A bat's voice is its livelihood. Chirping and squeaking at just the right frequencies lets it echolocate food and stay alive. Sounding pretty isn't the point—except when it is. For the first time, scientists think they've found a bat species in which females choose mates based on their voices. Even if a lower-frequency squeak might be better for finding prey, males with the highest squeaks are the sexiest. The horseshoe bats, or Rhinolophidae, are a family of nearly 80 species with bizarre noses. Often generously described as "leaf-shaped," the nose looks more like a many-layered tree fungus growing out of the bat's face. The bats live in Europe, Africa, Asia and Australia. Throughout this family of bats, the larger species make lower-frequency echolocation calls. Think of a tall, barrel-chested bass singer in a choir compared to a smaller tenor (if the whole choir were singing above the range of human hearing, that is.) But one species, Rhinolophus mehelyi, breaks the pattern. Its highest echolocating calls are much higher—by about 30 kHz—than scientists would expect based on its size. In this choir, the basses are singing the tenor part. Sébastien Puechmaille, a researcher at the Max Planck Institute for Ornithology at the time, thought sexual selection might explain R. mehelyi's falsetto voice. After all, bats spend their waking hours in the dark, so they don't have much chance to look around for the most attractive mates. Do females pick their partners based on whose call is the highest? Puechmaille and his colleagues captured wild R. mehelyi bats from two caves in Bulgaria. To learn what female bats find sexy, they tested the females in a large box with two compartments. A speaker in one compartment played recorded echolocation calls from higher-frequency bats (110 to 112 kHz), while a speaker on the opposite side played lower-frequency calls (104 to 106 kHz). The speakers played 100 calls, alternating between low-frequency and high-frequency. Meanwhile the scientists counted how many times the female bat landed in each compartment (presumably looking for the attractive male she'd heard there). They saw that female bats picked the high-frequency side twice as often. It seems higher male voices are irresistible to females of R. mehelyi. Usually, when female animals choose mates based on how they look or sound—bright tail feathers, big antlers, booming ribbits—it's because the showiest males are also the fittest. Their flashy features demonstrate that they're healthy and have good genes, making them desirable fathers. Puechmaille weighed and measured the male bats in the study to see whether this held up. He found that larger, heavier bats had higher peak frequencies in their echolocation calls. When females choose the higher-voiced males, they're also choosing bigger and stronger mates. Finally, Puechmaille asked, do these seemingly fit and attractive males have more offspring? This is trickier to measure, but the researchers estimated the answer by analyzing the DNA of bats from one cave. By comparing the bats' genetic markers, they could measure how closely each animal was related to all the others. Bats that have produced more babies should have more relatives hanging around. The DNA showed that males (but not females) with higher-frequency calls were more closely related to the rest of the colony. This suggests that those tenor-voiced basses are, in fact, fathering more young. That's not to say males should practice singing soprano to make themselves as sexy as possible. Previous research showed that R. mehelyi's extra-high echolocation call shrinks the distance from which it can spot its prey. It's a case where sexual selection may be working "against natural selection," the scientists write. So male bats must balance attracting mates with being able to find food. They can't live off their looks, after all.
Puechmaille, S., Borissov, I., Zsebok, S., Allegrini, B., Hizem, M., Kuenzel, S., Schuchmann, M., Teeling, E., & Siemers, B. (2014). Female Mate Choice Can Drive the Evolution of High Frequency Echolocation in Bats: A Case Study with Rhinolophus mehelyi PLoS ONE, 9 (7) DOI: 10.1371/journal.pone.0103452