Driving New Jersey’s highways during rush hour is usually a stressful experience. Speed demons weave in and out of overcrowded lanes, clueless drivers whip across traffic to make exits they have almost missed, and it’s generally best to assume that any other driver might do something extremely stupid at any given moment. Among all this motor madness, though, cues such as turn signals, horns, and brake lights help drivers avoid plowing into one another, and other mammals which find themselves in crowded, fast-moving conditions have developed their own ways of avoiding collisions.
Just as driving down I-287 at 5 PM requires drivers to deftly perceive what is going on around them, bats must also be aware of their surroundings when emerging from their roosts. Not only must bats navigate through the swarming mass of their kin, but predatory birds often take advantage of the concentrated masses of flying mammals to catch some dinner on the wing. The ability of bats to echolocate – using the echos of chirps and buzzes to detect prey, predators, other bats, and objects – has generally thought to be a liability in such crowded conditions. With so many bats so close together any individual bat’s attempt to echolocate could be jammed by similar calls from another, and sometimes echoes will start to return before the bat has even finished emitting the call. It is, as assessed in a different fashion by Nachum Ulanovsky and Cynthia Moss, a “cocktail party nightmare.”
As proposed by a new study on Brazilian free-tailed bats (Tadarida brasiliensis) published in the Journal of Mammalogy, however, this species may fine-tune its calls to get around the of auditory overload. In order to figure out how Brazilian free-tailed bats fly out of their roost without constantly smacking into one another, a team of scientists led by University of Tennessee researcher Erin Gillam studied the calls of bats as they left two different roosts in Texas (one of about 200,000 bats and the other of 17,000 bats) during the summer of 2006. Both audio and infrared video were recorded when bats began emerging from the caves, and together this information was analyzed to determine whether the bats were altering their calls as the number of individuals exiting the cave changed.
When the scientists analyzed their audio recordings, they found that the bats leaving the caves made two main types of short, frequent calls which differed substantially from the calls the bats typically made while foraging. The acoustic structures of these calls were most similar to the calls of bats flying through cluttered forest habitats or bats about to snag an insect; they were calls suited to detecting nearby objects. As proposed by the authors of the study, these calls allow for more accurate targeting at close range, thereby reducing the potential of calls from other bats to jam an individual’s ability to echolocate. Even though the bats did not change their calls with number of bats leaving the cave as the authors had suspected, they were using a distinct kind of call most frequently associated with moving in cramped conditions.
Despite the observation that bats used accurate, close-range calls when exiting caves, however, many questions remain about the reasons why bats use these particular sounds. For example, just why the bats should use two similar types of calls is unclear. Gillam and co-authors hypothesize that it might have something to do with obtaining different types of information, with one call directed towards the column of bats and the other directed away from the column to detect predators or other objects in the sky. Then again, at least one of the calls is very similar to calls made by the same species of bat during social interaction. There could be some kind of social component to the squeaks – perhaps the chiropteran equivalent of “Hey, I’m flyin’ here!”
Image: Brazilian free-tailed bats (Tadarida brasiliensis) exiting Carlsbad Cavern in New Mexico. From Wikimedia Commons.
Gillam, E., Hristov, N., Kunz, T., & McCracken, G. (2010). Echolocation behavior of Brazilian free-tailed bats during dense emergence flights Journal of Mammalogy, 91 (4), 967-975 DOI: 10.1644/09-MAMM-A-302.1