Bats About Our Town

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Bat Basics

All seven species of local bats described here are Vesper bats (Vespertilionidae). These are bats that emerge from their roosts around sunset and feed on insects during the night. Larger bats tend to eat larger insects, such as beetles and dragonflies and moths. Smaller bats tend to eat mosquitoes, midges, and small moths.

Bats Are Flying Mammals

Bats are the only mammals that can fly. With some changes a human being could fly as well. Imagine that your arms became very short, your thumbs stayed pretty much as is, and your fingers became as long as your body, with a thin and strong webbing connecting these fingers and extending to your legs and the space between your legs. This webbing is used to scoop the air, and the shape of the scoop can change according to how you flex your hand and move your legs. This method of flying may sound awkward. In fact it is aerodynamically very efficient, allowing you to make great flying maneuvers in the air.

Yuma Bat Catching a Moth

(Click on the image to see a larger image)
Photo © Merlin Tuttle

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How Bats See at Night

Bats have eyes with stereoscopic vision as we do, and their night vision is much better than ours. Bats also use a specialized hearing skill called echolocation to navigate as they fly through the air. Whales and dolphins are famous for using echolocation in the ocean to “see” objects and to communicate.

A Vesper bat flies open-mouthed because it is emitting high frequency sounds that bounce off objects up to 30 feet away. Hearing the echoes with both ears, the bat can place objects and judge their size, much as having two eyes allows human to have depth vision. While looking for prey, the sound pulses are slow, speeding up when an insect is located to help the bat home in and catch the insect. Bats use their mouths, wings, and tail to scoop up insects, sometimes several per minute. As they quickly maneuver to gulp down prey, they have a fluttering, seemingly random flight.

Local bats emit sounds in the 20,000 Hz range and above. Humans hear in the 20 Hz to 20,000 Hz range, so we cannot hear these bat calls. Each bat species has a characteristic frequency range and type of call. This is how researchers out at Woodard Bay, where Yuma Bats and Little Brown Bat females roost together in a maternity colony, distinguish the two species. Although the bats look very much alike, their calls, when viewed on a computer screen, look quite different.

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A small handheld instrument called a bat detector creates sounds audible to humans, and research versions of these instruments also can record and slow down these calls.

On this website, each bat call has been slowed down ten times for the sound clips. Most bat calls available on the Internet are not slowed down, merely translated into a lower frequency range, which makes them sound confusingly alike.

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How Bats Drink Water

Bats drink water while flying low over a quiet stretch of water—a lake, a pond, a stream.  If possible, they drink at night just after emerging and before they begin serious hunting of insects.  They cannot drink salt water, so the bats emerging at Woodard Bay over Puget Sound have to fly a bit before getting a drink.  The photograph clearly shows the tongue of the Townsend’s Big Eared Bat lapping at the water surface.

Townsend’s Big Eared Bat Flying and Drinking Water

(Click on the image to see a larger image)

Photo © Merlin Tuttle

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Long Life and Steady Habits

Bats are very small—the Little Brown Bat weighs one third what an ordinary house mouse weighs—yet they live long lives. A Little Brown Bat was found 34 years after being banded, and we do not know how old it was when first captured. Locally, a pregnant Little Brown was caught and released near Roy, Washington in 2008. She had a band on her wrist dating back to 1992. She was caught near the same spot as in 1992, showing how stable the association between a bat and a particular territory can be.

Bats are difficult to study, and the life-span information for most species is completely unknown. It is likely that most species, like the Little Brown Bats, live a long time.

Males and Females: Separated Half the Year

During the spring and summer, female bats are occupied by their pregnancies and then by nursing their pups. The pregnancy requires a lot of extra food, and producing milk to nurse a pup needs even more food. The females congregate around especially rich food sources in the warmer areas of our county, such as Capitol Lake in the center of downtown Olympia. In addition, the females group together in roost sites such as hollow trees, barns, bridges, and attics. They share body heat with one another and conserve calories. Depending on the species, the roosting colonies may be relatively small or quite large. The female Yuma bats form especially large colonies, and the Woodard Bay colony is the largest known in Washington State.

Big Brown Bats Roosting in a Building

(Click on the image to see a larger image)

Photo © Merlin Tuttle

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Silver-haired and Hoary Bats are exception to this roosting pattern for female bats. They roost individually in trees among the foliage, using their silver-tipped hair for camouflage. They give birth to twins, not single pups like other local bats. Both their thick fur and larger size help them conserve heat.

The male bats disperse to cooler, less insect-rich areas. These males can afford to use a distinctive bat survival strategy called torpor. When temperatures drop, a bat’s metabolism slows down, allowing the use of fewer calories to survive. Frequent torpor would not be good for the females during pregnancy, nor for the new pups.

If a human mother were to give birth to a baby as large as a Yuma bat pup, that baby would weigh forty pounds! Yet the pregnant mother Yuma bats of Woodard Bay fly up to 20 miles round trip to Capitol Lake each night to feed. When the pup is born, the mother typically makes two such trips per night.

After 4 to 6 weeks depending on the species, bat pups begin flying and hunting with their mothers, following them to the places their species uses for surviving the winter.

Fall brings males and females together again. Some or all of the Big Brown Bats, California Bats and Silver-haired Bats may remain in our locality for the winter. The Yuma and Little Brown Bats migrate to unidentified sites, probably to higher and colder areas. Townsend’s Big-eared Bats find caves and mines. As temperatures drop, the bats clustered together in their winter hibernation sites enter torpor, the state of slowed-down metabolism. They have enough stored fat to see them through most winters, with little to spare. If disturbed, they may use too many calories flying around their roost site, and may not survive.

There is one exception to this pattern. The large, strong Hoary Bats fly as far away as Mexico or possibly Central America, where they do not need to hibernate. Solitary during the summer, they join together in groups for the journey south.

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Bat Pregnancies Adapt to Weather Conditions

The fall reunion is the time for mating before the bats begin to hibernate. The female bats receive the sperm but conserve it in their bodies until spring, when fertilization occurs. The pregnancy begins close to the time when the female bats emerge to hunt the new springtime insects hatching out. If a spring is particularly cold, as occurred in 2008, the pregnancy is delayed and the female emerges later to hunt—in this case, two weeks later. And if the newly pregnant female encounters even more bad weather, her fetus develops slowly and is born after a longer pregnancy than in warm, insect-abundant spring times.

There is a danger for pups born late in a cold, rainy springtime. They may not be able to put on enough weight in the summer to live through their first winter. They need a long warm period in the fall, an Indian Summer, to fatten up on insects.

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