-off.
The overall objective of my PhD was to better understand how herbivorous prey animals manage the feeding/vigilance trade-off at a fine scale, considering the wide range of variables that may affect it, individual variation in this trade-off, and different functions of vigilance. To do this, I focused on the behaviours of two medium-sized herbivores as model species. I first followed 34 identified female eastern grey kangaroos (Macropus giganteus) for an entire year in Sundown National Park in southeast Queensland, filming their behaviour on a monthly basis in a wide variety of contexts. Second, I conducted a playback experiment on the behaviour of female impala (Aepyceros melampus) over two months in Hwange National Park in Zimbabwe to study their behavioural adjustments in social and predation contexts.
The first objective of this PhD (Chapter 2) was to examine seasonal variation in feeding rates and vigilance in eastern grey kangaroo over a year. I observed that, although many different factors affected these behaviours over short temporal scales, the main drivers of seasonal variation in behaviour were habitat for feeding rates, and group size for vigilance. Because habitat type and group sizes both were correlated with food patch quality, food resources clearly appeared to be the primary driver of variation in vigilance and feeding rates. These results suggest that the effects of group size and habitat use on the trade-off between food acquisition and safety appeared to be collateral drivers of the seasonal variation in food supply in our study system.
In Chapter 3 I used an experimental approach via a playback experiment to examine how playbacks of lions’ roars and male impalas’ social calls influenced vigilance, foraging and movements of female impala. The results showed that females responded to both types of signals, but in different ways. Lions’ roars stimulated antipredator behaviours (vigilance and movements) at the expense of foraging, whereas males’ vocalizations increased females’ movements at the expense of vigilance, but had no effect on bite rates.
In Chapter 4, I analyzed how predation risk, food availability and competition affected both the functions (antipredator and social vigilance) and the foraging costs of vigilance (with or without chewing, here called “vigilance while chewing” and “exclusive” vigilance) in kangaroos and how these patterns differed between seasons. My results showed that antipredator vigilance was mainly driven by the perception of predation risk decreasing with group size and distance between foragers. Social vigilance while chewing increased with group size, and the distance between foragers affected both types of social vigilance but with seasonal differences. Finally, in poor food conditions, kangaroos were more socially vigilant, while in good food conditions they appeared to reduce the cost of vigilance by increasing their vigilance while chewing when patch quality increased.
Finally, in Chapter 5, I studied between-individual variation in feeding rates, vigilance, and their trade-off in eastern grey kangaroos in relation to ecological (food patch richness), social (group sizes) and physiological (reproductive states) conditions. I did not find differences between individual female kangaroos in how they adjusted their behaviours in relation to ecological and social conditions, but there were differences among individuals in their behavioural adjustments in relation to their reproductive states. This suggests that between-individual behavioural variation is not necessarily repeatable across different contexts.
This thesis covers different aspects of how prey animals adjust their trade-off between foraging and vigilance in relation to their environment, group mates, and individual characteristics. It shows that animals constantly adapt their behaviour and strategies according to the situation they experience, in order to balance the acquisition of food and social information with staying safe. It is therefore important to take these factors into consideration while studying this trade-off. In addition, my results showed that behavioural adjustments vary depending on the temporal scale, and can vary between individuals, and that social behaviour has a major effect on vigilance and foraging activities. These aspects need to be further studied in the future.