On Summer Nights, Some Bats Like to Jam

Mexican free-tailed bats “jam” each others’ echolocation calls to discombobulate competitors

Mexican free-tailed bats can be real jerks to their friends. Photo: Nickolay Hristov

Some bats are famous for their altruism. Vampire bats that don’t manage to find a life-giving blood meal can squeak for help and receive a regurgitated snack from their neighbors. But not all bat species are so community-oriented. Recordings show that Mexican free-tailed bats essentially yell at their competitors so loudly that other bat’s echolocation calls are jammed, preventing them from snagging their insect prey.

Previous work revealed that tiger moths and perhaps other insect species can exploit vulnerabilities in their killers’ echolocation calls by issuing disorienting ultrasonic clicks. Until now, though, scientists did not suspect bats of using this seemingly rude strategy to get one up on fellow insect-eaters, and experts wrote off any bat-on-bat jamming as a simple mistake. Most bats, in fact, can alter the frequency of their calls to politely avoid infringing on their neighbor’s wavelength turf.

Now researchers from Wake Forest University and the University of Maryland report in the journal Science that Mexican free-tailed bats, at least, are one exception to the good-manners rule.

These winged mammals often live in vast colonies of a million individuals or more, and they have a sophisticated social system that includes at least 15 different calls used to communicate with one another. In the summer of 2009, the team was working in the field in Arizona when they picked up on a strange call that they could not place. To a human ear, the amplified mystery call sounded a bit like the noise a balloon makes as an annoying party guest slowly lets the air squeak out. Perplexingly, they noted that that whiney sound almost always turned up only when another bat was engaged in a “feeding buzz”—a series of successive chirps that become more frenzied as the bat homes in on a tasty flying morsel.

Could it be that the weird call was an intentional act of sabotage by a hungry, covetous neighbor?

The researchers began investigating this hunch, returning to the Arizona field site for three summers in a row and establishing two other sites in New Mexico. They tracked bats with a spotlight (which had no noticable effect on the bats or the insects’ behaviors) and a video recorder and recorded sounds using a bat detector. They also transformed the calls in real time into sounds discernable by the human ear. Using these tools, the team identified and tracked individual bats, analyzing their movements, calls and predatory successes and losses. The data allowed the team to reconstruct each bat’s flight path in 3D.

Of the nearly 70 whiney calls that they recorded, all overlapped with another bat’s feeding buzz. The calls typically began less than two-tenths of a second after the would-be diner started to issue its predatory chirp. Like a game of Marco Polo, the annoying screech consistently answered the feeding buzz until the victimized bat either caught the insect anyway or gave up. In the presence of that jamming signal, hunting bats were up to 86 percent less likely to capture an insect in Arizona and 77 percent less likely to do so in New Mexico than they would be if they were just left alone. The whine only prevented insect capture when it directly overlapped with the feeding buzz, supporting the hypothesis that the call garbles the other bat’s chirp and prevents it from receiving feedback. 

You can hear those competing calls for yourself, slowed down 20 times:

Just to make sure they were indeed interpreting their observations correctly, the team returned to the field with a broadcaster in hand. First, they tethered juicy moths to clear lines of string to lure the bats in. Then they played various calls, from the whiney jamming sound to random bursts to no sound at all, and studied how those recordings affected the bats that came in for the kill. Compared to random noise or no noise at all, bats that were subjected to the whine playback were 73 percent less likely to actually snag the moth on a string, they found.

The team observed that the trickster using the jamming sound did not go on to snag the meal. They think the mentality might be more along the lines of less-for-you-equals-more-for-me-later. This back-and-forth competition continues throughout the night, with bats alternating between pursuer and jammer. Rather than run away to a less competitive airspace or attack their loud neighbors, bats simply stood their ground and gave it another go, so that at the end of the night, each bat had tallied up a number of wins and losses. Like friends retiring from a game of pick-up soccer, presumably the bats don't hold any grudges when they return to their crowded roosts, seeking some much-needed shut-eye before beginning the whole noisy dance again the following night. 

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