Sean Markey
National Geographic News
December 4, 2002
Chemistry may help keep a relationship together, but male
tree-hole frogs found in the lowland rain forests of Borneo
use physics to attract their mates.
Scientists studying the mating calls of Metaphrynella sundana
have discovered that male frogs actively tune the pitch of
their calls to resonate inside hollow tree cavities—their
preferred mating habitat. Researchers likened the resonant
effects to those of organ pipes. When successful, male frogs
sound closer, louder, and presumably more attractive to prospective
female mates.
Researchers also found that males will expend extra energy
on their mating call—increasing the tempo and prolonging
the duration of each call—to take advantage of the favorable
acoustics.
"As far as we are aware, [this] is the first evidence
that an animal can actively alter its behavior, its call pitch,
to obtain…resonance…[to] produce this super-attractive
call, which is quite complex," said Björn Lardner,
one of two researchers to make the discovery.
Lardner, an animal ecologist at the University of Lund, Sweden,
and his research partner, Maklarin bin Lakim, of the Sabah
Parks Research and Education Division in Sabah, Malaysia,
conducted six months of field research in Borneo's Kinabalu
National Park. Lardner later analyzed audio field recordings
and other data at the Field Museum of Natural History in Chicago,
Illinois.
The discovery marks the first time an animal species has
been shown to alter its calling behavior to exploit resonance
effects. Some species of burrowing frogs and crickets are
known to dig burrows or cut baffles in leaves to specific
dimensions to create a resonant effect for their mating calls.
But such adaptations involve the insect or animal changing
their physical environment, not altering their individual
calling behavior.
Since such behavior was not previously known to science,
researchers have rarely looked for such traits in animals,
Lardner suggested. The findings raise the possibility that
similar overlooked animal behaviors await discovery, he said.
"The question that arises is, how common is this phenomenon?"
said Lardner. "Is it that we haven't looked for it in
animals? Could it be that animals are using [other] signals
in more complex ways of exploiting the physics of the environment
in which they live?"
An Unexpected Discovery
Borneo tree-hole frogs grow up to one inch (2.5 centimeters)
in size and are endemic to the lowland rain forests of Borneo.
They mate and breed in tree cavities formed by rotting broken
limbs. Frequent rainfall can partially fill these cavities
with water. Female tree-hole frogs lay their eggs in these
miniature, sheltered standing ponds after mating.
To study the frog's mating-call strategies, researchers Lardner
and bin Lakim recorded more than 300 mating call events by
frogs in their natural habitat.
The pair also captured a male tree-hole frog and placed it
in an opaque plastic cylinder partially filled with water.
A drainage tube allowed the researchers to gradually lower
the water level inside the cylinder to learn how the frog's
mating call was affected by changing acoustic properties of
the hole.
The researchers returned the following evening to record
the mating calls of the male frog.
Analysis of those field recordings demonstrated that when
male tree-hole frogs colonize a new hole, they can emit a
series of calls to "sample" its acoustic properties.
With each successive call, the frog can presumably adjust
his call pitch to increase the volume it hears inside a tree
hole. Male frogs eventually reach the resonant frequency of
the tube if it lies within reach of their individual vocal
range.
If resonance is gained, the male frog reaps a benefit. During
their experiment, Lardner and bin Lakim found that the male
frog increased the volume of its mating call by 10 to 15 decibels
by leveraging the resonant frequency of the tube.
The physics of sound state that doubling the distance to
the source of a sound decreases its recorded volume by six
decibels. So the volume gained through resonance is significant,
Lardner said.
"By exploiting the resonance effect, [male frogs] will
appear to sit closer and/or they will sound more attractive
to the female," said Lardner. "Not only will they
be heard from a longer distance and attract females from a
longer distance, but presumably—we can only guess—the
female also appreciates a powerful, strong call as a sexy
trait that indicates a powerful male." The researchers
also found that when they drained water from the tube over
a 28-minute period, the captive male frog changed the pitch
of his mating call by up to 115 hertz to stay within the resonant
frequency of the cylinder.
The vocal range of individual male tree-hole frogs vary,
much like humans. Natural tree hole habitats also vary in
shape, size, and resonant frequency. Luck therefore plays
a role in whether or not individual male frogs can gain resonance
in a given hole.
When male frogs do find a tree hole with resonant frequencies
that lie within their vocal range, they pull out all the stops,
Lardner said, increasing the calling rate and pulse duration
of their mating calls.
"When they hear, 'Oh tonight I have a bonus effect because
I am in a hole that is good, then I [will] really invest extra
energy in these other call aspects," Lardner said.
An "Exciting Finding"
Ronald Heyer, a research zoologist with the division of reptiles
and amphibians at the Smithsonian Institution's National Museum
of Natural History in Washington, D.C., said that there have
been a number of important discoveries involving frog calls
in recent years.
Commenting on these latest findings on Borneo tree-hole frogs
via e-mail, Heyer wrote, "This study adds yet another
layer of understanding about communication in frogs, involving
a completely unanticipated, unexpected, exciting finding."
"[It] seems to indicate that there is quite a bit of
control over vocalization possible by individual frogs, which
will make us have to again rethink what we understand about
frog communication in general," Heyer wrote.
A summary of Lardner and bin Laksim's research appears in
the December 5 issue of the science journal Nature.
Copyright National Geographic
Reprinted from http://news.nationalgeographic.com/news/2002/12/1204_021204_TreeFrogs.html
