Graduate student Erin Fraser applied today's technology to bat specimens collected over the past 135 years to begin unraveling the migratory behavior of tri-colored bats. These are among the most common bats in much of eastern North America, yet relatively little is known of their seasonal travels.
|Tri-colored bat (Perimyotis subflavus). Photo courtesy of M.B. Fenton.
Fraser's Ph.D. research at Western University in London, Ontario, indicates a fundamentally different picture of the species' migratory patterns than was previously assumed. The work was funded in part by Bat Conservation International Student Research Scholarships.
Known until recently as eastern pipistrelles, the little tri-colored bats roost during the summer months alone or in small female-only colonies in buildings or foliage. In the autumn, they travel to "swarming sites," usually caves or abandoned mines. Each night, hundreds, sometimes thousands, of bats gather in and around swarming sites in a spectacular sight. This is when mating occurs. Tri-colored bats hibernate in caves or abandoned mines during winter.
Tri-colored bats are generally believed to be short-distance migrants who move across the landscape in all directions as they travel among sites. However, some researchers suspect these bats may migrate across significantly longer distances as "latitudinal migrants" that travel south in the autumn and return northward in the spring.
Biologists in the past mostly studied migratory bat movements by catching bats, attaching small bands to their wings, then hoping to recapture a few of those banded bats months later. The analogy of seeking needles in haystacks comes to mind.
The addition of stable isotope analysis to the biologist's toolkit has made another approach to migration studies possible. This technique measures the relative abundance of two stable isotopes of hydrogen (protium and deuterium) in a tissue sample. As a general rule, the ratio of deuterium to protium in rainwater decreases as the air mass moves from the tropics toward the poles.
As an animal's tissue grows – fur in this case – it incorporates the stable hydrogen isotope composition of its food and drinking water. Once new fur has grown, this ratio will not change. Since many bat species replace their fur annually during summer/fall, the stable hydrogen isotope composition of fur collected outside of the period of fur growth can provide information about the bat's location when new fur was growing.
Obtaining hair samples from tri-colored bats throughout their far-flung range is a daunting challenge. The team turned to four museum mammal collections across North America. They provided tiny fur samples from specimens in their collections. Collection information for the 184 samples in the study spanned all seasons at latitudes across most of the species' known range. They were collected as far back as 1878. The samples were analyzed at the Laboratory for Stable Isotope Science at Western University in London, Canada.
Fraser said the results of her study indicated that 24 of 73 males (33 percent) captured outside the period of new fur growth were south of the location where they grew their fur; only one male showed evidence of having moved north. Five of 32 females (16 percent) sampled during the non-molt period indicated some southern movement and two may have moved north.
"Our research indicates that the migratory behavior of tri-colored bats is more complicated than previously assumed, with substantial variation between sexes and across the species' range," she said. " At least some individuals appear to engage in substantial latitudinal migrations that had not been previously documented for this species."
Fraser, who's now an Assistant Professor in Biology/Environmental Science at the Grenfell Campus of Memorial University in Canada, notes the generous assistance of the Royal Ontario Museum, Louisiana University Museum of Zoology, Harvard Museum of Comparative Zoology and Cornell Museum of Vertebrates.BCI Members can read the full story of Erin Fraser's research in the Spring 2013 issue of BATS magazine. You can help BCI support important scientific studies through its Scholarship program and other critical bat-conservation efforts at www.batcon.org/donate.