  Social foraging shows unexpected features such as the existence of a group
size threshold to accomplish a successful hunt. Above this threshold,
additional individuals do not increase the probability of capturing the prey.
Recent direct observations of wolves in Yellowstone Park show that the group
size threshold when hunting its most formidable prey, bison, is nearly three
times greater than when hunting elk, a prey that is considerably less
challenging to capture than bison. These observations provide empirical support
to a computational particle model of group hunting which was previously shown
to be effective in explaining why hunting success peaks at apparently small
pack sizes when hunting elk. The model is based on considering two critical
distances between wolves and prey: the minimal safe distance at which wolves
stand from the prey, and the avoidance distance at which wolves move away from
each other when they approach the prey. The minimal safe distance is longer
when the prey is more dangerous to hunt. We show that the model explains
effectively that the group size threshold is greater when the minimal safe
distance is longer. Although both distances are longer when the prey is more
dangerous, they contribute oppositely to the value of the group size threshold:
the group size threshold is smaller when the avoidance distance is longer. This
unexpected mechanism gives rise to a global increase of the group size
threshold when considering bison with respect to elk, but other prey more
dangerous than elk can lead to specific critical distances that can give rise
to the same group size threshold. Our results show that the computational model
can guide further research on group size effects, suggesting that more
experimental observations should be obtained for other kind of prey as e.g.
moose.
