Contact: Bob Nelson For immediate release
(212) 854-6580 February 17, 1998
Females' Siren Song Initiates Courtship Duets
In African Frogs, Columbia Biologists Find
Amphibian Species Rare Instance of Female Making First Move
The female of a species of South African frog doesn't wait for suitors to
make the first move, according to new research by Columbia University biologists.
As her eggs become ready to fertilize, she begins a clicking song that initiates a
courtship duet with a nearby male that helps the partners find each other.
The female's aphrodisiac song, called "rapping," is a rapid series of loud
clicks that sound like a Geiger counter exposed to a lode of uranium. It is among
the rare instances in the animal kingdom where the female takes the first step.
Conventional wisdom in biology is that males advertise for mates and
females answer: peacocks display, peahens admire; cocks crow, hens attend. It
is extremely uncommon for females to initiate courtship, and "advertisement
songs" that indicate receptivity had been thought to be the exclusive province of
males. The male South African clawed frog, Xenopus laevis, had been thought to
advertise his availability first, with a distinctive 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 of the
National Academy of Sciences, contradicts this view. The research, which also
sheds light on how hormones may affect the nervous system, was conducted by
Darcy B. Kelley, professor of biological sciences; Martha L. Tobias, senior
research scientist, and Sandya S. Viswanathan, a graduate student in
neurobiology and behavior, all at Columbia.
A female frog faces an urgent problem: there is a span of only about 24
hours in which her eggs are ready to receive sperm. She must attract a suitor
during that period, or lose her progeny. Mating takes place at night in murky
ponds crammed with frogs, making it difficult for receptive females to locate
males, even though the males vocalize continuously during breeding season. If a
receptive female can't locate the male, she sings her rapping song; the male
responds with a different song, an answer trill, and mating follows. If a male
approaches an unreceptive female, she responds with a different song, a slow,
monotonous ticking, which silences him.
"Our best guess is that duetting makes it clear to both parts of the receptive
pair who and where the other member is," said Dr. Tobias, who conducted the
initial field work near Cape Town. "One could think of the female's
advertisement call as a general love song and the male's answer call as a
serenade to a specific love object."
Because visual cues are absent or nearly so and the mating period is short,
vocalization is an ideal way for female frogs to find a mate. The authors suggest
these factors may have necessitated a reciprocal signaling 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 on opposite
sides of a visually opaque but acoustically transparent barrier and using a
hydrophone to record their calls. Receptive females swam directly towards
vocalizing males, and on reaching the barrier began rapping. That song had a
dramatic effect on males, which launched into intense bouts of answer trilling
and swam rapidly in search of the source.
The research team was not certain that duetting was what brought male
and female frogs together until they recorded the female rapping song and played
it to male frogs in a fiberglass tank. The males again responded with an answer
trill and tried to swim toward the sound, in one case even attempting to mate with
the loudspeaker. Ms. Viswanathan conducted these experiments at Columbia's
biological sciences department.
Dr. Tobias discovered the female's mating call and coined the term for it.
"When Darcy came to South Africa, I demonstrated the female behavior 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 brain and
male muscles are specialized for his prolonged courtship songs and that the
secretion of male sex hormones, androgens, are responsible for establishing the
numbers and types of cells required for the vocalizations. Connections between
male vocal neurons and muscle fibers are weak, but strengthen with use. Dr.
Tobias had demonstrated that an isolated male vocal organ can be stimulated to
produce the advertisement call. Though the researchers understood the neural
and anatomical basis for frog songs, they had no idea how the animals used the
songs until they were able to conduct field work.
The discovery of rapping highlights the role of another hormone, estrogen,
produced by females. Estrogen circulating in sexually receptive females acts on
the neurons that control their mating song to strengthen the connection between
neurons and vocal muscles, the scientists have found. "The effect resembles the
action of estrogen in the mammalian central nervous system," Professor Kelley
said. "That's a topic of considerable interest to scientists examining hormone
effects on brain function, since estrogen has already been shown to delay the onset
of Alzheimer's disease and improve verbal memory in women."
The work was supported by the National Institutes of Health.
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