John Muir commented that every time he saw a grizzly bear the wilderness got bigger. Others faced with such an encounter might readily agree with the Scottish-born American naturalist who explored large parts of the western United States. Yet today’s predators are smaller than those of the past,and the reason seems to have something to do with the speed of their prey.
Large predators gain tremendous benefits from being big. Killing other animals and protecting or stealing a carcass from others tend to be easier since their teeth and claws are larger. Handling struggling prey is safer as the larger predator has less trouble holding the prey firmly and preventing it from striking out at the hunter’s vital organs. These attributes were even more important in the past since life was tougher for predators then.
A new study,however,reveals that excessive size can interfere with predators’ hunting. Daniel MacNulty of the University of Minnesota and his colleagues have found that for predators which need to run after their prey,being big can be more of a hindrance than a help.
Tracking the pack
Using a combination of radio-tags and direct observations,the researchers analysed 94 wolves as they hunted elk in Yellowstone National Park,Wyoming. They made a detailed record of their hunting and kills between 1995 and 2003. The weights of the individual wolves were estimated based upon models derived from body-mass measurements taken concurrently from 304 wolves,including 86 of the 94 wolves in the study. Weights ranged from 22 to 64kg.
The researchers report in the Journal of Animal Ecology that although on the whole big animals are indeed better at handling and killing prey,the largest wolves struggled in the chase. The odds of successful handling and killing improved by 7 per cent for each 1kg increase on average. Up to 39kg each extra kilogram of body mass increased the success of a pursuit by 19 per cent; but above that threshold each kilogram decreased the success by 1 per cent.
Big animals did indeed have longer strides and ran faster,but only up to a point-39kg for the Yellowstone wolves. Beyond that,the strain of being large overcomes the benefits of a longer stride,says Dr MacNulty. This restricts the size of wolves,since becoming too large would ultimately leave them unable to hunt.
The researchers conclude that smaller carnivores would have had an advantage in environments with predominantly fast prey,and bigger ones where prey was slower and larger. As cave bears and wolves from a few million years ago were bigger than their descendants,they were probably quite a bit slower than they are today. Not that it would make the wilderness seem any smaller.
© The Economist Newspaper Limited 2009