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My name is Carl Gerhardt. I am a Curators' Professor of Biological Sciences at the University of Missouri.
My research is focused on the Eastern grey tree frog, Hyla Versicolor, and Cope's grey tree frog or Hyla Crysoscelis.
Despite the fact that these are two different species,
to both humans and frogs they appear identical.
There is one straight-forward way of telling the species apart:
listening to them.
During mating season - usually between April and August - grey tree frogs gather in
the bushes and trees around ponds.
There, the males generate mating, or "advertisement," calls or "trills."
As more males arrive and begin to trill, the volume rises and creates what we call a "chorus."
(Frog chorus)
This frog chorus, often quite loud,
may attract other male and female frogs to the area.
Females do not rely on eyesight to choose a mate; only on sound.
What they are listening for in a suitor's sound is pretty specific.
Females of both species tend to listen for longer, more sustained trills.
Sustaining an advertisement call is quite taxing on the male;
it can be likened to a human running a marathon.
Thus, a longer call tends to signal the presence of a more robust, healthier male.
When it comes to females pairing up with the correct species, however, a second
criterion comes into play.
We've discovered that this has nothing to do with the call length or pitch;
what is most important to the female, and what is completely non-negotiable, is that the
call contains the right trill rate for her species.
The Cope's grey tree frog has a high trill rate of about fifty pulses per second.
(Rapid trilling sound)
The Eastern grey tree frog has a slower, more melodic call with a trill rate of about
twenty pulses per second.
(Slower trilling sound)
Males that produce the wrong pulse rate are completely unacceptable to the female.
This selectivity prevents the two different species from mixing, which
would create hybrid, and thereby sterile, offspring.
So do otherwise identical tree frogs know what trill rate to produce?
Frogs do not use active cognition; in other words, producing calls with a
certain trill rate is not a behavior they have consciously adopted.
Nor have they learned it by trial and error.
These circuits are affected by chromosome number.
One of the species is what biologists call "polyploid," meaning they have extra sets of chromosomes.
Cope's grey tree frog - the faster triller - is "diploid," it has two sets of
twelve chromosomes, or twenty four total.
The Eastern grey tree frog is "tetraploid." It has four sets of twelve
chromosomes or forty eight total.
Along with more chromosomes, the Eastern grey tree frog - the slower triller - also has
physically larger cells.
These larger cells affect the frogs muscular fibers and call-generating circuits
thereby producing a slower trill-rate call.
New results also suggest that the cell size change affects the female trill rate,
remarkably in a parallel way.
This has important implications for changes in hearing in all kinds of animals.