Food snitches threaten rare dogs.

It's a dog's life, and it's even harder than we thought.

African wild dogs need so much energy during a day--more than twice what some scientists had thought--that losing just a quarter of their food to thieving hyenas may threaten the dogs' survival. Martyn L. Gorman of the University of Aberdeen in Scotland and his colleagues, who have taken the first energy measurements of these dogs in their natural habitat, report the findings in the Jan. 29 Nature.

The new data could guide efforts to preserve the vanishing species, which has dwindled to some 5,000 animals in the wild, Gorman says. His data bolster the contentions of some field biologists that dogs and the food-stealing hyenas don't mix.

The measurements may also jolt zoologists into taking another look at the energetics of hunting. "I think what we've done in the past was really underestimate how hard you have to work to catch your own food," Gorman says. Much of the previous work relied on captive animals in laboratories.

Gorman and his colleagues injected water containing distinctive isotopes of hydrogen and oxygen into six free-ranging dogs. By measuring the isotopes in blood samples taken 24 hours later, the researchers could estimate the dogs' rate of respiration; from the respiration rate, they calculated that each animal used an average of 15.3 megajoules of energy per day. Earlier calculations predicted 6.7 megajoules per day for any active dog, whereas studies of working border collies found that these dogs need 8.2 megajoules.

Joshua Ginsberg, a researcher from the Wildlife Conservation Society based at the Bronx Zoo in New York, says biologists have long suspected that wild dogs don't cope well with food theft. "What we haven't known is that they are such incredible energy machines."