By Aislinn Maestas
In 2024, the U.S. Fish and Wildlife Service (USFWS) and its partners fostered a record 27 Mexican wolf pups into wild dens. Now entering its 10th year, the effort is helping to improve the genetic diversity of the wild population of Mexican wolves and, in turn, bringing the subspecies closer to recovery.
Fostering is a method used by the Mexican Wolf Recovery Program to place more genetically diverse wolves in the wild to address the threat facing the subspecies. It begins with selective breeding by the Saving Animals From Extinction (SAFE) Mexican Wolf program to produce genetically diverse pups.
The SAFE Mexican Wolf program is a cooperative effort between the U.S. Fish and Wildlife Service and the Association of Zoos and Aquariums (AZA), as well as non-AZA facilities. SAFE zoos and wolf facilities house more than 300 Mexican wolves under human care at 60 SAFE facilities across the United States and Mexico. Wolves are bred each year at a handful of SAFE facilities with the explicit intent of producing pups for foster efforts.
Within 14 days of whelping (being born), SAFE-born foster pups are transported from their facility to a site in New Mexico or Arizona. This often happens with the help of LightHawk, a non-profit group with a network of 300 volunteer pilots who donate their expertise, time, aircraft, and fuel to support conservation work. SAFE facilities are located across the United States, and foster pups have been flown from as far away as New York, Illinois and Missouri.
Once the foster pups arrive in the Mexican wolf recovery area, they are hiked to a wild Mexican wolf den. Both wild-born and foster pups, who are similar in age and size, are placed together in the den. The entire process takes place over a single day and requires dozens of people to develop plans, find dens, provide transportation and care for pups.
Once humans have left the area, the breeding female usually moves all the pups to a new den a few hundred yards away and spends the next several months caring for her new foster charges alongside her own. With the help of her packmates, the pups will be raised with the skills and knowledge needed for a life in the wild.
“The process of fostering is complex, requiring intense coordination between state and federal agencies, LightHawk and our SAFE partners,” said Allison Greenleaf, senior biologist with the U.S. Fish and Wildlife Service. “But the science of fostering is quite simple. Wolves are family-oriented animals, and the female wolves have extremely strong mothering instincts. Those instincts are what allow us to add pups to wolf dens and know the breeding female will take care of them.”
Why genetics matter
All wild and captive Mexican wolves in the U.S. and Mexico are descended from only seven founding individuals. As a result, genetic issues, including inbreeding, remain a threat to the subspecies. Improving the genetic diversity of the population remains a top priority for the recovery program.
The SAFE population of Mexican wolves has more genetic diversity than is currently represented in the wild. While the wild population has shown a healthy growth trajectory for more than a decade, continued releases from captivity are necessary to address genetic threats.
The Mexican Wolf Recovery Plan establishes genetic criteria to ensure that gene diversity available from the captive population has been incorporated into the wild population. This is accomplished through the release of a sufficient number of wolves to ensure that 22 released wolves survive to breeding age (i.e., a pup that lives to age 2 or an adult that lives for a year following its release).
To date, a minimum of 20 fostered pups have been documented surviving to breeding age. This minimum number represents fosters that have been captured or documented surviving through DNA analysis.
Other indicators of success include the fact that at least 13 fostered wolves have successfully bred and produced litters in the wild. Fostered Mexican wolves have produced at least 30 litters, and several of those offspring have gone on to produce pups of their own.
“It has taken time, but we are seeing fostering yield results as genetic indicators in the wild stabilize and improve,” said Maggie Dwire, deputy Mexican Wolf Recovery coordinator for the U.S. Fish and Wildlife Service. “Studies have shown that the wild population continues to produce large litters, has demonstrated rapid population growth and has weak evidence of inbreeding depression. This indicates our efforts, including fostering, are working to mitigate the threat of inbreeding in the wild population.”
Climbing toward recovery
In 2024 the wild population of Mexican wolves saw its eighth year of consecutive growth, with a minimum of 286 wild wolves in the United States. One major driver of success for the recovery program is having the flexibility to choose the right management tool for each goal, adapting as necessary.
For example, “When we were focused on reintroduction, we needed to release entire packs,” said Dwire. “However, once we established a population of Mexican wolves that was reproducing and growing on its own, we embraced the opportunity to switch tactics to achieve recovery.”
Fostering has been found to be a less controversial technique for accomplishing genetic goals compared to releasing adults from captivity. It does not require putting wolves into new areas but rather adds numbers to existing packs within an established home range. Also, the pups are raised as wild wolves, with all the skills and knowledge needed to survive, including a natural fear of humans. Finally, fostering allows for a wider geographical distribution of releases from the SAFE program, with fosters having been placed into 48 wild dens across Arizona and New Mexico.
The USFWS and its partners are already preparing for the 2025 foster season.
“Our foster program is a model for how to address conservation challenges through innovation,” said Greenleaf. “I was here when we did the first foster 10 years ago, and last year was lucky enough to put the 100th foster pup into a den. I marvel at how much we have learned and the knowledge we are now passing to others. This work is hard, but the rewards make it worth it. Plus, the pups are really cute.”
This article was originally published in the Summer 2025 edition of International Wolf magazine, which is published quarterly by the International Wolf Center. The magazine is mailed exclusively to members of the Center.
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Aislinn Maestas is a public affairs specialist with the U.S. Fish and Wildlife Service.
