Early one morning this spring, veterinary techs at Chicago’s Brookfield Zoo bundled a litter of 9-day-old Mexican wolf pups (Canis lupus baileyi) — each weighing just under two pounds — into a cloth carrier. The sleeping pups were then loaded onto a small private plane: Three would go to southeast Arizona, and three to New Mexico’s Gila National Forest, where U.S. Fish and Wildlife Service biologists had recently located the den of AF1578, a 6-year-old female lobo they identified as an ideal foster mother.
Just days earlier, AF1578 had given birth to a wild litter. Now, deep in the Gila, scientists carrying three pups approached her den, and their sounds were enough to spook AF1578 into temporarily fleeing. The scientists moved in, wearing gloves and long-sleeves to minimize the chance that their human smell would linger. They put the captive-born, microchipped pups in with their new wild-born foster siblings. It was essential that all the pups smell alike, so, before leaving, they coated the foster and wild pups in each other’s scents — a critical step that required a small amount of the puppies’ urine. A uniform scent increased the chances that AF1578 would accept the new additions as her own.
From a hiding spot in the piñon and juniper, a wolf released a howl that echoed across the woodland and then faded into a whistle on the wind. Mama sounds upset, the team mused. Time to go. Now, the fate of those captive-born wolves would depend on AF1578’s maternal instincts — whether she’d be able to parent all the pups in the den, including the ones she hadn’t given birth to.
THIS SPRING MARKED 25 years of the U.S. Fish and Wildlife Service’s efforts to revitalize endangered Mexican wolf populations in the U.S. It’s also the eighth year that the agency has introduced captive-born wolf pups into wild dens in a “cross-fostering” program. It’s a strategy that depends on unseen variables — postpartum hormones and pheromones. “We found this little lever in their life history and their biology,” said Susan Dicks, a Fish and Wildlife veterinarian. The “hormone soup” of the early maternal time is powerful, she said. “They just can’t resist pups.”
“They just can’t resist pups.”
By the 1970s, Mexican wolves in the Southwest were nearly extinct. The early 20th century was notable for its profound hostility toward predators in general; ranchers hired trappers to kill any wolves that might threaten their livestock, and the federal government even sponsored programs to incentivize killings. Then, in 1973, the Endangered Species Act passed, and, in 1976, Mexican wolves were listed as a protected species. It wasn’t until 1998, however, that the wildlife agency introduced captive-bred Mexican wolves to the wild. By the end of 2022, the population in Arizona and New Mexico had grown to at least 241, according to the agency.
Previous releases of family groups into historic lobo range were met with anger by local ranchers; between 1998 and 2022, more than 130 Mexican wolves were killed, according to the Sierra Club. In 2016, it began its cross-fostering program, hoping that it would lead to greater public tolerance — and eventually more wolves. Although there are still human-caused wolf killings each year, Aislinn Maestas, a spokeswoman for the agency, said it’s easier to convince ranchers to accept new pups in an already established pack than an entire new family that requires new territory to roam. This year, 16 pups were fostered into the wild, bringing the total number released under the program to 99. The agency hopes the wild wolves will pass their knowledge on to the captive-bred pups, while the newcomers introduce valuable genetic diversity into the population.
Quantifying the program’s success, though, is a challenge. Wolf pups can’t be tracked with a collar — they’re too small, Maestas said. But Fish and Wildlife has so far confirmed that 14 of the 83 captive-born pups released between 2016 and 2022 survived to breeding age. The agency estimates that 50% of the captive-born pups released each year survive for at least a year, a mortality rate consistent with that of their wild-born siblings. Based on gene-monitoring data, the agency says the program has begun to stabilize — and in some cases slightly improve — the gene pool since its lowest point in 2018.
But advocates at the Center for Biological Diversity interpret the numbers differently: They say that cross-fostering hasn’t improved the gene pool enough to matter. Today’s lobos are as genetically related as siblings, said Michael Robinson, a senior conservation advocate at the center. He and his colleagues want Fish and Wildlife to go back to releasing well-bonded family packs, which they believe would improve survival and genetic diversity. In a related fight, the group sued the agency in 2022, alongside Defenders of Wildlife, challenging the standards it uses to define the lobo’s recovery.
On the Gila National Forest this spring, when the scientists placed the new foster pups in the den with their new wild siblings, they took three of the wild-born pups with them. That way, the new mother wouldn’t be burdened with extra mouths to feed, and her pups’ genetics could be introduced into other wild packs. They hiked back to the trailhead, toting the three pups in the cloth carrier, taking them to a new den and another mother. As the scientists’ footsteps faded away, they hoped that AF1578 — herself a cross-foster who grew up in the wild — would return to her den and embrace both sets of pups, the captive-born foster babies and her own natal offspring. And, eventually, lead them all into the wild.
Cecilia Nowell is an Albuquerque-based journalist reporting at the intersection of environment and gender. We welcome reader letters. Email High Country News at editor@hcn.org or submit a letter to the editor. See our letters to the editor policy.
This article appeared in the print edition of the magazine with the headline Smell matters.
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