Frog hearing and communication

Frogs and toads produce a rich variety of sounds, calls, and songs during their courtship and mating rituals. The callers, usually males, make stereotyped sounds in order to advertise their location, their mating readiness and their willingness to defend their territory; listeners respond to the calls by return calling, by approach, and by going silent. These responses have been shown to be important for species recognition, mate assessment, and localization. Beginning with the pioneering experiments of Robert Capranica in the 1960's (Capranica, 1965) using playback techniques with normal and synthetic calls, behavioral biologists and neurobiologists have teamed up to use frogs and toads as a model system for understanding the auditory function and evolution. It is now considered an important example of the neural basis of animal behavior (Neuroethology), because of the simplicity of the sounds, the relative ease with which neurophysiological recordings can be made from the auditory nerve, and the reliability of localization behavior. Acoustic communication is essential for the frog's survival in both territorial defense and in localization and attraction of mates. Sounds from frogs travel through the air, through water, and through the substrate. The neural basis of communication and audition gives insights into the science of sound applied to human communication.

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Sound communication

Behavioral contexts

Frogs' hearing capabilities help them identify mates, intruders to their territories, predators and unknown sources. Different sounds are received and processed differently. Sounds from other frogs often have higher frequencies than sounds from approaching predators, and frogs’ circuitry is adapted to this difference. Some frogs, such as bullfrogs monitor high and low frequency sounds together. This is because bullfrogs have low frequency calls, and must distinguish between other bullfrogs and predators. Long,(1999).

The presence of predators elicits an alarm call. It may be to distract the predator or since it is often done after the frog is caught, it may warn other frogs. Long,(1999). Mating calls and alarm calls elicit the most activity in the thalamus. Mudry & Capranica, (1987).

Frogs that do not need to call to mate are called satellite males. However, they search out the territories of dominant males and mate with the females that are attracted to the dominant male’s call. In some species, females make a release call to communicate to the males to stop mating with them. Biologist have name the species of frogs’ main vocalization an “advertisement call,” because it announces a frog’s presence. It is the same call to attract females and to deter males from approaching it. Some frogs vary the pattern of their calls in different contexts.

Behavioral ecology

Frogs are more often heard than seen, and humans, as well as other frogs rely on their calls to identify them. Depending on the region that the frog lives in, certain times of the year are better for breeding than others, and frogs may live away from the best breeding grounds when it is not its species’ mating season. During the breeding season, they congregate to the best breeding site and compete for call time and recognition. Species that have a narrow mating season due to ponds that dry up have the most vigorous calls. Long,(1999).

Calling strategy

Male-male competition

In many frog species only males call. Each species has a distinct call, though even among the same species, different dialects are found in different regions. Although humans cannot detect the differences in dialects, frog distinguish between regional dialects. For example, male bullfrogs can recognize the calls of their direct territorial neighbors. By ignoring the calls of these neighbors they save energy, and only vocalize aggressively with to an intruder’s call. In this way, calls establish territories, but they also attract females. Long,(1999). Males may have a solitary call for times when there is no competition that uses less energy. During other times, when a frog must compete with hundreds or thousands of other frogs to be heard, together they perform a chorus call where each frog calls in turn, successively. The most important feature of the chorus is the shared pattern. Through this pattern, few individuals calls are drowned out. One frog’s call may be dominant and trigger the calls of the responding frogs in symphony. Interestingly, calling is linked to physical size and females may be attracted to more vigorous calls. Long,(1999). Frogs in the same region chorus within their species and between different species. Frogs of the same species will retune their frequency so it is distinct from other frogs of the same species. Different species of frogs living in the same region have more dramatically different call frequencies (Narins). The frequency and durations of different species' calls vary similarly to the preference of that species' females. The neural circuity of females of different species varies.

Male-female interactions

Like, the males, females can distinguish the minute differences between individual frogs. However, males and females are attuned to different parts of the advertisement call. For example in co qui’s, males are more attuned to the low frequency “co” and females more attuned to the high frequency “qui.” Narins and Capranica (1980). In fact, the order of the parts does not matter. Similarly in the tungara frog, the female basilar papilla is biased towards a lower-than-average “chuck” portion of a male call (McClelland, Wilczynski, and Rand). Experiments that measure the vocal responses and approaches shows these attenuations.

Mode of sound communication

Calls are often sent through the air, but other mediums have been discovered. Some species call while they are under water and the sound travels through the water. This is adaptive in a region with many species competing for air time. Narins has found female frog species that use solid surfaces, such as blades of grass and logs, upon which they tap rhythmically to attract mates. Also, Feng has found that some species of frogs used ultrasound.

Sound production

The smallest frogs must consume lots of energy to produce calls. In addition, vocalizing muscles can make up 15% of a male spring peeper’s body mass, while the same muscles are only 3% of females. Frogs produce sound from the air sac below their mouth that from the outside, is seen to inflate and deflate. Air from the lungs is channels to the air sac. The air sac resonate the sound to make it louder. The larynxes of males are larger and more developed in males, though not significantly different from females’ (McClelland, Wilczynski, and Rand).

Sound perception

  Frogs have satisfactory hearing capabilities with a range of roughly 400 to 4000 Hz. Frogs’ ears are located on the sides of their heads, behind their eyes, and look remarkably different from humans’ external ears. The often translucent membrane is an eardrum or a tympanum. Long,(1999). Female tympanic membranes are larger in diameter than males of some species (McClelland, Wilczynski, and Rand). This may be linked to their reliance on sound to find mates.

Neurobiology

The inner ear has two sound-receiving areas that contain specialized hair cells, tuned to different frequency ranges. One of these areas, called the amphibian papilla, contains hair cells tuned to low and medium frequencies and the other area, called the basilar papilla, contains hair cells tuned to high frequencies. The area of the thalamus where complex auditory processing occurs is called the auditory thalamic area. When both high and low frequency hair cells are stimulated, then a smaller response occurs in the thalamus. This shows that too many sounds at once are less useful to the frog than a specialized response. Mudry & Capranica, (1987). Which level of frequency the frog wants to focus on is additionally controlled by a unique structure in the frog’s inner ear. When the frog is focusing on low frequency sounds being picked up from the ground’s vibrations, it readjusts the inner ear by linking it to the skeleton. The sound is propagated through its body, via the feet and shoulder bones to the auditory nerve. This helps the frog detect large terrestrial predators. Conversely, when the frog is focusing on high frequency sounds, such as calls from other frogs, especially in tree frogs, muscles in the middle ear adjust the inner ear so it can ignore low frequency noises that distract it when it is seeking a mate. Long, (1999).

Sound localization

Biologist believed that frogs ears are placed too close together to localize sound accurately. Frogs cannot hear short, high frequency sounds. Sound is localized by the time difference when the sound reaches each ear. The “vibration spot” near the lungs vibrates in response to sound, and may be used as an additional measure to localize from. Long,(1999).

Applications of Frog Neuroethology

Dr. Feng’s work applies the neuroethology of frog communication to medicine. A recent project on hearing aids is based on how female frogs find their mates. Females must recognize the male they choose by his call. By localizing where his call is coming from she can find him. An additional challenge is that she is localizing his call while listening to the many other frogs in the chorus, and to the noise of the stream and insects. The breeding pond is a very noisy place, and females must distinguish a male’s calls from the other noise. How they recognize the sound pattern of the male they are pursuing from the surrounding noise is similar to how intelligent hearing aids help people hear certain sounds and cancel out others. The underlying neural mechanisms are fast neural oscillations, and synaptic inhibition to cancel out noise. The timing and frequency of the sound also play a part in frog communication and may be used in Feng’s work. He also studies bat echolocation to create intelligent hearing aids. He is also working on cochlear implants. Feng, 2007.

See also

• Neuroethology
• Frogs
• Umwelt
• Vision in toads
• Animal echolocation

References

• Capranica, Robert R. (1965) The Evoked Vocal Response of the Bullfrog. MIT PRESS, Cambridge, MA. (110p. )
• Albert S. Feng. Neuroscience Program University of Illinois at Urbana-Champaign. 17 Dec 2007 *Long, Kim. Frogs A Wildlife Handbook. Boulder, Colorodo: Johnson Printing, 1999.
• Mundry, KM, and RR Capranica. "Correlation between auditory evoked reponses in the thalamus and species-specific call characteristics. I Rana catesbeiana." Journal of Comp Physiology 160(1987): (4):477-89.
• McClelland,BE., W. Wilczynski, and AS. Rand. Department of Psychology, University of Texas, Sexual dimorphism and species differences in the neurophysiology and morphology of the acoustic communication system of two neotropical hylids.
• Narins, PM, and RR Capranica. "Neural adaptations for processing the two-note call of the Puerto Rican treefrog, Eleutherodactylus coqui." Brain Behavioral Evolution 17(1)(1980): 48-66.