Tri-colored bats are among the most common bats in much of eastern North America, yet relatively little is known of their seasonal travels among summer roosts, swarming sites and hibernacula. Our research goal was to apply new technology to old museum specimens (collected between 1878 and 1986) to investigate the unusual migrations of tri-colored bats – making us feel kind of like cold case detectives.
Our study, funded in part by Bat Conservation International Student Research Scholarships, indicates a fundamentally different picture of migratory behavior than has been previously assumed for this species.
Known until recently as eastern pipistrelles, the little insect-hunting tri-colored bats (Perimyotis subflavus) 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 for as long as several weeks, hundreds, sometimes thousands, of bats gather in and around swarming sites in a truly spectacular sight. This is when mating occurs.
Tri-colored bats hibernate during winter, also in caves or abandoned mines. As with a number of other bat species, females store viable sperm throughout the hibernation season, then ovulate and become pregnant after emerging from hibernation in the spring.
Although there is little information about tri-colored bats’ movements among summering grounds, swarming sites and hibernacula, they are generally believed to be short-distance, regional migrants who move across the landscape in all directions as they travel among sites.
However, seasonal variation in abundance and sex ratio within local populations has led some researchers to suspect these bats may migrate across significantly longer distances as “latitudinal migrants” that travel south in the autumn (in this case to hibernation sites), then return northward in the spring. Another species considered to be a latitudinal migrant, the hoary bat (Lasiurus cinereus), may migrate more than 1,250 miles (2,000 kilometers) one way.
A better understanding of the migratory patterns and range of the tri-colored bat is essential for effective conservation decisions. Not only is the species among those battered by White-nose Syndrome, but tri-colored bats account for a substantial proportion (perhaps as much as 25 percent) of bat fatalities at some wind-energy facilities. Current evidence suggests bats are most likely to be killed by wind turbines during long-range migrations across latitudes.
Biologists in the past mostly studied migratory bat movements primarily by banding, which involves 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 is now widely used for investigating the origins of many migratory animals. In 2004, Paul Cryan of the U.S. Geological Survey used it to describe hoary bat migrations (see “Chemistry and Migration Mysteries,” BATS Fall 2004).
This powerful technique measures the relative abundance of two stable isotopes of hydrogen in a tissue sample. Different isotopes of the same element have the same number of protons, but vary in atomic mass because of differing numbers of neutrons in the nucleus of each atom. Hydrogen, the simplest element, has two stable isotopes: protium and deuterium.
When water-saturated air masses form in the tropics and move across the landscape, deuterium (the heavier stable hydrogen isotope) condenses to form precipitation more readily than protium. Although other factors are also involved, 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.
The average stable hydrogen isotope compositions of precipitation during different seasons have been estimated and published for locations around the world at waterisotopes.org, which we utilized for our study.
Obtaining hair samples from tri-colored bats throughout their far-flung range is a daunting challenge. We received help from four museum mammal collections across North America. They provided tiny fur samples from the specimens in their collections, along with information on when and where each was obtained. Collection information for all of our 184 samples spanned all seasons at latitudes across most of the species’ known range. We conducted our stable isotope analyses at the Laboratory for Stable Isotope Science at Western University in London, Canada.
Based on data from male bats, we calculated that new fur growth occurs between June 23 and October 16, although sample limitations suggest the end date could be as early as September 9. We assume the same dates apply to female tri-colored bats. The stable hydrogen isotope composition of bat fur collected outside of the time of new fur growth can help us to infer north-south movements of individual bats.
Our results 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. For the most part, the distances traveled by female migrants appeared to be less than those of male migrants.
Based on these results, we conclude that, contrary to previous hypotheses, at least some tri-colored bats of both sexes do indeed engage in longer-range, north-south migrations that have more typically been associated with hoary bats, eastern red bats (Lasiurus borealis) and silver-haired bats (Lasionycteris noctivagans).
All of the female migrants and most of the male migrants were captured at the northern end of the species’ range, north of 40 degrees (roughly the latitude of Philadelphia). This seemed surprising, as we found that bats that were already near the northern extent of the known range appeared to have migrated south – from locations even farther north.
Also, the bat whose stable isotope composition indicated that it had traveled the farthest also had the most northern stable isotope signature. This male was collected in southwestern Ontario and its stable isotope signature indicated it may have grown its fur at a higher latitude than has been previously recorded as within the range for this species.
Why would bats at the northern extent of the species’ range be making north-south migratory movements? We found this migratory pattern more often among males than females. Sex-biased migratory behavior is common among bats, but it usually involves females moving farther than males. Our finding that male tri-colored bats are more likely to be long-range, latitudinal migrants may reflect reproduction-based energy requirements.
Females ovulate and become pregnant after emerging from hibernation in the spring and face increased energy needs throughout pregnancy and lactation in early to mid-summer. It may not be energetically possible for females to undertake a long migration at that time, and more northern latitudes may not be the best place to give birth and raise their pups.
Conversely, male reproductive costs occur in late summer when they produce sperm, and they may benefit from migrating to more northern summer habitats before that occurs.
In winter, bats may need to migrate south from extreme northern latitudes to hibernacula where winters are shorter and hibernation is less energetically expensive.
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. At least some individuals appear to engage in substantial latitudinal migrations that had not been previously documented for this species.
ERIN FRASER completed her Ph.D. at Western University in London, Ontario, where she used stable isotope techniques to study migration in several species of North American bats. She is currently an Assistant Professor in Biology/Environmental Science at the Grenfell Campus of Memorial University in Canada.
The author gratefully recognizes these mammal collections, which provided fur samples for this study: Royal Ontario Museum, Louisiana University Museum of Zoology, Harvard Museum of Comparative Zoology and Cornell Museum of Vertebrates.