Australian biologists say animals that have been feared extinct are often rediscovered, and conservation efforts too often focus on creatures that will never be found again.
A study by the University of Queensland's Dr Diana Fisher and Dr Simon Blomberg appears today in the journal Proceedings of the Royal Society B.
They examined 187 species of mammal that have been thought to have become extinct since 1500, and found 67 have since been rediscovered.
Dr Fisher says the study found mammals whose habitats have been partly destroyed are the most likely to be found again.
Mammals thought to have been killed off by introduced predators, disease, or human hunting are least likely to be rediscovered.
Fisher says knowing the factors that cause extinction are important in assessing the likelihood of success in saving threatened species.
"It's fruitless putting a lot of effort into big charismatic animals that are very unlikely to be found," she says.
"You could get a better conservation outcome by focusing more effort on the sort of things that will be found."
She says it makes more sense to focus on species whose habitats have been cleared.
"It's not too late to find a lot of these species somewhere else where they might be persisting," she says.
"They might not hang on forever if they don't get found and protected."
Fisher says the thylacine or Tasmanian tiger is one example of a species which there is little hope of finding again.
"For mammals, [the thylacine] is the only one that most people have ever heard of, and it's something that attracts a lot of attention," she says.
"Since the year that the last one died ... there's been at least 25 really major searches and a whole lot of amateurs looking for it since.
"They haven't found it, and it's pretty obvious that it's extinct ... but if you don't find something you can keep looking forever."
Fisher says Australia has one of the highest rates of mammal extinction in the world, but many Australians are unaware of which animals are close to extinction.
The success stories are often mammals which are less familiar to the public, like the bridled nailtail wallaby.
Fisher says this small wallaby used to be widespread from Victoria up to Charters Towers in Queensland, but by 1930 was thought to be extinct.
"In 1973 in central Queensland at a cattle property, a local guy was fencing the place in preparation for clearing it," she says.
"He saw this animal; it's got a distinctive marking, so luckily he recognised it.
"At that stage they had just been showing Elizabeth Gould's prints of extinct animals in the Women's Day magazine that his wife had at home. He told the Parks Service and they bought the place."
She says the bridled nailtail wallaby has since been reintroduced to new sites and is thriving.
"It's pretty unlikely it would have survived had that place been cleared," she says.