The population of little brown bats has leveled off before it hit the bottom of the cliff known as extinction, according to a state wildlife biologist, who spends part of her winters crawling through caves where the creatures sleep through the winter.

The population of little brown bats has leveled off before it hit the bottom of the cliff known as extinction, according to a state wildlife biologist, who spends part of her winters crawling through caves where the creatures sleep through the winter.

Over the past handful of years, their winter nap was not restful, for it was in that hibernaculum (Latin for a place where creatures hibernate) where the tiny, flying mammals faced extinction.

And it was a fungus that silently attached itself to the several species of bats, sapping their strength, wounding their wings and causing the mammals to wake from their winter torpor, only to die of starvation, thirst or exhaustion.

The fungus grew around the bats' noses and mouths and on their wings and came to be known as white-nose syndrome.

Discovered in February 2006 in a cave just west of Albany, N.Y., the fungus and the spoors from which it grows have spread to cover the eastern U.S., except for the southeast where bats don't hibernate and caves are few.

The fungus, and its resulting dead and dying bats, were discovered in New Jersey's Hibernia Mine in the late winter/early spring of 2008, one of two major bat hibernacula in the state.

Over the next five years, 90 percent or more of the little brown bats succumbed to white-nose. Also taking a big hit were northern long-eared bats, the eastern small-footed bat and the tri-colored bat. The tri-colored bat was formerly known as the eastern pipistrelle.

Today, less than 10 percent of those bat species' population remains, said MacKenzie Hall, wildlife biologist, who heads the Division of Fish and Wildlife bat program.

"But the good news is, the survival rate is back to normal," she said. "It's still devastatingly low; 90 percent of what it once was; but seems to be coming back."

What has been a surprise, and added considerably to human understanding of white-nose, is that big brown bats apparently weren't affected much, if at all, by white-nose syndrome.

Hall said there appear to be a couple of theories of why, tied in with where and how the big browns sleep, and surprisingly, what they eat.

Big browns are the bats most likely to spend warm weather days in attics of human homes, a climate where white-nose fungus doesn't exist.

Eating and sleeping

In winter, the big browns do hibernate in caves, but tend to sleep near the entrance where more drastic temperature swings occur.

White-nose, which also goes by the scientific name of Pseudogymnoascus destructans, needs a temperature rage of low 40 to mid-50 degrees to thrive. That's the temperature deep inside the cave where the smaller bats sleep.

Big brown bats tend to hang out closer to the cave entrance where the temperature swing is greater.

"They will even come out and fly around on warm winter days," Hall noted, and with a larger body mass, they have the ability to survive below-freezing temperatures for a spell.

Researchers have put on a full court press since the destruction of the bat population became apparent, and Hall said a team at Fordham University has also found the big brown's wings have fatty acids built up in them from the type of insects they eat. Those fatty acids tend to repel the fungus so there is no wing damage.

With fewer "competitors" flying through the night skies, the population of big brown bats has grown even as the population of their smaller cousin species plummeted.

Genetic savior

While the big browns' living and eating habits saved them, it is genetic traits in the other species that appear to be key to their recovery, since the surviving parents seem to be passing along their natural immunity to their offspring.

But recovery will not be quick since female bats only have one pup per year.

"It could be 30, 40 years before the population is back to where it was before white-nose," said Hall.

In the meantime, four bat species have been recommended for inclusion on the state's list of endangered species.

Those are the little brown, eastern small-footed, tri-colored and northern long-eared species. The long-eared species was already on the federal threatened list prior to white-nose.

The process for formal listing on New Jersey's list has just begun, Hall said and the ensuing rule-making process could take a year or more.

"The crisis now is there are so few bats," Hall said. "We were close to ground zero but that has eased. Now is the lag time (before full population recovery)."

She said there are some actions the public can take, leading off with public education and "some tolerance of bats."

While the major caves, such as Hibernia Mine in Morris County, and several old mines in the Delaware Water Gap National Recreation Area, have been closed to human access, humans still should take precautions to thoroughly clean their clothing and equipment when they visit other caves.

Where next?

This spring, white-nose was confirmed in a bat in Washington state, more than a thousand miles from the nearest known occurrence. That distance was likely covered by a human carrying spoors of the fungus from one cave to another.

Initially, it was believed humans gave the fungus the transportation boost it needed, but Hall said it appears that most of the spread is by bats themselves carrying the spoors.

There is also concern, based on this past winter's discovery of the fungus in northern Texas, that white-nose could spread quickly to the south into Mexico and skirt around the southern Rockies into California. Hall said the public in New Jersey can help out by not disturbing bat colonies around the house.

"Behind a shutter, they're not going to be a problem," she said. "If they get into your house, contact a professional. They know the best methods to exclude them."

She noted homeowners can also install commercial or home-made bat houses if they have suitable habitat. The bat houses are designed to hold up to 100 bats, the size of maternity colonies.

"Agriculture needs them and even a residential neighborhood needs them," she said. "They are the bug-eaters of the night."

Bruce A. Scruton can also be contacted on Twitter: @brucescrutonNJH or by phone: 973-383-1224