Group size effects of a dangerous prey species on predator attack decision and capture success

Background/Question/Methods

Predation is a classic explanation for the evolution of grouping behavior in prey. However, our understanding of how prey grouping behavior affects predator attack decision is limited to prey species that are relatively easy to catch once detected and unlikely to injure attacking predators. Studies have shown that predators preferentially attack large groups, despite decreased capture success, possibly due to increased detection of larger prey groups. Dangerous prey, or prey that are difficult to catch and likely to injure an attacking predator, often require many attack and capture attempts, and increased overall handling time, in order for the predator to be successful and avoid injury. Predators that hunt dangerous prey may, therefore, make different attack decisions than those that hunt prey that are difficult to find but relatively easy to catch. Here, we used a unique dataset derived from direct observations of wolves (Canis lupus) hunting bison (Bison bison) in Yellowstone National Park to determine how prey group size and composition influence predator attack decision and capture success with a dangerous prey species. We also quantified the relationship between capture success and the age and sex of selected individuals.

Results/Conclusions

Our results suggest that predators that hunt dangerous prey show a preference for attacking small prey groups, despite an initial increase in capture success with group size. The probability that wolves attacked bison decreased linearly with bison group size and wolves were over 4 times more likely to attack cow/calf groups (mixed groups) of bison than bull groups. Interestingly, the effect of group size was less influential on the decision to attack bull groups than mixed groups. There was a non-linear relationship between capture success and herd size of mixed groups. Capture success initially increased up to a group size of 24, and then decreased after that. There was slight evidence that wolves were more likely to capture an individual in a mixed group than a bull group. However, there was strong evidence that an attack was over 28 times more likely to be successful if a calf was selected rather than a cow or bull. The decision to preferentially attack small prey groups may be due to the increased energetic costs of handling difficult to capture prey, and/or the increased risk in attacking a formidable prey species.