Free The Universal Sense by Seth Horowitz Page B

acoustic social behavior, because it tells the story of sound and sex.
    Xenopus laevis frogs live for love songs. Like all frogs, they depend on phonotropism —homing in on the calls of the oppositesex—to be able to find each other in the murky ponds they call home in the wilds of southern Africa (or in the somewhat less murky water of the lab aquarium). Unlike in most other species of frogs, the females do as much calling as the males, and it’s the females who are in control. Xenopus laevis doesn’t have complex singing apparati—it makes calls by using its laryngeal muscles to snap two cartilaginous disks together to create castanet-like clicks. It doesn’t offer much in the way of tonal repertoire, but this type of signal doesn’t require passing air over the vocal system and messing up the sound with bubbles, and clicks spread well through water, without distortion. Besides, as with all love songs, it’s in the timing. Males produce a relatively wide range of calls, from slow amplectant calls—ticking away every few seconds—when they are in a loving embrace with a female to occasional chirps (especially when they are picked up) and growling click trains. But their most important call in mating is their advertisement call , a half-second-long sequence of clicks, slow at first and then followed by a rapid burst, repeated at rates of up to a hundred times a minute.
    An advertisement call is exactly what it sounds like—it’s a signal to try to attract females in the area and to warn off other males, and it is heard most often in response to a female’s call. Female members of X. laevis have only two different songs—rapping and ticking, also made up of differently timed clicks—but they control the males’ behavior. Ticking calls are quite slow, only about four clicks per second, and females sing this when they are not sexually receptive. A male hearing a ticked-off ticking female will often move away from her, for reasons that should be obvious. Rapping is a call females sing when they are sexually receptive; it is a series of clicks about three times faster than ticking, eleven to twelve clicks per second, that actslike an acoustic aphrodisiac for any male in the area. Even playing a recording of a female X. laevis rapping song will make any male in the area approach the source and try to mate with whatever is making the sound. This often requires the lab tech to pry it off an underwater speaker and do extensive cleanup afterward.
    While in human singing, the ability of the singer is based on a great many physiological, cognitive, and behavioral factors (especially practice, or else Auto-Tune), in frogs the males’ and females’ songs are based more on physiological hardwiring. One of my favorite lab experiments of all time was called the vox in vitro or “song in a dish” by Darcy Kelley and Martha Tobias. Tobias and Kelley removed the larynx from male and female frogs along with part of the laryngeal nerve. When they stimulated the nerve at the appropriate rates, they found that they could actually make the disembodied larynx create sexually specific songs without the rest of the frog, but that even by changing the stimulation rate, they could not make an adult female larynx call quite as fast as a male’s larynx. This is due to sexual differentiation in the type of muscle fibers in the larynx. Males’ laryngeal fibers are fast-twitch or Type II muscle fibers. This type of fiber is metabolically suited for high-speed but relatively short-duration activity. Female laryngeal fibers are primarily slow-twitch or Type I muscle fibers and have greater endurance but contract more slowly. The difference between the two is based on developmental exposure to sexual hormones. Exposure to greater concentrations of androgens (of which the best-known is testosterone) during development changes the type of muscle fiber that will be expressed. However, sexual differentiation of the larynx is not the driving force