Zoo life can be deadly for elephants, researchers concluded in a study that found wild elephants live longer than their captive sisters. The average African female elephant lived to be just under 17 in a zoo but female elephants living natural lives in Amboseli National Park in Kenya lived an average of 56 years, they found.

Stress and obesity are the likely killers, Ros Clubb of Britain's Royal Society for the Prevention of Cruelty to Animals and colleagues found.

"In zoos, the welfare of African elephants (Loxodonta africana) and Asian elephants (Elephas maximus) has long caused concern," they wrote in Friday's issue of the journal Science.

"Infanticide, Herpes, tuberculosis, lameness, infertility, and stereotypic behavior are prevalent, and zoo elephant populations are not self-sustaining without importation."

The international team of researchers studied more than 4,500 individual elephants, including about half the global zoo population.

"Neither infant nor juvenile mortality differed between populations, but adult females died earlier in zoos than in Amboseli," they wrote.