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Across North America, dichlorodiphenyltrichloroethane (DDT) and other pesticides had a significant impact on bats from the 1940s through the '60s. Since the ban on DDT in 1972, bat populations had been slowly recovering, until a fungal disease appeared three-plus decades later.

Bat populations crashed again, when was discovered in a New York cave in 2006. The fungus that causes the disease spread rapidly across much of the United States, and the number of bat species that hibernate in caves and mines plummeted.

In 2012, the U.S. Fish & Wildlife Service estimated that 5.7 million to 6.7 million bats had perished in the eastern United States and Canada since the disease was first discovered. It initially 90% to 95% of many bat populations.

These mammals play a vital ecosystem role. The nine bat species found in southern New England are insectivores, meaning they eat insects such as mosquitoes and some moths humans would label pests. It's been estimated that an individual bat can eat 600 insects an hour. Nearly 70% of bat species feed primarily on insects. Some eat fruit, rodents, frogs, and fish.

Peter August, a retired professor of natural resources at the University of Rhode Island who spent his undergraduate and graduate years studying small mammals, including bats, noted these flying warm-blooded vertebrates provide a number of that benefit humankind.

They pollinate plants—"If you like margaritas, they pollinate the agave plant that makes tequila," he said—dispense seeds, provide biological control services that includes consuming corn earworm moths, a major agricultural pest, and provide medical insight.

"The little brown bat weighs 8 grams and can eat 10 grams of insects a night," August said.

Through pollination, bats also help grow avocados, bananas, and mangoes. In all, some 300 species of fruit depend on bats for pollination. Their presence in significant numbers also allows farmers to use less pesticides.

August noted bats don't get cancer, so by them humans can "get a window into anti-cancer issues."

They are the only mammals capable of true flight, and there are some 1,400 different species worldwide. In the United States, about 50 species of bats can be found. Bats come in all shapes and sizes, from the tiny bumblebee bat that weighs less than a penny to flying foxes that can have a wingspan of up to 6 feet.

Bats aren't blind, as their eyesight is similar to that of humans, and only three—the common vampire bat, the hairy-legged vampire bat, and the white-winged vampire bat—feed solely on blood, mostly that of birds. You would have to travel much further south than southern New England to find one.

Bats aren't "rodents with wings." They are in their own mammalian order called Chiroptera, which in Greek means "hand wings." Although they are small like rodents, bats are more closely related to primates and humans than they are to mice or rats.

Bats are extremely long-lived for their size, but also have one of the slowest reproductive rates for animals their size.

August noted bat pups, typically born in late spring/early summer, have a 90% mortality rate. He said pups are clumsy feeders, inefficient hunters, new at flying, and they "just don't have the ability to put on all that fat mass to get through that first winter."

Those bats who survive the first year can live up to 35 years, compared to rats that live 1-2 years. Female bats usually only give birth to one pup a year; rats give birth to many offspring, multiple times a year.

The following is a look at the bats in southern New England listed as a species of concern, threatened or endangered:

: Listed as a species of concern in both Massachusetts and Connecticut. These bats are usually solitary, occurring together only to mate and when caring for their young. They may also be loosely drawn together while feeding and during migration. Like neotropical migrant songbirds, tree bats migrate south for the winter, returning north in the spring. They begin migrating south as early as August but primarily move in October and November and return by March and April.

As with most other tree bats, eastern reds are threatened by the precipitous decline in larger moths due to parasitism by the nonnative wasp . The wasp was introduced to North America in 1906 to control two invasive forest insects: gypsy moths and browntail moths.

In Massachusetts, this bat has been recorded in 103 of the state's 351 municipalities and in all 14 counties. This species is considered a hardwood-forest dweller, although they forage over a wide range of habitat types, including orchards, parks, cemeteries, residential areas, harvested forest cuts, woods/field edges, streams, and the edges of lakes and ponds. They generally roost in the foliage of hardwood trees.

: Listed as a species of concern in both Massachusetts and Connecticut. This species, the largest bat in southern New England with a wingspan of nearly 17 inches, can fly up to 60 mph. It measures 4-6 inches in length and weighs 0.6 to 1.4 ounces. Females are about 4% larger than males.

Since they don't emerge until well after sunset, fly high and fast, and live singly, hoary bats aren't well reported. They have been documented in 55 of 351 Massachusetts municipalities and in 12 of the state's 14 counties.

They are most often associated with coniferous forests, but they are also found in woodlands, farmlands, fields, over ponds, and in residential neighborhoods with mature trees.

: Listed as a species of concern in both Massachusetts and Connecticut. They are the most uncommon of the three species of tree bat in Massachusetts, with records from only 33 of the 351 municipalities and in 11 of 14 counties.

They hunt along the borders of mature hardwood forests and fields, over ponds, along streams, and in hardwood clear-cuts. Farther north, they are associated with mature spruce/fir habitats. They often roost solitarily in hollow trees, crevices in rocks and cliffs, and under loose bark.

: Listed as endangered in both Massachusetts and Connecticut. This species is one of the tiniest bats in the eastern United States with a forearm length of only 1.2-1.4 inches and a total length of 2.8-3.7 inches. It has a distinctive black "masked" face, tiny hind feet, and a flattened skull that helps it squeeze into tight rock crevices.

For more than 40 years, the only record of this bat occurring in Massachusetts was a single specimen found in a Hampden County mine in 1937 and preserved in Harvard University's Museum of Comparative Zoology. From 1980 to 2017, a few more individuals were documented hibernating in three mines and caves, and since 2018, more locations, including several summer records have been documented. The first documentation of breeding didn't occur until 2024.

They often roost singly but may be found in small groups. In the summer, they emerge at dusk and fly slowly and erratically, gleaning insects in the forest canopy.

Facing significant threats from white-nose syndrome, this species relies on the protection of undisturbed caves and mines for its survival.

: Listed as endangered in both Massachusetts and Connecticut. In summer, females with young roost together in maternity colonies. These small summer colonies, ranging from a couple to a few dozen bats, are usually under the loose bark of standing dead trees, but occasionally are under the bark of live trees, particularly shagbark hickory, or in cavities. Females bear one young per year, in late June. The young bats are weaned in about a month and can be flying by mid-July.

They begin migrating back to their winter hibernacula as early as late July, but most arrive back in September and October. Unlike many bat species that hibernate in North America, nearly all Indiana bats hibernate in just a few select caves and abandoned mines. These large aggregations of hibernating bats are very susceptible to disturbance by humans in winter and, in recent years, to infection with white-nose syndrome.

The last hibernating colony of these bats in Massachusetts was in a mine in the town of Chester in 1939. As their population slowly increased in New York and they returned to Vermont, there was hope that this species would soon be rediscovered in Massachusetts.

: Listed as endangered in both Massachusetts and Connecticut. This species was once the most abundant bat in Massachusetts, but its population declined by more than 99% after the onset of white-nose syndrome.

In the summer, these bats emerge at dusk from daytime roosts for the first in a series of nighttime flights. They use echolocation to locate the insects on which they feed, often catching them in their tail or wing membranes. By the end of summer, the bats have accumulated stored fat, often comprising up to 45% of their body weight. They begin to "swarm" around the entrances of caves and are thought to be testing the air of possible hibernacula. By the end of October, the bats enter hibernation sites.

The (not state listed in southern New England) is the only other bat commonly found in houses.

: Listed as endangered in both Massachusetts and Connecticut. This species has specialized flight and is highly adapted for maneuvering through dense forest canopies. Unfortunately, this bat has been severely impacted by drastic population declines due to white-nose syndrome, which disrupts hibernation and depletes stored energy.

Their long tails and large wing membranes allow these bats to fly slowly and navigate through cluttered environments. These special adaptations enable them to glean prey from foliage, in addition to catching insects on the fly. These bats locate resting insects through a combination of passive listening and the emission of high-frequency echolocation calls.

: Listed as endangered in both Massachusetts and Connecticut. This is the only species of bat in the Northeast with 34 teeth. All the species of Myotis have 38, while the big brown bat and all the tree bats have 32.

They are the earliest bats to emerge to feed in the evening. They use echolocation to locate insects, often catching them in tail or wing membranes. Their foraging area is small relative to other bats, but they may travel up to 5 miles from their roosting site to feed. They have been known to migrate up to 85 miles to hibernation sites.

Their numbers were reduced by heavy pesticide use in the mid-1900s but were steadily on the rise again until the outbreak of white-nose syndrome in the winter of 2007-08.