Bamboo, an Asian elephant, briefly moves a large ball around at the Woodland Park Zoo, in Seattle. Elephants, one of the most popular animals at American zoos, could become very hard to find over the next few decades as aging pachyderms die and the rest congregate in the handful of zoos capable of caring for the large animals. Facilities like Seattleís have already made the difficult and controversial decision to move their elephants to another zoo so they can join a larger herd.

SEATTLE — Visitors flocked to the Seattle's zoo to watch the two elephants, Chai and Bamboo, as they used their long trunks to play with balls and snack on carrots and apples.

The elephants would sometimes exhibit other behavior. Chai would pace from side to side and bob her head up and down — a sign, animal activists say, of the stress of being confined inside the 1-acre area. It is common behavior, and a growing number of people feel the giant animals — hard-wired to roam free across thousands of square miles in Africa and Asia — don't have a place in American zoos.

Seattle's Woodland Park Zoo recently made the difficult and controversial decision to close its popular elephant exhibit and move Chai and Bamboo to a larger zoo in Oklahoma so they can join a larger herd. Others, like the Bronx Zoo in New York City, say they are moving in the same direction, but will wait until one or more of their existing herd dies. And zoos like Detroit have already retired their pachyderms to one of two U.S. animal refuges.

Many zoos can't give elephants the space they need. They are also social animals that prefer to live in a herd, but that is hard to provide as elephant numbers dwindle, both in captivity and in the wild.

As of December 2014, there were 159 African elephants at 39 North American zoos and 139 Asian elephants at 34 zoos in the U.S. and Canada, according to the Maryland-based Association of Zoos & Aquariums. The group In Defense of Animals says 21 zoos in North America have closed their elephant exhibits since 1991.

At the same time, national guidelines adopted in 2011 would require some zoos to close their elephant programs by 2017 if they cannot increase their herds and expand their facilities. Some North American zoos have as few as one elephant, said Rob Vernon, spokesman for the Association of Zoos & Aquariums, which drew up the guidelines. The new rules encourage a herd of at least three elephants.Woodland Park Zoo, which has had elephants in its care since 1921, started discussing the future of its herd after a third pachyderm died in 2014. A community task force recommended in 2013 that the zoo bring in more elephants and expand its facilities, but zoo President and CEO Deborah Jensen said those goals were not achievable.In the end, Seattle decided its best choice was Oklahoma, where 36-year-old Chai and 48-year-old Bamboo may become the old “aunties” of the herd.There's very little consensus about what is the right choice. Bringing more elephants to the U.S. no longer seems feasible. Breeding programs are only minimally successful.

Chai, an Asian elephant, stands in her enclosure at the Woodland Park Zoo in Seattle. Elephants, one of the most popular animals at American zoos, could become hard to find over the next few decades as aging pachyderms die and the rest are sent to the handful of zoos capable of properly caring for them. Facilities like Seattle have already made the difficult decision to move their elephants elsewhere.