In captivity, nomadic animals like polar bears have a higher infant mortality rate and show more abnormal behavior than naturally sedentary animals do, according to British researchers who analyzed 40 years of scientific data.
The problem is so widespread worldwide that most zoos need to improve their confinement conditions by building larger, more complex exhibits for roving animals, said Georgia Mason, an Oxford University zoologist and the study's lead author.
Alternatively, zoos could phase out roaming animals.
"As their animals die and need replacing, zoos should just try replacing them with smaller-ranging species because they're more likely to be successful," Mason said.
Zookeepers have long known that confining wide-ranging animals can disrupt their natural lifestyle. In the last decade, British zoos have gradually stopped replacing polar bears when they die, because zookeepers found breeding difficult and noticed erratic behavior in captive polar bears, Mason said.
The study, published in Thursday's issue of the journal Nature, was partly funded by the Universities Federation for Animal Welfare and six British zoos including those in Bristol and Edinburgh.
Zoos, particularly those in North America, have taken steps in the last few years to build more natural, spacious habitats, said Michael Hutchins, director of conservation for the American Zoo and Aquarium Association, who was not part of the study.
Zoos now give animals variety in their enclosures such as borders for them to patrol and structures for them to climb, he said.
The British researchers studied 35 captive species and compiled data on infant mortality and on which animals paced back and forth from more than 1,000 scientific articles published since the 1960s. Pacing is usually a sign that zoo animals lack stimulation in their enclosures.
They looked at mortality data from over 500 zoos worldwide and pacing results from 42 zoos in the United States and Europe.
The study found captive polar bears and lions were the most at risk of infant deaths and pacing. Polar bears, which cover an average natural range of about 31,000 square miles, or the size of Portugal, fared the worst. They showed an average 65 percent infant death and spent 25 percent of their time pacing in a typical zoo habitat one-millionth of their normal range area.
Stay-at-home animals such as the grizzly bear and American mink showed lower infant mortality and pacing rates in captivity.
The study did not address how zoo infant mortality compares to infant deaths in the wild or why wide-ranging animals show higher infant mortality than stay-at-home captive animals, although researchers suspect poor maternal care is a factor.
In New York City's Bronx and Central Park zoos, care is taken to ensure that polar bears have enough room and variety in their enclosures to prevent abnormal behavior, said Richard Lattis, senior vice president of the Wildlife Conservation Society, which oversees the two zoos.
In the Central Park Zoo, polar bears roam a 5,000-square-foot exhibit that includes a pool and gravel pits for them to dig for buried treats like frozen fish. An ice machine in the back generates a pile of ice to mimic Arctic conditions.
"We should really be aware and conscious in zoos that animals have requirements that we cannot match in space, but we can provide opportunities and challenges that both physically and mentally address those needs," Lattis said.
By Alicia Chang