The Túngara frog is a tropical species of frog, upon which many mate choice studies have been performed. The male Túngara frog has a characteristic ‘whine-chuck’ mating call which has been exploited in these studies.

The whine portion of the call is typically a high frequency sweep, where as the chuck is a lower frequency repeated pulse. The chuck is produced by a fibrous mass in the vocal chords of the male, in studies where this fibrous mass was removed surgically; it was shown that the male still attempts to produce the chuck, despite being unable to.

Sexual selection affects the complexity of male mating call. It has been shown that the females prefer those males who produce the whine-chuck call, compared to just the whine call alone. This explains why the males will still attempt to produce the chuck portion of the call despite being unable to, as it is beneficial to their reproductive success, i.e. males producing the chuck are much more likely to be reproductively successful.

Sensory Bias (Ryan, M. J. 1990)

Female Túngara frogs are more sensitive to mating calls which are different from the population average, specifically mating calls which are lower in frequency. A lower frequency mating call is produced by a larger fibrous mass (responsible for the chuck portion of the mating call); a larger fibrous mass typically means a larger male and thus a male more likely to be of greater genetic quality.

However, it is believed that the female basilar papilla may be tuned in a way that makes them biased in favour of lower frequencies. This would explain the preference towards males who produce lower frequency mating calls.

It is suggested that this is an example of sensory exploitation by the males i.e. the male Túngara frogs have evolved the trait to exploit pre-existing biases in female senses. This would require that the lower frequency chuck-whine mating calls evolved after the sensory bias of the female basilar papilla. Males who evolved the chuck portion of the call (which satisfies the female sensory bias) therefore had better reproductive success, thus the evolution of the chuck was selected for. The reasoning behind this theory is that a closely related species Physalaemus coloradorum (a species considered to be less as evolutionarily advanced) does not produce the chuck sound in their mating calls.

Phonotaxis experiments were performed on the female Túngara frogs, which confirmed preference of the lower frequency chuck calls. Preference has thus allowed for sexual selection to occur.

The optimum frequency of the chuck portion of the mating call is the frequency at which the highest stimulation of female neurones occurs. Frequencies lower than the optimum decrease female neuronal stimulation on slightly, but interestingly frequencies higher than the optimum see a much greater decrease in neuronal stimulation.

Female Mate Choice (Ryan M. J. 1980)

Nearly all animals can only mate with conspecifics, i.e. the same species and specific mate recognition systems are in place in many species to ensure that this is true. Such systems allow species to ensure they recognise only appropriate partners. Conspecific signals (such as the mating call of male Túngara frogs) must therefore somehow encode a species identity.

In certain regions such as near the equator, there can be multiple similar species of frogs (in this example, three, from the Hyla genus) all performing mating calls within close proximity of one another. Despite this, hybridisation does not occur nor does interspecific clamping (clamping of the male to the female before mating). Therefore the male mating calls must somehow sufficiently describe the species identity.

A phonotaxis experiment was conducted on female H. ebraccata (one of the species which searches for a mate in the close proximity of other species within the same genus). During the experiment, there were three possible call types played to the female, either:

• Conspecific
• Heterospecific
• Noise (or silence)

During each experiment, two of the above were played from speakers at separate locations, the female thus had a choice between the two and she was observed to see which speaker she moved towards. The results were that the females primarily preferred the speaker relaying the conspecific call of male H. ebraccata (the expected result). But interestingly, when given a choice between heterospecifics (i.e. no conspecific call was played) the females did show a preference towards H. phlebodes. This showed that the females were in fact able to differentiate between heterospecific mating calls. In order of preference:

1. Conspecific calls – H. ebraccata
2. H. phlebodes mating calls
3. Other heterospecific calls
4. Noise / Silence

Backwell, P.R.Y. & Jennions, M.D. 1993, “Mate choice in the Neotropical frog, Hyla ebraccata: Sexual selection, mate recognition and signal selection”, Animal Behaviour, vol. 45, no. 6, pp. 1248-1250.

Gridi-Papp, M., Rand, A.S. & Ryan, M.J. 2006, “Complex call production in the túngara frog”, Nature, vol. 441, no. 1, pp. 38.

Ryan, M.J., Fox, J.H., Wilczinski, E. & Rand, A.S. 1990, “Sexual selection for sensory exploitation in the frog Physalaemus pustulosus”, Nature, vol. 343, no. 6253, pp. 66-67.