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 Seattle's Woodland Park Zoo have already made the controversial decision to move their elephants to another zoo so they can join a larger herd. Others, like the Bronx Zoo in New York City, say they are moving in the same direction. And zoos like Detroit have already retired their pachyderms to one of two U.S. animal refuges.

The Association of Zoos & Aquariums says 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.