From foxes to cheetahs, Saint Louis Zoo scientist helps mammals reproduce and thrive
A Saint Louis Zoo scientist is partly to thank for the recovery of three subspecies of foxes native to the Channel Islands off the coast of California.
Earlier this week, the U.S. Fish and Wildlife Service announced it plans to de-list the foxes of San Miguel, Santa Rosa and Santa Cruz islands from its endangered species list. It also plans to "downlist" the Santa Catalina island foxes to "threatened."
Dr. Cheryl Asa, director of research for the Saint Louis Zoo, was part of the "big and complicated" effort that brought the foxes back from the brink. She said not only did the entire island ecology have to be restored, but also the foxes proved to be finicky when breeding to boost their numbers.
"The zoo is interested in contributing wherever we can in endangered species recovery, and this worked for me because I had a long history of specializing in canid reproductive physiology," the wildlife reproductive expert said. "I've worked with foxes, wolves, bush dogs and fennec foxes."
Food chain reaction
The foxes' plight was part of a "fascinating ecological story," Asa said, starting with organic chemical spills off the coast of Los Angeles. The accumulating toxins caused the eggs of marine life to thin, drastically reducing the food source for the bald eagles that lived on the same islands as the foxes.
The bald eagles took off for more plentiful shores, and without the more dominant species around, golden eagles moved in on the real estate. They fed well off of feral pigs leftover from the islands' ranching days.
"Newborn piglets are a perfect size package of meat for golden eagle hatchlings," Asa said. "But as the piglets got bigger, they got too big for the eagles to use in that way, so the eagles switched to foxes, which are also very small. They are about cat-sized, five pounds or so."
Asa said because the foxes hadn't historically had a predator like the golden eagles, they didn't know how to protect themselves. As a result, their numbers began "crashing."
Bringing back balance
A team lead by biologist Tim Coonan from the National Park Service, which was responsible for the islands, began work to save the foxes back in the 1990s.
First, it had to live-capture the golden eagles and move them off the islands, as was done with the feral pigs. The team then reintroduced the bald eagles, whose nests thrived thanks to cleaned-up water. The foxes were put into captive pens to begin a breeding program.
"So the island came back into a kind of ecological balance, which was then worth putting foxes back in," Asa said. "If you had tried to just reintroduce them, it wouldn't have been as successful."
But some of the fox pairs just would not, well, get it on. The team needed a veritable wildlife love doctor, and that's when Asa came in.
"Reproduction isn't one event, it's a sequence of stages and everything has to work properly for you to get babies," she said.
Not-so frisky foxes
Asa and her team monitored hormones, ovulation, stress levels, compatibility, and other measures to help figure out why the foxes weren't successfully reproducing.
One surprising reason why the foxes weren't very frisky was due to stress. Asa said it's normal for wild animals placed in captivity to react to such a big change, but usually their offspring would be "more calm and adapted" to life in monitored colonies.
"We were seeing an opposite trend in the foxes," she said. "The youngsters born in captivity seemed less acclimated and seemed more nervous about it. We were able to modify some husbandry and routines and find ways that they settled in."
With these changes and more insights from Asa and her team, the project was better able to get the foxes to breed.
By 2008, the island foxes were being released back into the wild, and their numbers have dramatically rebounded. The U.S. Fish and Wildlife Service calls it the fastest recovery of an endangered mammal in the U.S.
Asa's work with the island foxes was also an "eye opener" for her thinking on zoos' management of small populations of species.
Zoos must take care to avoid inbreeding and other risks due to the small number of animals they have in captivity and the corresponding small gene pool available. Asa said that's where population biologists and geneticists use computer programs to manage the genes in the population.
"After doing these analyses, they will make recommendations for individual pairs - like this animal would be best paired with that animal because the genetic outcome would be best for the population," she said.
But in the case of the island foxes, they simply did not like their hand-selected partners.
"Those individuals seem not to want to breed together, so some species are pickier about who they mate with and we think that was going on with the foxes," she said.
The choosy foxes inspired Asa to think about using "mate choice" - or giving a female animal an option of a few genetically suitable, eligible bachelors.
"If we offer (a female) two or three choices that would be good or at least okay for the population, if she finds one among them that she likes, well, then everybody’s better off, because she’s actually producing offspring," Asa said, "whereas if she was had been given only one choice who was best for the population and refused to mate at all, that wouldn’t add to the success of the population management."
The process proved too complicated to use with the island foxes, but Asa and the Saint Louis Zoo began holding workshops and symposiums so other institutions would consider mate choice "as an important component of breeding recommendations for zoos."
Checking choice in cheetahs
Currently, the Saint Louis Zoo is investigating mate choice by using urine as a proxy for male cheetahs. Asa said the zoo is looking to see whether female cheetahs showing a preference for specific urine samples means they're more willing to mate with that male.
Most mammals (save primates) pay a lot of attention to urine and chemicals in the odor, she said. A recently completed study showed females do pay attention to male urine samples and showed a difference in how long they spent investigating urine from a closely related male, a moderately related, and an unrelated male.
"That was something the population geneticists were interested in – if the females were given free choice, would they make a good genetic choice?" Asa said. "So that showed they can at least tell the difference based on genetic distance from themselves, and seem to show a preference for males unrelated."
Asa said the zoo's work on mate choice is just beginning, but the method could be a boon for species' gene pools in zoos.