Living in dark caves and hunting at night, bats use sound to navigate the world around them. New research shows that some bat species use echolocation for much more than avoiding obstacles and detecting prey. A study led by Mirjam Knörnschild and Kirsten Jung of the University of Ulm, Germany, published this week in Proceedings of the Royal Society B, describes one bat society that uses ultrasonic calls to distinguish among individuals, defend territory, and serenade potential mates.
The researchers traveled to Costa Rica, home of the greater sac-winged bat (Saccopteryx bilineata), and used a handheld ultrasound recorder to capture the bats’ calls. They sampled a total of thirteen male and fourteen female bats in three different wildlife preserves, showing that the bats’ use of echolocation is consistent across the species, rather than unique to a single colony. The researchers used a variety of sound analysis techniques to determine differences between individual bats’ voices as well as differences between male and female calls.
Greater sac-winged bats have a complex social structure that makes it important for them to communicate identifying information to each other from a distance. They live in harems: one male will cohabit with and protect a group of two to eight females. Younger and weaker males live alone but still establish and defend their own territory. Both male and female sac-winged bats loyally defend their day roosts and hunting territories, although females are not always sexually faithful to their harem masters.
Like human voices, every bat voice is different. On average, female bats have higher-pitched voices than males, even though female sac-winged bats are generally larger than males. Female calls are also shorter in duration than male calls.
Male bats use these differences in pitch and call length to determine the sex of an approaching bat, and they respond accordingly. After a night of hunting insects, the males return to the day roost first to welcome females and protect against invasions from other males. Because males that lead harems are always the first bats back to their day roost, the researchers could study the ways they responded as other bats approached the roost.
When a male stranger flew toward the day roost, the resident male bat responded with an aggressive, threatening pattern of calls. When a female bat approached the roost, however, the resident male would respond with a courtship call, regardless of whether the female was a stranger or part of his harem. The male bats consistently distinguished between male and female visitors and greeted them according to their sex. Moreover, the males began their threats or seductions while their visitors were still too far away to be identified by smell – they could only have figured out their visitors’ sexes from clues in their echolocation calls.
This new study reinforces previous discoveries about the sophisticated ways that mammals use echolocation to communicate. In an interview with Discovery News, lead researcher Mirjam Knörnschild said that she believes most bat species use echolocation to communicate social information, and “the same is probably true for dolphins.” Other recent studies have shown that bottlenose dolphins develop signature whistles that they use like names, and that baby bats “babble” as they learn to speak the language of echolocation.
This discovery also reveals the extent to which mammals devise uses for their sensory abilities that go far beyond their original adaptive purpose. When a bat screeches, it might be searching for a hot date as well as for a meal.