The pine marten, a North American member of the weasel family, occasionally works together in packs of hundreds to bring down large prey. Documented accounts of martens bringing down deer are common, and there are reports of moose succumbing to the chihuahua-sized animal. One unverified claim has the creatures killing a grizzly bear. Attacks generally result in numerous deaths of martens, but they breed quickly and the losses are made up with ease. These attacks are not the norm, as martens usually eat mice and other small animals. No one knows exactly what causes the communal behavior, but it is universally agreed that it is good that it is so rare.