“20% of the people do 80% of the work.” – Pareto Principle
Among social predators, hunting success initially increases with group size but then levels off and even decreases with larger groups. This is a commonly observed trend that has been noted in everything from insects to felines, wolves and chimps.
There are two hypotheses for the observed decline in performance among large groups: 1) the interference and 2) the free-rider hypothesis. Renamed for fun:
- Too Many Cooks in the Kitchen – proposes that overcrowding is responsible for reduced hunting success (Hn ). It also predicts that the greatest drop in (Hn ) is expected with inexperienced hunters as they are more likely to interfere with each other. With interference, we would expect to see wolves getting in each other’s way in all phases of the hunt but the decline should be greatest with inept hunters.
- The Freeloaders – suggests that hunting success is non-linear because individual hunters withhold meaningful participation and take part only to be close enough to enjoy the spoils. It predicts that the level of competence is immaterial, and that withholding effort will be higher in hunting phases with greater individual cost (e.g. injury). The last prediction is that non-breeding wolves have greater incentive to freeload.
With individually known wolves (N=94), the authors collected hunting behavioral data for 8 years (1995-2003). Using the same basic methods briefly described in Wolves Start Getting Old at 2. This time the analysis focused on how group numbers affected individual and pack (Druid Peak, Leopold, Mollie’s and Rose Creek) hunting success.
As in the previous study, the individual wolves were scored on whether they completed each of the 3 predatory tasks. If a wolf successfully participated in sequential foraging states then it was scored as a successful completion of the predatory task.
- approach => attack-group = Attacking
- attack-group => attack-individual = Selecting
- attack-individual =>capture = Killing
Group Hunting Success
Graphs (a)-(c) show the non-linear effects of group size on the three hunting tasks. Graph (d) shows that the overall probability of success reaches a maximum with a group of 4 wolves.
Individual Hunting Success
Crunching the data for individuals indicates that individual performance decreases with group size. For any individual wolf, the probability that it will kill an elk for a given encounter mirrors what we see in the group analysis.
The folkloric view held by some is that of wolves as supreme social predators with some magical qualities beyond those of other animals. The reality is that wolves are just like any other group predator.
Like other social carnivores, birds, insects and primates – including (specially?) humans, bigger hunting groups does not increase hunting success. Increasingly larger groups show ever decreasing levels of cooperation. Token participation allows the individual to partake in the benefits of the group, while withholding effort minimizes the individual cost.
A similar pattern of reduced cooperation with increasing numbers has been reported in group conflicts among free-ranging dogs (Bonanni et al 2010).
The authors give three lines of evidence that support the withholding hypothesis.
- Individual performance decreased as group size increased regardless of age or hunting ability [recall that these are individually known wolves]
- Individual decline in performance is linked to the danger of the task. [more danger meant more withholding]
- Decline in individual performance with increasing group sized related was slower in wolves with pups.
The authors also checked the data to see if fatigue from a previous hunt or the previous task could account for a wolf’s under-performance, for example, a wolf that took part in selecting might be too tired to participate in killing. The data showed the opposite was true and that a wolf was more likely to participate in a task if it had performed it earlier that day.
Breeder wolves were also more likely to select and kill than non-breeder wolves. Wolves with dependent pups were less likely to withhold effort regardless of group size.
One finding also went against the alternative explanation of inference. The rate of decline was greater among competent groups than less competent groups; this is the opposite of what is predicted by the interference hypothesis.
Many social also maintain groups several times larger than their optimal hunting numbers indicating that hunting does not play a role in the formation of large groups.
For wolves the optimum hunting group size is 4, greater numbers don’t make them better hunters.
The authors conclude:
“Regardless of the mechanism(s) involved, the widespread tendency for hunting success to level off with increasing group size suggests that the influence of group size on hunting success per se is unlikely to promote the formation and maintenance of large predator groups.”
I was unable to find papers on why free-loaders are tolerated – reasonable given the newness of this paper. It might be that Canis lupus can’t keep track of cooperators and free-loaders, or that they lack capacity to formulate a punishment for cheaters. It might also be that they are gaining some fitness benefit in other non-hunting related tasks like defending a kill or maintaining territory.
That’s the way it goes with research, you get one answer which leads to a whole bunch of more questions.
Bonanni, R., Valsecchi, P., and Natoli, E. (2010). Pattern of Individual Participation and Cheating in Conflicts Between Groups of Free-Ranging Dogs. Animal Behaviour 79: 957-968.
MacNulty DR, Smith DW, Mech LD, Vucetich JA, Packer C (2011). Nonlinear effects of group size on the success of wolves hunting elk. Behavioral Ecology 23:75–82. doi:10.1093/beheco/arr159
MacNulty DR, Mech LD, Smith DW (2007). A Proposed Ethogram of Large-Carnivore Predatory Behavior, Exemplified by the Wolf. USGS Northern Prairie Wildlife Research Center. Paper 105. http://digitalcommons.unl.edu/usgsnpwrc/105