There may be many more "extinct" mammals waiting to be rediscovered than conservation biologists previously thought.

Categorizing a mammal species as extinct has rested upon two criteria: It has not been seen for more than 50 years, or an exhaustive search has come up empty. But "extinct” species occasionally turn up again, and some species have disappeared more than once. Australia’s desert rat kangaroo, for example, was rediscovered in 1931 after having gone missing for almost a century, only to disappear again in 1935 when invasive red foxes moved into the area of the remaining survivors.

In order to determine how often extinct species had been rediscovered, University of Queensland scientists Diana Fisher and Simon Blomberg created a dataset of 187 mammal species that have been reported extinct, extinct in the wild, or probably extinct since 1500, as well as those which have been rediscovered. They also looked at historical data on the threats that caused species to become extinct – or brought them close to it – including habitat loss, introduced species and overkill by humans.

It turns out that rumors of the extinction of more than a third of these species have turned out to be premature, the scientists report in Proceedings of the Royal Society B Sept. 29. At least 67 species – a little more than a third of those presumed to be extinct – were later found again. And in most cases, these were animals that had been hardest hit by habitat loss.

Humans and invasive species have been significantly more efficient killers. It's rare that a species reported extinct due to one of these causes has been seen again.

“If you think that a missing species is extinct and the main cause of decline was introduced predators such as feral foxes, cats or rats, then you are very likely to be right,” Fisher said. But, she added, “If the main cause of decline was habitat loss, you are quite likely to be wrong if you say that it's extinct, unless it was restricted to a very small area.”

As an example, Fisher cites the Malabar civet, which was thought to be extinct due to habitat loss in 1929 but survived in marginal areas at least until 1987 when it was last seen on a cashew plantation. Unfortunately, that animal was killed by villagers, and no more have been seen since.

The team found species that were relatively sparsely distributed over a larger range were more likely to turn up again. But mammals of any particular evolutionary group or body size weren't more likely to be rediscovered.

“ I was a little bit surprised that body size was not important,” Fisher said. “I thought that small species might not be found so often, because they don't attract much attention, but that wasn't the case.”

With these findings in hand, conservation biologists may be better able to target species that are more likely to still be out there somewhere. While species hunted into extinction – such as the Stellar’s sea cow – are almost certainly gone forever, individuals of other species may still exist. Whether we find them again or not seems to be directly influenced by how hard we look.

According to Fisher and Blomberg, one or two searches for a missing species aren't likely to succeed, but missing species that were the subject of three to six searches have often been rediscovered. Chances do not continue to get better past this point, though. Species that have been the subject of more than 11 searches, such as the Tasmanian tiger and the Yangtze dolphin, have not been found.

We may hope for the rediscovery of such charismatic species, but the chances of finding some of the lesser-known species that haven't been looked for yet are significantly better. Among the good candidates for rediscovery Fisher lists are the Montane monkey-faced bat of the Solomon Islands, last seen on Guadalcanal in 1990, and Alcom’s pocket gopher, which was abundant in a high-elevation forest in Mexico in the late 1990’s but hasn't been seen since.

“We should be trying to protect the habitat of recently extinct species,” Fisher said. “But this is not easy, because we don't know where they might be rediscovered. It is not necessarily near where the species was last seen.”

Gilbert’s potoroo, for example, disappeared sometime around 1879 but was rediscovered in 1994 at Two People’s Bay in Australia in a reserve that had been set up to protect an endangered bird. Because many rediscovered species had populations that were spread over a wide area, ecologists have a lot of ground to cover in their search for “extinct” mammals.

Images: 1) Desert rat kangaroo. John Gould/Wikimedia Commons. 2) Tasmanian tiger. Smithsonian Institution/Wikimedia Commons. 3) Gilbert's potoroo.John Gould/Wikimedia Commons.

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