Amphibian Species Rare Instance of Female Making First Move
The female of a species of South African frog doesn't wait forsuitors to make the first move, according to new research by ColumbiaUniversity biologists. As her eggs become ready to fertilize, she begins aclicking song that initiates a courtship duet with a nearby male that helpsthe partners find each other.
The female's aphrodisiac song, called "rapping," is a rapid seriesof loud clicks that sound like a Geiger counter exposed to a lode ofuranium. It is among the rare instances in the animal kingdom where thefemale takes the first step in courtship.
Conventional wisdom in biology is that males advertise for matesand females answer: peacocks display, peahens admire; cocks crow, hensattend. It is extremely uncommon for females to initiate courtship, and"advertisement songs" that indicate receptivity had been thought to be theexclusive province of males. The male South African clawed frog, Xenopuslaevis, had been thought to advertise his availability first, with adistinctive trill, and then to grab in succession the nearest females,releasing those that were not ready to lay eggs.
The new work, reported in the Feb. 17 issue of the Proceedings ofthe National Academy of Sciences, contradicts this view. The research,which also sheds light on how hormones may affect the nervous system, wasconducted by Darcy B. Kelley, professor of biological sciences; Martha L.Tobias, senior research scientist, and Sandya S. Viswanathan, a graduatestudent inneurobiology and behavior, all at Columbia.
A female frog faces an urgent problem: there is a span of onlyabout 24 hours in which her eggs are ready to receive sperm. She mustattract a suitor during that period, or lose her progeny. Mating takesplace at night in murky ponds crammed with frogs, making it difficult forreceptive females to locate males, even though the males vocalizecontinuously during breeding season. If a receptive female can't locatethe male, she sings her rapping song; the male responds with a differentsong, an answer trill, and mating follows. If a male approaches anunreceptive female, she responds with a different song, a slow, monotonousticking, which silences him.
"Our best guess is that duetting makes it clear to both parts ofthe receptive pair who and where the other member is," said Dr. Tobias, whoconducted the initial field work near Cape Town. "One could think of thefemale's advertisement call as a general love song and the male's answercall as a serenade to a specific love object."
Because visual cues are absent or nearly so and the mating periodis short, vocalization is an ideal way for female frogs to find a mate.The authors suggest these factors may have necessitated a reciprocalsignaling system in this species.
The team's first experiment, performed by Dr. Tobias in Cape Town,involved placing receptive male and female frogs in a concrete pond onopposite sides of a visually opaque but acoustically transparent barrierand using a hydrophone to record their calls. Receptive females swamdirectly towards vocalizing males, and on reaching the barrier beganrapping. That song had a dramatic effect on males, which launched intointense bouts of answer trilling and swam rapidly in search of the source.
The research team was not certain that duetting was what broughtmale and female frogs together until they recorded the female rapping songand played it to male frogs in a fiberglass tank. The males againresponded with an answer trill and tried to swim toward the sound, in onecase even attempting to mate with the loudspeaker. Ms. Viswanathanconducted these experiments at Columbia's biological sciences department.
Dr. Tobias discovered the female's mating call and coined the termfor it. "When Darcy came to South Africa, I demonstrated the femalebehavior and song to her. That night the four of us, husbands included,sat around thinking up names for the song," she said. "I hit on 'rapping'because it does mimic the sound, which is something like a Geiger counter."
Professor Kelley had already shown that the Xenopus male's brainand male muscles are specialized for his prolonged courtship songs and thatthe secretion of male sex hormones, androgens, are responsible forestablishing the numbers and types of cells required for the vocalizations.Connections between male vocal neurons and muscle fibers is weak, butstrengthen with use. Dr. Tobias had demonstrated that an isolated malevocal organ can be stimulated to produce the advertisement call. Thoughthe researchers understood the neural and anatomical basis for frog songs,they had no idea how the animals used the songs until they were able toconduct field work.
The discovery of rapping highlights the role of another hormone,estrogen, produced by females. Estrogen circulating in sexually receptivefemales acts on the neurons that control their mating song to strengthenthe connection between neurons and vocal muscles, the scientists havefound. "The effect resembles the action of estrogen in the mammaliancentral nervous system," Professor Kelley said. "That's a topic ofconsiderable interest to scientists examining hormone effects on brainfunction, since estrogen has already been shown to delay the onset ofAlzheimer's disease and improve verbal memory in women."
The work was supported by the National Institutes of Health.
Materials provided by Columbia University. Note: Content may be edited for style and length.
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