Prey depletion and the effect of group size on cooperative hunting in African wild dogs

Cooperative hunting can favour group living by increasing the probability that a hunt will end with a kill, increasing the size of prey that can be taken, increasing the probability of killing multiple prey or decreasing the distance moved and energy expended. Across a broad range of taxa, environments and hunting behaviours, one of the most consistent benefits of cooperation is an increase in the mass of prey that can be killed. African wild dogs, Lycaon pictus, are cursorial hunters that cooperatively search for, capture and kill prey that are typically ∼1.5–2× their own mass (and sometimes as much as 10×). Prior research with wild dogs has shown that cooperative hunting favours group living though all of these mechanisms. However, most ecosystems with appreciable wild dog populations are now affected by prey depletion due to bushmeat poaching, which disproportionately reduces the density of large prey such as wildebeest, Connochaetes taurinus, relative to smaller prey such as impala, Aepyceros melampus, or puku, Kobus vardonii. Here, we applied a Bayesian structural equation model to data from high-frequency triaxial accelerometers in 13 wild dog packs in two ecosystems to test whether prey depletion altered the effect of group size on hunting movements and energy expenditure. Contrary to prior results from an ecosystem with abundant prey of all sizes, we found that larger packs made larger movements and expended more effort when hunting. Also contrary to prior results, we found that large packs did not kill larger prey (N = 346 kills). Our results suggest that prey depletion reduces the benefit of hunting in large groups by reducing opportunities to kill large prey, thereby necessitating increased movement and energy expenditure. Anthropogenic effects are now altering fundamental ecological relationships such as the costs and benefits of variation in group size. Although the behavioural consequences of shifting ecological baselines will often be difficult to detect, understanding these effects is increasingly important for the conservation of endangered species like the African wild dog.