Bat Cave

March 21, 2010

Visiting Lower Michigan’s Only Bat “Cave”
By Mike Terrell
It may not be your typical “caped crusader” bat cave -- in fact, it’s
not a cave at all, but Northern Michigan bats don’t seem to care. Up
to 20,000 bats hibernate every winter in the cavernous insides of
Tippy Dam’s spillway, located on the Manistee River between the
villages of Brethren and Wellston.
When Tippy Dam was completed in 1918, it included a spillway; as is
the case with all dams. The spillway is used when a river is in high-
water or flood-stage to take care of excess water flow that isn’t
being passed through the hydraulic turbines to produce electricity.
Tippy Dam includes four large spillway gates, but it has an unusual
design that no other Consumers Energy dam system has.
“The design created large hollow chambers within the dam structure,
and the chambers remain within a few degrees of the water temperature
held behind it,” said Lorren Hannah, Manistee River Hydro Supervisor.
“The temperature in the chamber remains a few degrees above freezing,
and there’s moisture on the walls and small pools along the bottom of
the chamber. Openings in the structure allow access to the bats as
well as a constant air exchange. It’s very much like a natural cave,”
he said.
“The stability of the Manistee River watershed – dominated by sandy,
water absorbing soil – has made it necessary to spill water through
the chambers only once in the nearly 100 years Tippy Dam has been
operating, and that was during the summer of 2008,” added Dave
McIntosh, a Consumers river hydro engineer.
Having to flush water through the spillway in winter – a highly
unlikely occurrence – would likely result in high bat mortality, they
pointed out.
Apparently it didn’t take the bats long to discover that. Routinely,
16,000 to 20,000 bats hibernate in the chambers during the winter
months. Although bat usage had been documented for decades by
Consumers employees, nobody paid much attention to them until the
1990s when the company enlisted the aid of Dr. Allen Kurta, an Eastern
Michigan University biological professor known for his work with bat
species, and, in particular, the endangered Indiana bat.
For the past 16 years, Dr. Kurta, along with EMU students in his
biology classes, has made a pilgrimage to Tippy Dam every couple of
years to study and catalog the bats in the cave during hibernation;
and again in the fall when the bats are still active, gathering for
hibernation.
“The first time I saw this I was amazed at the number of bats that
hibernate in here, said Dr. Kurta during a winter study on March 13. “
The population has remained very stable over the last 16 years. I
would estimate that there are approximately 18,000 in the spillway
chambers this winter.
“What I’ve been most amazed about over the years is finding a few
Indiana bats and Eastern pipistrelle bats among the population. That
was startling to me the first time I discovered it.”
Eastern pipistrelle bats are common throughout northern Mexico and
most of the United States, but typically not in Northern Michigan.
They hadn’t been found north of Berrien County before, and the same
was thought of the Indiana bat. “A few of those bat species are
finding their way north to hibernate, which might indicate climate
change,” he added.
In Michigan’s UP, old mine shafts and natural caves have been found to
serve as bat hibernacula, but Tippy is the only one documented in the
Lower Peninsula.
As Dr. Kurta and his students handle the hibernating bats to catalog
their species and band them, they are careful to make sure the bats do
not become too active so they won’t use up excess fat reserves stored
for winter hibernation.
“We handle them quickly and carefully to reserve their fat and
energy. Normally they will quickly go back into a hibernation state
once we leave,” he assured. “It won’t be a problem for them.”
Consumers Energy is also on board to minimize contact with the bats
during their hibernation state, according to Hannah.
“We enter the spillway chamber only when necessary during the winter
months, which isn’t often,” he stressed. “Spillway gate tests are
only done during the summer as well as any maintenance we might need
to perform. We pretty much leave them alone except for this study,
and that’s only done every couple of years.”
That also goes for the property around Tippy Dam that Consumers owns.
Many of the local bats, like the brown and northern bats, spend their
summer roosting in trees around the pond. Consumers prohibits tree
cutting on its property around the dam from May to October; the time
when the bats will return to the cave-like spillways.
Dr. Kurta said the bat population looks healthy and appears to be
stable. Many of the bats they catalog have been banded during
previous visits. One of the Indiana bats they examined had been
previously banded in 1996, and they say that of the 18,000 or so bats
wintering here, probably less than 30 are the endangered species; like
looking for a needle in a haystack.
Bats will live approximately 20 years and will return year-after-year
to the same hibernation spot unless interrupted for some reason,
according to the professor.
Bats are fascinating creatures, although we seldom see them. They are
the only mammal that can fly and are most helpful to mankind. They
devour insects at an incredible rate. One small brown bat can eat up
to 3,000 mosquitoes in a single summer night. They also help
pollinate a variety of flowers and transport many plant seeds.
They won’t suck your blood and evaporate in sunlight, like so many of
the myths we grew up with.