Mosquitoes nipped at the skin as dusk fell over the Mashpee River Woodlands on September 25, the last traces of daylight fading into overcast skies and cloaking Trout Pond in shadows.

From where the pond trickled under River Road, an old dirt path that skirts the edge of Trustees of Reservation conservation land, the only audible sounds were the chitter of evening crickets and the rush of Route 28 behind the trees.

“Normally you can’t hear bats,” Mashpee resident Greg Auger said.

In his hand, Mr. Auger held a bat detector, an ultrasonic microphone that crackled like a radio when he turned it on. Static buzzed from the device as he aimed it out over the pond, fiddling with the frequency knob.

Then, out of the static came a distinct clicking sound—“ch, ch, ch”—over the speaker. A bat!

Somewhere fluttering among the darkened treetops, a bat chirped at a frequency inaudible to human ears, the bat detector translating those high-frequency chirps to a lower pitch that can be heard by humans.

Echolocation

Mr. Auger said he first started coming to this spot in the Mashpee woodlands to observe bats after his brother introduced him to Donald R. Griffin, the scientist who discovered the sensory world of bats and coined the term “echolocation” in 1944.

Since his undergraduate years at Harvard University, Dr. Griffin studied a large colony of bats that used to roost in the Old Indian Meeting House in Mashpee before the historic building was restored.

In his 1958 book “Listening In The Dark,” Dr. Griffin described banding more than 1,000 little brown bats at the meeting house between 1934 and 1937. His early studies tracked the banded bats from their summer colony in Mashpee to caves and mine tunnels in western Massachusetts, Vermont and Connecticut, where the animals migrate in the winter.

Decades later, in the early 2000s, Dr. Griffin could still be found studying the little brown bats in Mashpee. With Mr. Auger, an IBM retiree and an aficionado of high-tech video equipment, Dr. Griffin spent many nights in the Mashpee woodlands using military-grade infrared cameras to observe the bats’ feeding habits.

“Don and I developed a way of videoing the bats and insects,” Mr. Auger said. “He was seriously into citizen science.”

From River Road in the Mashpee River Woodlands, Mr. Auger and Dr. Griffin would aim their recording equipment out over Trout Pond or into the densely wooded glade opposite the pond, where dozens of bats would swoop and swarm in close proximity.

“What’s really interesting is how many bats you would see—20 to 30 at once—finding prey without killing each other,” Mr. Auger said. “One of the things we were trying to figure out is how they get 20 to 30 in there without hitting each other.”

A science documentary called “Super Bat,” by the French director David Korn-Brzoza, captured a night of field studies conducted by Dr. Griffin and Mr. Auger in Mashpee.

“July 27, 2003, very warm night; we’re set up with our rig aimed out over the glade and it’s 82 [degrees Fahrenheit] at 7:32,” Dr. Griffin said in the documentary. “We hope the bats show up.”

Encompassed by darkness, Dr. Griffin and Mr. Auger dangled a moth tied to a fishing line just in front of an infrared camera. Then—“ch, ch, ch”—their bat detector picked up the chirps of a bat as the animal swooped at the moth.

“I am still very much intrigued by how they use their sonar for very difficult operations; how they discriminate echoes from something they’re interested in, like whether it is a good tasty insect or not, from all kinds of other echoes that they have to be getting,” Dr. Griffin said in the documentary. “We don’t really know how their brains can make that kind of discrimination; it’s an intriguing mystery.”

Dr. Griffin died in November 2003, just months after the documentary shoot. His National Academies of Sciences biographical memoir notes that “in the last weeks of his life Griffin was out ‘night after night’ on Cape Cod ‘still trying to learn more about bats.’”

Little Brown Bats

Dr. James A. Simmons, a professor of biology at Brown University and a longtime colleague of Dr. Griffin, is a leading bat researcher studying echolocation for the US Navy.

Last week, in an interview with The Mashpee Enterprise and Mashpee Community Television, Dr. Simmons recalled studying bats with Dr. Griffin and Mr. Auger in the field in Mashpee and talked about ongoing research into echolocation.

“Echolocation was discovered during the very end of World War II and its relationship to radar was very quickly understood,” he said. “These bats here in Mashpee played an important role because understanding how bats use their sonar—for example, when a bat flies along and detects an insect with its sonar, how it goes through an interception maneuver to catch the insect—teaches you a lot about how their echolocation system works.”

Ever since Dr. Griffin discovered echolocation in bats, the military has been interested in the phenomenon because of its similarities to radar and sonar, Dr. Simmons said.

“The difference, so to speak, is military radar and sonar compared to bats is amateur and the bats are professionals; their whole lives and their view of the world is coming through this system,” he said.

Many of the studies in Mashpee focused on little brown bats, or Myotis lucifigus, the species of bat that lived in the Old Indian Meeting House. Little brown bats hunt small insects such as mosquitoes, Dr. Simmons said.

“We spent a lot of time making recordings of bats doing interception maneuvers and flying around,” Dr. Simmons said. “They chase each other; bats have dogfights in the dark. Now there’s a problem: How do you have two animals, both using sonar, conduct dogfights with each other?”

Field studies by Dr. Griffin helped discover how bats home in on their prey using echolocation, the chirps from the bats growing faster as they close in on an insect, he said.

“Bat sounds are very short, and when they come out of the bat detector they sound like ‘ch, ch, ch,’ little clicks; to the bats they’re chirps, they go ‘chirp, chirp, chirp,’ but we hear ‘ch ch ch,’” Dr. Simmons said.

“So what you’d hear is ‘ch, ch, ch, ch,’ and then when they detect an insect they go ‘chchch prrrrr,” as they home in and extend their tail to catch their prey, he said. Once the insect is in the bat’s mouth, the bat can resume echolocation, even while chewing.

Little brown bats are important for controlling insect populations, but a fungal disease known as white-nose syndrome has led to a steep decline in the species, Dr. Simmons said.

Once the most abundant bat species in Massachusetts, little brown bat populations have declined by more than 99 percent in recent decades, according to the state’s Natural Heritage & Endangered Species Program.

“They’re very vulnerable because although they’re here in Mashpee, they go north to caves in New England to hibernate and that is where they get exposed to the disease and die,” Dr. Simmons said of little brown bats. “Bats that are infected with this fungus, they wake up periodically during the winter and use up their fat storage, so that by the time they get to spring they’ve used up all their fat and they’re dead.”

Mashpee’s Bats Today

As night settled over the Mashpee River Woodlands two weeks ago, Mr. Auger traded the hand-held metal bat detector for an iPad.

“The technology has changed unbelievably,” he said, as he plugged a small red square—another bat detector—into the iPad. “Way back when Don first started, the equipment had to be taken out on a Jeep on a trailer.”

On the iPad screen, he opened an app that displayed a spectogram, a graph that plots high-pitch audio frequencies as they vary over time. As a bat passed somewhere in the darkness, the high-pitched chirps it used for echolocation appeared as ticks on the graph, sweeping downward from about 70 kilohertz to about 40 kilohertz.

The app recognized the pitch of the chirps as belonging to a little brown bat. Then, a few minutes later, the app detected the chirps of a big brown bat, which emits a slightly lower pitch.

The next night, Mr. Auger went to the meeting house, which he said is no longer a roost for the colony of bats once studied by Dr. Griffin. The historic building was sealed up when renovations were completed in 2009.

He did detect both little brown bats and big brown bats in the area, though. Some of them likely roost in the old bat boxes on telephone poles near the meeting house, he said.

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