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Since the time of Aristotle, humans have sought to unlock the secrets of aging and remedy the ravages of time.
Age Defiance: Could Bats Hold the Secret to a Longer Life?
by Jennifer Amie
Bat species, which number more than 1,000, account for one quarter of all the mammals on earth. Bloodsucking varieties are stock characters in horror movies, but even their benign cousins,
with their leathery wings and sharp teeth, are feared and misunderstood. These nocturnal, flying mammals long have fascinated humans, and for the past six years, they have held a
special interest for University graduate student Anja Brunet. She believes that bats may contain the secret to longevity.
Scientists have examined the puzzle of aging in humans, flies, mice, and birds—but Brunet felt that one promising subject had been overlooked. "I put two and two together and
thought it might be interesting to look at the question of aging in bats," she says. She decided to make it the focus of her work in the Department of Ecology, Evolution, and
Behavior, where she recently earned her Ph.D.
Many theories seek to explain why living creatures age and how their life-spans are determined. In the early 1900s, German physiologist Max Rubner observed that smaller animals generally
have shorter lifespans than larger animals. Mice, for example, live fewer than five years while elephants can live to the ripe old age of 80.
Mice and other small creatures also tend to have something in common: a high metabolic rate. Larger animals, like elephants, have a much lower metabolism. Metabolic rates represent an
expenditure of energy. Could it be that the faster you spend your energy, the shorter your lifespan?
While this rule of thumb seems to hold true for many animals, bats appear to be an exception—and Brunet wondered why. Since the 1950s, researchers have been keeping records of how
long bats live, and they have discovered some lifespans of more than 20 years—a remarkable record of longevity for a relatively small mammal. In comparison, shrews, a relative
of bats, live only one to two years.
Brunet began to investigate the role of metabolic rate, or energy expenditure, in bats' longevity. One factor that might distinguish bats from shrews is that bats have several significant
ways of conserving energy, the most important of which is hibernation. In Minnesota, bats hibernate for six months, slowing their metabolic rate dramatically. Their heart rates drop
from more than 250 beats per minute to 20.
Even outside of the hibernation season, bats slip into a state called "daily torpor." "When bats hang upside down and roost, they allow their body temperatures to drop to
ambient temperature," says Brunet. "They're not expending energy on keeping their bodies warm."
Could these energy savings be responsible for bats' longevity? As it turns out, the answer is not that simple. "Plenty of tropical bats do not hibernate, yet they live to be 20
years old," explains Brunet. "And animals such as ground squirrels hibernate, but aren't long-lived."
As part of her investigation, Brunet compared the metabolic rates of wild shrews that she captured in Glencoe, Minnesota, and little brown bats that she captured in North Oaks. Both
are small mammals, and the two species are closely related—yet their lifespans differ by as much as 30 years.
One might expect that bats' metabolic savings, gained through torpor and hibernation, could be responsible for their longer lives. However, bats also require a tremendous amount of energy
for flight. "Active bats have very high metabolic rates," says Brunet. "Flying uses a lot of energy. Only during hibernation do they have extremely low metabolic
rates." Overall, Brunet discovered, bats and shrews have very similar metabolic rates.
Brunet's comparison shows that metabolism alone cannot explain bats' longevity, although it is a contributing factor. Evidence indicates that hibernating animals do extend their lives by
conserving energy, and hibernating bats live longer than non-hibernating bats. Genetics and environmental factors also are known to affect aging, but Brunet wondered whether there
was yet another force at work.
She turned to another theory: the "free radical" theory of aging, first described by Denham Harman in 1956.
Free radicals are harmful chemicals generated as byproducts of energy production within cells. Specialized structures within cells, called mitochondria, produce energy by burning sugar
in the presence of oxygen. This process is necessary to sustain life, but it results in "leftover" electrons. Typically, oxygen picks up these electrons and is transformed
into water in the process. Sometimes, however, oxygen molecules pick up an incomplete number of electrons and become highly corrosive free radicals.
Free radicals start reactions in the body that destroy DNA, protein, and cell membranes. This damage, scientists believe, is what leads to the degenerative effects of aging. "It's a
paradox of life," says Brunet. "As we consume oxygen we produce both energy and free radicals. We depend on oxygen to live, but it might also be destroying us." It
stands to reason that animals with longer lives may be sustaining less damage from free radicals, possibly because their mitochondria are able to produce energy more efficiently,
with fewer of the byproducts that lead to free radicals.
"There is quite a bit of correlational evidence that longer lived organisms produce fewer free radicals than short-lived organisms," says Brunet. "I became intrigued and
began to wonder: Can we attribute the longevity of bats to the free radical theory of aging?" To answer this question, Brunet turned to the same bats and shrews whose metabolic
rates she had compared. She isolated mitochondria from the cells of the bats and the shrews and measured their production of free radicals. The short-lived shrews produce twice as
many free radicals as the bats. "It seems that the bats have more efficient mitochondria than the shrews, which could explain why they live so much longer despite the
similarities in their metabolisms," says Brunet.
With these discoveries, Brunet's research is off to a promising start, and she plans to continue her work during a postdoctoral research appointment at the University of Idaho, where she
will work with researcher Steven Austad.
Austad studies longevity in birds, which, like bats, have long lifespans and high metabolic rates. Also like bats, they fly. "It is possible that flight is associated with
longevity," says Brunet. "Maybe the energy required for flight requires the most efficient mitochondria and, in turn, decreases the free radical damage that leads to
aging." |