The Odorrana tormota is the first amphibian known to communicate in ultrasound. Source: Wikipedia
On Friday, Professor Peter M. Narins from the UCLA’s department of Integrative Biology & Physiology and Ecology & Evolutionary Biology presented his findings on ultrasonic communication in amphibians. Prior to Narins’s research, no amphibians were known to detect or emit ultrasound.
The first frog found to communicate in ultrasound was the Odorrana tormota. This clear- chinned amphibian is a distant relative of the bull frog but is distinct for its clear and recessed tympanic membrane. The tympanic membrane is a thin membrane behind the frog’s eyes that separates the outside from the frog’s inner ear. It also converts vibrations in the air to vibrations in the fluid.
Narins believes that the Odorrana tormota developed ultrasonic communication because of its loud environment. The Tau Hua creek of Anhui province, China, where the Odorrana tormota resides, produces intense background noise at all levels of the human audible range. This makes communication difficult unless adaptations are made. The Odorrana tormota is unique, for many other species of frogs have in similarly loud environments adapted visual cues and communications instead.
The Odorrana tormota is morphologically adapted to receive and produce ultrasound. The frog’s recessed ear drum allows its middle ear bone to be shorter than other frogs’. As a result, the shorter and less massive middle ear bone can respond to high frequency waves. Also, the Odorrana tormota’s transparent tympanic membrane is thinner by 3-4 micrometers. The thinness and less massive tympanic membrane allows the hearing organ to vibrate in response to higher frequencies.
In order to protect its thinner tympanic membrane, the Odorrana tormota is also the first amphibian found with a closing Eustachian tube. The Odorrana tormota’s diet includes many long, spindly insects that could puncture its thin tympanic membrane during consumption. By closing its Eustachian tube, the Odorrana tormota closes the pathway connecting its mouth to its ear. The Odorrana tormota can close its Eustachian tube because its skull hyoid connection rotates, while other frogs possess solid skull hyoid connections.
It is less clear how the frog is able to produce its ultrasound croaks. Narins and his colleagues have found that steadily increasing the pressure of air through the frog’s larynx produces the Odorrana tormota’s complex call. This means that very little brain activity is needed to reproduce the ultrasonic croak, as the larynx has immense structural complexity.
While in China studying the Odorrana tormota, Narins found that the Rufus- faced warbler also produces ultrasonic calls; this would be the first bird recorded at such high frequencies. Several moths in the same region were found to respond to the Odorrana tormota’s ultrasonic calls, demonstrating their ability to hear above human levels. Another frog, the Odorrana livida, lives in the Tau Hua creek region. This frog was found to also communicate in ultrasound but at lower levels than the Odorrana tormota. The Odorrana tormota hears up to 34,000 Hertz while the Odorrana livida hears up to 22,000 Hertz. Any sound above 20,000 Hertz is considered ultrasound, as it is above the threshold of human hearing.
After China, Narins studied the Huia cavitympanum in the Nyips River of Borneo, Malaysia. The Huia cavitympanum and the Odorrana tormota are not closely related, yet the Huia cavitympanum also communicates in ultrasound. Narins hypothesizes that the Huia cavitympanum adapted higher frequency communication because of the broad ambient noise from its environment. The two animals may be a case of convergent evolution.