In the News: Earth Notes: Mexican Gray Wolf Cross-Fostering

Mexican gray wolves were once common throughout the southwest United States and into central Mexico. But their populations were decimated in the nineteenth and twentieth centuries as human settlement exploded. Prey decline, habitat degradation, and federal predator control programs all but wiped out the animals. By the 1970s, Mexican wolves had almost completely disappeared from the wild.

Cross-fostered wolf pups at the Chicago Zoological Society

CREDIT CHICAGO ZOOLOGICAL SOCIETY

Some remained in captive breeding programs, and in the late 1990s the U.S. Fish and Wildlife Service reintroduced them in eastern Arizona and New Mexico. Though their numbers have grown, the animals have been plagued by a lack of genetic diversity, which limits reproduction.

Cross-fostered Mexican gray wolf pups in a wild den.

CREDIT USFWS

The endangered southwestern populations have struggled to gain a foothold. Mexican wolves remain the rarest subspecies of gray wolf in North America, with just over a hundred in the wild.

To help them along, biologists have applied an experimental technique in recent years called cross-fostering. They swap out wild or captive-born pups into wild litters, and the young are then raised by surrogate parents. In theory they’ll eventually spread their genes to the greater population.

Twenty-two Mexican wolf pups have been cross-fostered in the wild so far, and officials say initial results are promising. But biologists have confirmed that only three pups have reached breeding age, and the fates of many others are unknown.

Some conservation groups are critical of cross-fostering. They’ve called for more captive-born adult wolf releases, arguing that a boost in genetic diversity is needed immediately.

Still, wildlife officials are forging ahead and plan more cross-fostering this year—in hopes that the effort will help bring Mexican gray wolves back from the brink of extinction.