狼群经常被描绘成一种合作狩猎的模式，然而并非所有的灰狼（Canis lupus）都会一起合作捕获它们的猎物。
根据日前发表在《行为生态学》杂志上的一项研究结果，一些狼竟然也吃白食。
研究人员通过对生活在美国黄石国家公园中的狼群进行研究后发现，随着狼的数量的增加，狼群捕猎的成功率并没有像预期那样增加；与此相反的是，一旦它们的数量超过4只，其捕猎成功率便会呈现出平稳状态。
这似乎是因为大型狼群中往往会有那么几个吃白食的家伙——它们通常都是那些不用养育后代，因此也就不用冒着生命危险供养幼崽的狼。
黄石灰狼恢复项目的生物学家Daniel Stahler指出，这些不劳而获的狼貌似也会参与捕猎的合作，但直到杀死猎物之前，它们实际上只作出了非常小的贡献。
这样一来，它们也能够像一个大家庭的成员那样一道参加周末的晚宴——这些家伙大多数只知道吃，但绝不会把家里的培根也带过来。（生物谷 Bioon.com）
doi:10.1093/beheco/arr159
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Nonlinear effects of group size on the success of wolves hunting elk
Daniel R. MacNultya, Douglas W. Smithb, L. David Mechc, John A. Vucetichd and Craig Packera
Despite the popular view that social predators live in groups because group hunting facilitates prey capture, the apparent tendency for hunting success to peak at small group sizes suggests that the formation of large groups is unrelated to prey capture. Few empirical studies, however, have tested for nonlinear relationships between hunting success and group size, and none have demonstrated why success trails off after peaking. Here, we use a unique dataset of observations of individually known wolves (Canis lupus) hunting elk (Cervus elaphus) in Yellowstone National Park to show that the relationship between success and group size is indeed nonlinear and that individuals withholding effort (free riding) is why success does not increase across large group sizes. Beyond 4 wolves, hunting success leveled off, and individual performance (a measure of effort) decreased for reasons unrelated to interference from inept hunters, individual age, or size. But performance did drop faster among wolves with an incentive to hold back, i.e., nonbreeders with no dependent offspring, those performing dangerous predatory tasks, i.e., grabbing and restraining prey, and those in groups of proficient hunters. These results suggest that decreasing performance was free riding and that was why success leveled off in groups with >4 wolves that had superficially appeared to be cooperating. This is the first direct evidence that nonlinear trends in group hunting success reflect a switch from cooperation to free riding. It also highlights how hunting success per se is unlikely to promote formation and maintenance of large groups.
