Hybrid kittens born to panthers brought into the area from Texas have "about a three times higher chance of becoming adults as do purebred ones," reports a paper planned for publication in January 2006 in the British journal Animal Conservation. "Hybrids are expanding the known range of habitats panthers occupy and use," the paper adds.
In view of the paper's importance for management decisions, its authors and the journal's editors agreed to make the full text available now, said Stuart Pimm of Duke University, the principal author.
"There are hugely difficult controversies that I'm certain will erupt as soon as the paper is published," Pimm, the Doris Duke Professor of Conservation Ecology at Duke's Nicholas School of the Environment and Earth Sciences, said in an interview.
"A lot of scientists said this kind of genetic rescue would not work. They said if a species is rare, and its range is restricted, just adding individuals from the outside is not going to work. Some thought it would be a waste of time, a waste of money."
Pimm was himself skeptical about the success of such a rescue attempt in his 1991 book, "The Balance of Nature? Ecological issues in the conservation of species and communities." He acknowledged in his interview and in the new paper that "I was wrong."
The paper's other authors are Luke Dollar, a doctoral student in Pimm's research group, and Oron "Sonny" Bass, Jr., a biologist who has studied panthers at Florida's Everglades National Park for nearly 20 years.
The research was funded by a grant from the Doris Duke Charitable Foundation.
The authors extensively reviewed the locations and movements of both purebred and hybrid panthers that researchers had tracked by radio.
In 1995, separate pairs of Texas female panthers fitted with radio collars were released in each of the four sections of South Florida where Florida panthers were known to be living, the authors wrote. Five of these eight cats are known to have produced hybrid kittens. By 2003, all three of the surviving Texas adults had been removed from the wilds, since wildlife managers "wanted to keep outside genetic exposure to a minimum," Pimm said.
Beginning in 1992, researchers also briefly captured kittens and inserted microchips into them to track their whereabouts and fates. Of the 118 purebred and 54 hybrid kittens that received microchips, 13 purebred and 20 hybrids survived long enough to also receive adult radio collars, according to the paper.
The researchers followed how well the kittens born to purebreds survived and compared them to the hybrids -- those that had Texas mothers or grandmothers. "More than three times as many hybrid kittens appear to reach adulthood as do purebred ones," the authors wrote.
According to Pimm, native Florida panthers were "becoming famous" for an array of genetic abnormalities, including low sperm counts and heart defects. "It turns out that purebred kittens don't survive very well," he said. "They die in very high numbers. Once you get new blood, those defects disappear from the population. Hybrids are better than inbred animals."
The paper also notes that "hybrid cats are beginning to expand their ranges to areas previously thought to be unsuitable," meaning environments "once supposed to be unable to support them."
Purebred Florida panthers have been confined largely to protected areas north of interstate 75 and west of State Highway 29. "I think they were surviving in this area because they had the least trouble there," Pimm said. In contrast, hybrid offspring have been moving successfully south and east into new sections of the Everglades National Park and Big Cypress National Preserve, he said.
Pimm, whose research group works in the Everglades each year, said some scientists have erroneously tended to "look at a map and say, 'because this species isn't here [on the map], then it can't be here.'"
Pimm noted the case of a panther biologist who he said was dismissed from his government post for arguing that developers should not be given permits to clear areas where the endangered species has not been seen before.
"Our results support his contention that the panther is a generalist. You certainly don't want to give up areas to developers by assuming that panthers cannot occupy them," said Pimm, who noted that an expanded range for Florida panthers could raise several such social issues.
While female hybrids seem to be surviving considerably better than their purebred counterparts, "hybrid males have shorter lives than purebreds," the paper also reports. The authors suspect many of the males are killed by older purebred males in the area. Others migrate into areas with "fewer cats but other dangers from human settlement," Pimm said. "Young males get into trouble whatever species they are."
Pimm acknowledged the hybrid panthers are genetically different from purebreds. But he and his co-authors noted that the federal government has decided that offspring hybrids will still be considered "Florida panthers" eligible for endangered species protection.