How Pack Size Factors into Wolf Hunting Success
In 2014, The Department of Wildland Resources (DWR) out of Utah State University, conducted a study in which they attempted to determine the influence of group size on the success of wolves hunting their prey. The study was conducted to test the hypothesis that predators in large groups are more cooperative when hunting a formidable target. The following post will give an overview of this study, to provide the reader with an in-depth look at the successes of one of nature's’ most alluring predators, the wolf.
Introduction to the Scientific Study
Within a society of social predators, an increased ability to capture prey is a frequently cited benefit to group living. However, recent research suggests that the benefit of improved hunting success (defined as the probability of capturing prey) is more commonly achieved in small groups (MacNulty 1). There are numerous studies discussing how carnivore hunting success peaks at 2-5 hunters, then levels off, or even declines across larger group sizes. These studies suggest the formation and maintenance of large groups is unrelated to prey capture, however, predators that hunt more difficult prey may not abide by these rules.
Bison maintain a higher level of difficulty for wolves to hunt over their year-round prey, elk. Bison are more difficult to kill than elk due to the fact they are larger, more aggressive, and more likely to injure or kill the wolves hunting them (1). As a result, bison require relatively more time to subdue, which is characteristic of dangerous prey (1). Groups of wolves are more likely to attack bison than solitary wolves, but the effect of group size on the ability of wolves to capture bison is unknown. The Department of Wildland Resources measured the influence of group size on the probability wolves would actually capture a bison in comparison to the capture of an elk.
Methods Used to Study Wolves Hunting Bison
To sample the behaviour of wolves hunting bison, DWR observed hunting behaviour during biannual 30 day follows of 5 different wolf packs (2). Observation of the wolves was conducted from the ground, as well as from a fixed-wing aircraft. Two hundred and thirty nine wolves hunting bison encounters were recorded during the study (2). Encounters between the wolves and their bison prey are defined as at least 1 wolf orienting and moving (walking, trotting, or running) toward bison. After the initial encounter, progress was recorded by noting the foraging state (approach, watch, attack-group, attack-individual, capture) (2).
Group hunting success was measured by whether the wolves hunting were able to complete 2 tasks: approach - attack, equaling to attack - capture. A hunting group was deemed successful if the task was completed by at least 1 group member. If not, the group was considered to have failed (2).
Wolf Hunting Results
The influence of pace size on the success of wolves attacking and capturing bison was not a linear result. The top models of attacking and capturing included a linear spline for group size, indicating a threshold at which the effect of group size in hunting success suddenly changed (4). A simple linear relationship between group size and success was reasonably strong for attacking, but weak for capturing. The latter suggests that capture success may have increased across the largest observed group size of wolves hunting in a pack, which was 11-16 wolves (4).
The threshold group size was smaller for attacking than for capturing: 3-6 wolves for attacking, and 9-13 wolves for capturing (4). Beyond each threshold, group size had no significant role in their success. Whereas the threshold group size of wolves attacking bison and elk was the same (4 wolves), the threshold group size of wolves capturing bison (11 wolves), was nearly 3 times larger than that of wolves capturing elk (4 wolves) (4). So, the range of plausible threshold group sizes was similar when attacking bison (3-6 wolves) and elk (4-7 wolves), but higher when capturing bison (9-13 wolves) vs elk (2-6 wolves) (4). When taken together, these results indicate that bison capture success increased across group sizes. Elk success stayed constant and levelled off at a group size larger than that of wolves hunting bison. Given that solo bison capture success was 94% less than solo elk capture success, this pattern is consistent with the prediction that large groups are more cooperative when the success of a single hunter is very low (4).
The increase in capture success rate of wolves hunting bison can be attributed to a larger group size. This is due to enhanced cooperation, motivated by the very low capture rate of a single hunter (6). The evidence provided by this study, displaying how larger pack sizes of wolves were better at hunting more dangerous prey, offers rare empirical support for the hypothesis that a larger grouping of carnivores increases the variety of prey they can capture (6). However, because data on large groups hunting multiple prey species is scarce, it is difficult to determine whether the correlation between prey size and group size results from greater food requirements of large groups, or because large groups can indeed capture large prey more easily (6). Although the studies results do not address the relative importance of these two mechanisms, they suggest that improved hunting ability is a reasonable explanation (7).
This is a significant finding, because most empirical studies of group-size specific hunting success imply that the formation and maintenance of large predator groups is unrelated to prey capture. The study conducted by DWR clarifies that the benefit of improved hunting success could favour large groups in populations and species that hunt large, dangerous prey (7).
MacNulty, D. R., Tallian, A., Stahler, D. R., & Smith, D. W. (2014). Influence of Group Size on the Success of Wolves Hunting Bison. Plos ONE, 9(11), 1-8.