Cross-Fostering in the Mexican Wolf Recovery Program


Happy Wolf Awareness Week! Every October, Brookfield Zoo and other zoos and wolf centers around the country take time to celebrate wolves and share why they are an important and essential part of ecosystems in the United States and around the globe. This year, Wolf Awareness Week takes place from October 18th-24th.

In honor of this week, we wanted to share some information on the fostering technique used by biologists in the Mexican Wolf Recovery Program, which is helping to save the Mexican gray wolf from extinction. What is cross-fostering? Read on to find out!

A Brief History of Mexican Wolves
Mexican wolves once roamed the American Southwest in the thousands. As settlers moved westward in the late 1800s, they increasingly hunted the deer and elk that wolves needed to survive. With their natural diet diminishing, wolves sometimes turned to the settlers’ livestock as prey. In retaliation, hunters started targeting the wolves.

After decades of persecution and habitat loss, Mexican wolves were threatened with extinction by the early 1970s. In 1973, Congress passed the Endangered Species Act, the Mexican wolf was listed as an endangered species, and the U.S. Fish and Wildlife Service was tasked with heading the recovery efforts to save the species. After years of planning and organized breeding efforts at zoos in both the U.S. and Mexico, the recovery program returned 11 wolves to their historic range in 1998. Since then, the population has grown both through breeding of wild pairs, release efforts, and more recently, through a technique called fostering.

With just over 100 animals in the wild by the mid-2010s, biologists with the Mexican Wolf Interagency Field Team (IFT) began looking for more ways to increase the number of wolves in the wild while improving the genetic diversity of the population.

Red Wolves
Across the country in North Carolina, a comparable recovery program had been developed for red wolves. Red wolves had suffered a similar fate to the Mexican wolf. They faced persecution and predator control programs, and also lost much of their habitat to expanding human populations. Red wolves faced near certain extinction without intervention. Luckily, they were the first species designated as “threatened with extinction” under an early iteration of the Endangered Species Act that was passed in 1967. Recovery efforts for Mexican wolves mirrored those of the Red wolf program - breeding animals in zoos followed by re-establishing a population in the wild through release of zoo-born animals.

In the early 2000s, the red wolf program started placing young puppies from zoo-born litters into dens with wild wolf parents and their pups of the same age. This technique is called cross-fostering since the wild parents foster the pups, raising them as their own. The red wolf program saw huge success with this new method. They estimate that about 90% of the pups placed in wild dens survived. Cross-fostering allowed biologists to introduce new genetics into the wild red wolf population and also raise animals with the “street smarts” to survive in the wild. While it may seem simple at first, this process is a precise and highly coordinated effort.

Cross-fostering in the Mexican Wolf Recovery Program
The Mexican wolf program learned about the success of cross-fostering in red wolves and decided to borrow the technique. In 2014, the program attempted their first cross-fostering—this one from wild wolf to wild wolf. Two puppies of six in a litter born to female wolf named Ernesta were fostered into the Dark Canyon Pack. Ernesta lived here at Brookfield Zoo for two years prior to her released in the wild. The fostering was a success and the biologists learned a lot.

The next year, the team decided to cross-foster zoo-born puppies into a wild den. Timing is critical for cross-fostering, as the puppies from both the giving and receiving packs must be similar in age and development. A pack was identified which matched up to a wild pack’s whelping, or birth dates. Biologists set out to find the den site in the mountainous forests of the southwest. Even with an idea of where the den was located based on information from GPS collars, this task was no easy feat and the den could not be located. The puppies were then returned to their mother. Even though the pups did not make it into the wild, biologists learned a lot from this first captive to wild effort.

In 2016, the Interagency Field Team put that new knowledge to good use and made history in early spring when the first two puppies born at the Endangered Wolf Center in Missouri were cross-fostered into a wild den.


Brookfield Zoo’s Cross-Fostering Efforts
A few days after the important first successful captive to wild cross-fostering took place, a litter of puppies was born here at Brookfield Zoo. Just a few days later, two of the puppies made their way to Arizona and were cross-fostered into the Elk Horn Pack.

The next year, Brookfield Zoo’s pack made another contribution to the wild population when two more puppies were cross-fostered, this time into the New Mexico-based San Mateo Pack. To read more about Brookfield Zoo’s 2017 cross-fostering, check out the blog I wrote about my experience joining the puppies on their long journey: Cross-Foster Recovery Program – Mexican Gray Wolf Pups.

Since the first captive to wild cross-fostering efforts took place in 2016, 50 puppies have been fostered into the wild. One Brookfield-born cross-fostered pup, Blaze (2016 litter), has gone on to be a successful father and “foster-father” through the program. Another pup, Connie (2017 litter), has also successfully bred in the wild. 

This technique is proving very helpful in introducing new genetics into the wild population. Puppies cross-fostered into wild packs have about the same survival rates as wild-born puppies in the first year and longer-term survival rates are higher than releasing captive-born adult wolves.

2020’s Major Successes
2020 was set to be a big year for the recovery program. New Mexico lifted limits on the number of puppies that could be placed in the wild and Arizona increased their limit on placements to 12. The program prepared for the 2020 cross-fostering season by increasing the number of breeding pairs in zoos. 

When the pandemic hit and the United States started closing down, the Mexican Wolf Program was unsure if any cross-fostering efforts would occur this spring. But thanks to cooperation from the Interagency Field Team, Arizona Game and Fish Department, the New Mexico Department of Game and Fish, and zoos and wolf centers around the country just the opposite happened—20 puppies made their way into wild dens. A new record!

The puppies were born at zoos and wolf centers in Missouri, Kansas, New Mexico, California, and Arizona. Getting them to the recovery area, especially during a global pandemic was an undertaking made possible thanks to the generosity of donors with private planes and Lighthawk, a nonprofit organization that partners volunteer pilots with conservation organizations to help transfer endangered species to their new homes.

The Future of Mexican Gray Wolves

Even with the continued and growing successes of cross-fostering, Mexican wolves are not out of the woods. There are currently only 163 Mexican wolves in the wild. And these animals descend primarily from just seven wolves. Biologists and geneticists worry that the limited gene pool could make the population susceptible to inbreeding, a continued challenge facing Mexican wolf conservation. While Mexican wolves and the biologists fighting to save them from extinction have more hurdles to overcome, 2020’s successful cross-fostering efforts have pushed the program forward. The introduction of new animals with more genetic diversity brings new hope to the long-term survival of Mexican wolves in the wild.

Consider adopting our Mexican gray wolves to be part of their care and help support the efforts of the Mexican Wolf Recovery Program here at Brookfield Zoo! 

Written by Senior Animal Care Specialist Racquel Ardisana

Posted: 10/19/2020 12:25:08 PM by Sean Keeley

CZS & Brookfield Zoo

Since the opening of Brookfield Zoo in 1934, the Chicago Zoological Society has had an international reputation for taking a cutting-edge role in animal care and conservation of the natural world. Learn more about the animals, people, and research that make up CZS here at our blog.


Subscribe to our Blogs!