If an animal hasn't been seen in the wild for more than fifty years, it's technically extinct. But academic criteria haven't stopped some disappeared species from reappearing, confounding official estimates of which species are actually "gone forever" from their natural environments. A new study published by University of Queensland scientists, reported in detail by Wired's Brian Switek, explores this problem, and finds that humans don't have a great record of assessing which species are completely extinct. Analyzing a data set of 187 mammal species that have been reported or presumed extinct in the wild since the year 1500, the researchers found that nearly a third (67) of the species were later seen again.

The most likely type of mammal rediscovered were those that "were relatively sparsely distributed over a larger range ... But mammals of any particular evolutionary group or body size weren’t more likely to be rediscovered." Switek observes that the study's findings will be more than helpful for conservation biologists:

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.

Pictured: One example of an uncovered animal cited from a CNN report on the study was the Okapi, a forest giraffe that was thought to only be found in zoos and conservancies.

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Erik Hayden