8 million 'lost' bats maybe weren't: imaging and algorithms challenge famous Carlsbad estimate.

Eight million is a lot of bats to lose, and now a new unpublished study may explain what happened to the possibly lost bats of Carlsbad Cavern. Short answer: The famous 8 million bats never existed in the first place, a Boston University team says.

From spring to fall, the cavern in New Mexico still hosts as many as hundreds of thousands of migratory Brazilian freetailed bats that thrill visitors by boiling out of the cave at dusk for a night's foraging. All the roosting bats emerge in a dense plume that streams on and on, sometimes for an hour or three.

As with many wildlife spectacles, always present is the disturbing possibility that today's show is a mere wisp compared with the great Carlsbad bat clouds of yore. In 1937 V.C. Allison estimated the bat numbers by timing an emergence (14 minutes at great density; four minutes at half that) and eyeballing the stream's speed and size. About 8.7 million bats roosted in the cavern, he reported.

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Since then, estimates have varied but haven't topped a million. Consequently, some conservationists have raised alarms about Carlsbad's bats.

To count bats, Thomas Kunz of Boston University and colleagues set up thermal imaging cameras at the cavern in 2005. Margrit Betke, also of B.U., developed algorithms for analyzing the recordings.

In a series of counts in 2005, numbers varied from a low of not quite 70,000 to a peak about 10 times higher weeks later. Even at the peak, counts came up some 8 million bats short of the old estimate. The Boston team's modeling found that at most 50,000 bats could exit a choke point in the cavern per minute, limiting the number of bats in 1937 to 1 million. Carlsbad Caverns National Park has discounted Allison's numbers as excessive, says park biologist Renee West.

"That doesn't mean these bats aren't declining," says researcher Nickolay Hristov, now at Brown University. "The declines just haven't been as bad."