Endangered Mexican wolves roam the wilds of New Mexico, Arizona and Mexico. They also live in captivity. But their future may lie in a “frozen zoo.”
That’s the term of endearment scientists use for the bank of frozen wolf sperm and ovaries housed at the St. Louis Zoo in Missouri and Chapultepec Zoo in Mexico City – two cryogenic vaults where some of the most precious genes of the species are being held for future reproductive use.
Even as New Mexico continues to fight with the federal government over the U.S. Fish and Wildlife Service’s troubled program to reintroduce Mexican wolves to the wild, the scientists charged with breeding the species back into greater numbers are pushing on with the complex work of preserving the genetic diversity of a captive population that began with just seven wolves.
“Right now, our mandate is to preserve genes,” said Cheryl Asa, a reproductive physiologist who led the research program at the St. Louis Zoo for 30 years and now serves as a consultant. “This is looking into the future so that, as animals die who are genetically important to the species, their genes live on.”
The Mexican gray wolf, an apex predator native to the Southwest and Mexico, was listed as endangered in 1976 and the service has been working to recover the species ever since – often in the face of opposition from ranchers who live in and around the remote Gila region, where the wolves now roam, and are concerned about them preying on cattle. They also prey on mule deer and elk.
“These ecosystems evolved with predator and prey,” said Garrett VeneKlasen, executive director of the New Mexico Wildlife Federation, which represents sportsmen. “It’s a system of checks and balances. If we want to get our wildlife as healthy as it can be, we need to have all these species in adequate numbers on landscapes that can handle it.”
Since 2007, the quest to preserve Mexican gray wolf genes has included “vitrifying” the ovarian tissue of female wolves that are past reproductive age in hopes of one day being able to impregnate younger females through in vitro fertilization – although the technology does not yet exist to perform this technique in dogs or wolves. But researchers are getting close, Asa said.
“The ‘frozen zoo’ is what some of us call it,” said Maggie Dwire, assistant Mexican wolf recovery coordinator with the service. “We aren’t going to find new founders. We need to be very careful about retaining the genetic diversity we do have.”
Only female wolves past their natural breeding age of 12 years may be selected for spaying, Asa said. A scientist at the St. Louis Zoo with the title of curator of invertebrates, Edward Spevak, manages a computer program that determines which of the elder females is most genetically precious to the species’ survival.
Just before breeding season, during the last two weeks of January, when the wolves’ eggs are close to ovulation, they are spayed like a domestic dog would be spayed – an operation that might happen at one of the 51 institutions that hold captive wolves.
The wolf ovaries are wrapped in gauze, kept warm in a saline solution, packed in a container and immediately shipped in the cargo hold of the next passenger plane headed to St. Louis, Asa said.
In the St. Louis Zoo lab, scientists use a needle to draw out egg cells from each follicle; the remaining tissue is vitrified and banked, “kept in liquid nitrogen forever, or until they might be used,” she said.
With 251 wolves in captivity and space capped at 300 wolves, holding pens in the U.S. and Mexico are near capacity. That curbs their ability to breed as many wolves as they might otherwise, Dwire said, making the “frozen zoo” all the more important.
But wolf advocate Michael Robinson says that too few genetically valuable wolves are being released from captivity into the wild. Ten wolves, including six pups fostered into existing wolf dens earlier this year, have been released from captivity since 2009, according to Fish & Wildlife Service statistics.
The service placed two of the pups in dens in New Mexico – flouting a 2015 ban on wolf releases by the state Game and Fish Department.
“If these wolves had been released a decade ago, instead of stuck in pens due to politics, their great-grandpups would roam the Southwest today, embodying the genetic diversity that instead is being stored in freezers,” Robinson said in an email.
The most recent master plan for the captive population pins genetic diversity at 83 percent – as good as it’s probably going to get in the Mexican wolf population, according to its authors, survival plan coordinator Peter Siminiski and Spevak.
“When gene diversity falls below 90 percent of that in the founding population,” the report says, “reproduction may be increasingly compromised by … lower birth weights, smaller litter sizes and greater neonatal mortality” – all challenges for the Mexican wolf population, both in captivity and in the wild, where genetic diversity is even less.
In the wild, Asa said, “the animals are so dispersed that there is not enough mixing and matching. Wolves don’t know that we need them to balance the gene diversity.”
So wolf genes that would otherwise be lost are frozen, waiting for science to catch up.
“Assisted reproductive technologies are improving all the time,” Siminski said in an interview. “Our thinking back in the 2000s was, ‘we’re going to start banking as soon as we can.’ The time will come that these technologies will be available.”