Huia Hits the High Notes
When it comes to the senses, everything is relative. The world that we perceive every day comes to us through the filter of our sensory organs, neural pathways, and other physiological mechanisms for distinguishing between different sound frequencies, wavelengths of light, and other stimuli. Thus, the world that we live in appears to be a different place than, say, the world of a bumblebee, a bat, or my German shepherd.
A new paper in PLoS ONE provides a reminder of the fact that many animals communicate in ranges that are ouside of our slice of the sensory spectrum. Arch et al. (2009) report their findings that Huia cavitympanum, a frog found exclusively in Borneo, emits vocalizations that are entirely ultrasonic. (Ultrasonic, for humans, being anything > 20 kHz). While mammals are known to communicate ultrasonically (bats and rodents being the most common examples, but also think of high frequency “dog whistles”), non-mammalian vertebrates are generally thought to have limited abilities to detect high-frequency sound.
Emerging evidence suggests exceptions amongst tropical anurans, however. The first species to be shown to emit and perceive high frequency sounds was the frog Odorrana tormota, but the dominant frequency of these calls is still within the audible range. This is where H. cavitympanum is unique: Arch et al. showed that a subset of their calls are purely ultrasonic. This suggests that this is the first non-mammalian vertebrate shown to use exclusively ultrasonic signals to communicate. The authors used playback experiments, presenting male frogs with calls of different frequencies, and measured vocal responses. They found that the males responded selectively to ultrasonic calls of conspecifics.
Examination of the hearing mechanisms of both H. cavitypanum and O. tormota has shown that the state (open or closed) of the Eustachian tubes determines sensitivity to high frequencies, but that H. cavitympanum lacks the ability to close its tubes, which could explain the exclusive use of ultrasonic communication shown in this study. It is interesting that the ultrasonic calls make up only a small portion of the species’ vocalizations, but it appears at their auditory systems are most sensitive to these high frequencies.
Why, you may wonder, would a frog need to communicate in an extremely high-pitched voice? For small organisms, high-pitched calling is the most energy-efficient way to emit vocalizations. Organisms that call to advertise themselves to mates and competitors can “afford” to emit longer, louder calls if they use optimum frequencies. This is tricky, however, because high frequencies attenuate over short distances. There are also some features of H. cavitympanum’s environment that could serve as clues as to the adaptive significance of this unique vocalization style. These frogs tend to live in areas where rushing water is a constant background noise, so using ultrasonic communication can overcome the low-frequency auditory clutter from the environment.
The authors conclude that H. cavitympanum and O. tormota have converged on the ability to use ultrasonic sounds. If this has evolved twice, chances are it can do it again, so narrowing down mechanisms and selective pressure for the development of this communication style could give us clues as to where to look for additional species that are chattering at high-frequencies. Shall we get philosophical? . . . if a frog calls in the forest, but a human doesn’t hear the sound, does it really make a noise? Apparently, yes!