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Big preys reinforced emergence of big groups, new study suggests

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Posted November 13, 2014

Usually hunts organized by small groups are as efficient as attacks carried out by more populous ones. This fact raises some interesting questions related to the evolution of human societies. For instance, how bigger collectivities emerged from the small ones? Interestingly, foraging behavior of wolves suggests a possible answer to this intriguing puzzle.

Wild dogs Lycaon pictus, at a zoo in New Zealand. Image credit: Brian Gratwicke via Flickr, CC BY 2.0.

Wild dogs Lycaon pictus, at a zoo in New Zealand. Image credit: Brian Gratwicke via Flickr, CC BY 2.0.

New study conducted by American scientists shows that large pack of carnivores can be more effective than small ones, when hard-to-catch animals are chased. “Improved ability to capture formidable prey could therefore promote the formation and maintenance of large predator groups, particularly among predators that specialize on such prey,” the researchers explain.

“In many group-hunting taxa, ranging from insects to primates, hunting success fails to increase over larger group sizes despite apparent cooperation among hunters,” authors of the study published on PLoS ONE say. This observation can be explained by the fact that cooperation rates decline when group size increases, as animals have incentives to free ride.

Logic is simple: if prey can be captured by one or two wolves it is rational for others defect. However, scholars conjectured that the pay-off matrix should be different when a target animal is hard-to-capture. In such cases, efforts of solitary individuals will not suffice to catch a prey. Therefore, free riding should decline and large parties become more effective.

Picture: Behavior of wolves hunting bison: (a) approach, (b) attack-individual, (c, d) capture. Image courtesy of the researchers.

Picture: Behavior of wolves hunting bison: (a) approach, (b) attack-individual, (c, d) capture. Image courtesy of the researchers.

Unfortunately, this prediction has little empirical support. “Empirical research has yet to establish how group size-specific hunting success of large groups varies across prey species that are differentially vulnerable to predation,” the biologists note.

Daniel R. MacNulty who works as an Assistant Professor at Utah State University and his colleagues observed, how wolves living at Yellowstone attack bisons. Collected data were compared with previous results documenting hunts of elk, a target which is much easier to catch.

“Whereas improvement in elk capture success levelled off at 2–6 wolves, bison capture success levelled off at 9–wolves with evidence that it continued to increase beyond 13 wolves. These results are consistent with the hypothesis that hunters in large groups are more cooperative when hunting more formidable prey,” the researchers report.

This finding suggests that the emergence of sizeable groups was reinforced by hunts of large preys.

Article: MacNulty DR, Tallian A, Stahler DR, Smith DW (2014) Influence of Group Size on the Success of Wolves Hunting Bison. PLoS ONE 9(11): e112884. doi:10.1371/journal.pone.0112884, source link.

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