Two hoary bats are flying together across the arid landscape of New Mexico. My bat detector records not echolocation calls, but something else. And, unlike most of the hoary bats’ echolocation calls, these sounds are audible – I can hear them speaking. The bats, first one and then the other, seem to be exchanging similar vocalizations: social calls that I cannot begin to translate. This is surprising behavior for what’s considered a solitary species (except during migrations).
Echolocation, the remarkable biosonar system that bats use to “see” in the dark, has been studied for more than 70 years. Flying bats emit the ultrasonic calls into their path, then analyze the echoes that come bouncing back. But bats also produce other, less-studied vocalizations: the “social calls” they use to communicate with one another. These are often at lower, audible frequencies, and we are only beginning to translate these calls and understand their purpose.
Researchers have confirmed that bats gain information about the activities of other bats by eavesdropping on echolocation calls, and that they share information by producing specialized social calls. With support from a Bat Conservation International Student Research Scholarship, I am studying the surprising complex roles of social calls in the hoary bat (Lasiurus cinereus).
These are handsome bats, with long, dense, white-tipped fur. Hoaries are found throughout most of the United States and Canada and as far south as Argentina. The hoary bat is also the only land mammal native to Hawaii. These insectivorous bats roost alone or in family groups of a mother and her pups (usually twins) and prefer roosts in the foliage of trees, or occasionally in cavities of large trees or beneath peeling bark.
Their spring and fall migration routes have been reported at 250 to well over 1,000 miles (400 to 1,600 kilometers), and groups of hundreds of migrating hoaries have been reported. Many questions about the ecology and habits of hoary bats remain unanswered, and little is known about their behavior during migrations. Understanding how individual bats react to calls should give new insight into these mysterious social interactions.
Research on the hoary and other migratory tree bats has become especially urgent since these species are suffering very high fatality rates at wind-energy developments during their fall migration. Some researchers have suggested that mating or social behavior during migration may increase their vulnerability to wind-turbine collisions. A better understanding of the social behavior and ecology of these species could help guide conservation and mitigation strategies.
For example, acoustic deterrents are being developed by BCI and others to discourage bat activity at wind turbines and other potentially threatening structures. Our lab at Humboldt State University has found that broadcasting social calls of hoary bats can be effective at deterring bats of other species from ponds. But deploying these calls on wind turbines could be problematic until we understand how hoary bats respond.
My research is built around playback experiments, in which recorded calls are broadcast over speaker systems. While common in bird studies, call playback is not widespread in bat research. As my work shows, however, the technique can be a valuable method to explore species behavior, as well as a tool for luring particularly elusive bats for capture.
My first year of fieldwork was conducted in north-central New Mexico during the spring migration (May 15-June 15) and in northern California in the fall (August 31-October 18).
At each location, I placed my primary mist net where incidental captures were unlikely, such as against vegetation, then placed a loudspeaker about five feet (1.5 meters) off the ground behind the net. Additional nets were positioned across flyways, usually streams or ponds, to monitor bat activity in the area.
I broadcast two sound files of recorded social calls and created a control file by sampling the silence between calls so that the speaker would be operating but broadcasting only silence.
Bat detectors were used during the trials to record social vocalizations and determine species richness and activity. Echolocation calls were automatically classified to species, but since the software does not recognize social calls, I had to search through all recorded call files to identify them.
I conducted a total of 35 nights of trials the first year, 23 during the spring and 12 in the fall, and captured a total 374 bats of 15 species. Excluding nights with no hoary bat captures, and nights where I did not use my final selection of social calls, I was left with 10 nights in the spring and 10 more during the fall for a total of 104 total trials (52 control, 26 Laso2, 26 Laso3).
During the spring, I captured 18 hoary bats using playback and only one during the control trial without the broadcasts. In the fall, I captured 12 hoaries with call broadcasts and none without. Clearly, hoary bats are attracted to broadcasts of their social calls.
It originally had seemed likely that these social calls might be used primarily for mating behavior, as most of my original call files were recorded during late July and August, presumably when mating behavior starts. But the recordings of social calls during the spring migration, when only non-reproductive males were present, suggest this is not the case. I also frequently observed, during both seasons, groups of bats flying together and making social calls to each other. I recorded several examples of two bats making similar social calls back and forth. This may be coincidental, but it seemed to be a fairly common behavior among groups of hoary bats.
I am hesitant to draw firm conclusions from these preliminary data about why hoary bats are attracted to the social calls of their species. As has been found in other studies during the summer, they might emit social calls as a warning to other bats as they defend territories or foraging patches. But the hoary bats’ fast, direct flight and large-bodied prey suggest they do not forage primarily on clusters of insects, but hunt instead over broad areas where little would be gained by defending territory. Also, at least some hoary bats respond to social calls by investigating the source, which argues against a territorial purpose. And, of course, defending territory during migration would be a waste of energy.
In these trials, most hoary bats were captured directly over the loudspeaker. I often observed them flying in circles over it, which suggests that they aren’t shy about approaching other bats emitting these calls. I also observed groups of bats arriving together at a site. Social factors may underlie such hoary bat groups, and social calls may help to maintain group cohesion during migration. Much more research is required to examine this hypothesis.
Missing from my data is the response of females to call playback. Females were rarely present in New Mexico during the times I was there, and I was unable to locate any females during the fall migration in California. I am working to fill these gaps.
Also, it is thought that social calls are associated with mating, and their presence at wind developments could be indicative of mating behavior. However, after finding that social calls are used in other behavioral contexts, I urge caution interpreting behaviors based on acoustic data alone – at least until we learn much more about these complex and fascinating vocalizations.
GABRIEL REYES is a graduate student at Humboldt State University in the lab of Dr. Joe Szewczak, and works as a wildlife ecologist at H.T. Harvey and Associates.