Of all the reptiles, crocodiles are the most social and vocal. Adults hiss and bellow at each other. Babies chirp when they hatch. Crocodiles also listen for sounds to locate prey. A crocodile will lie nearly submerged in water for days to ambush a passing antelope.
Crocodiles’ hearing is most sensitive in the range of 100–3000 Hz, which on a grand piano corresponds to the middle of the second octave up to the middle of the seventh and highest octave. Crocodiles can detect frequencies as high as 8000 Hz.
Frequency affects which of two sound location cues crocodiles and other vertebrate species use. High-frequency sound is significantly attenuated by an animal’s head, with the result that a sound originating, say, from the right will register at a higher volume level at an animal’s right ear than at its left ear. Humans are among the species that use interaural level differences (ILDs) to locate sounds.
At lower frequencies, humans and other vertebrates use interaural time differences (ITDs). The crest of a sound wave can arrive at one ear before it reaches the other. Neural circuitry translates those microsecond phase differences into sound location cues—provided only one crest passes through the head at once. If the wavelength exceeds twice the diameter of an animal’s head, the phase difference is ambiguous and useless. (For more on ILDs and ITDs, see William Hartmann’s article “How we localize sound,” Physics Today, November 1999, page 24.)
Given the frequency range of crocodiles’ hearing, it’s conceivable that they, like humans and vampire bats, use both ILDs and ITDs. To find out, Léo Papet of the University of Lyon, Saint-Étienne in France and his collaborators designed and carried out an ingenious experiment. Their subjects were two young Nile crocodiles each about 60 cm in length from nose to tail. One of them is shown in the photo.
The experiment is conceptually simple: Put a crocodile in a tank and watch it react to and swim toward a sound source. In practice, deriving clear, quantitative results required several additional steps. First, the crocodiles had to be trained, with rewards of meaty morsels, to reliably swim toward a loudspeaker. To avoid the crocodiles merely looking for the loudspeaker, the training and subsequent experiments were conducted in the dark using an IR video camera. After seven weeks of training, the crocodiles located the sound and the meat 100% of the time.
Papet and his collaborators used three kinds of sound, all broadband chirps of 500 ms duration played four times over 11 seconds. One set of chirps, the control, included frequencies that the crocodiles could use for both ILDs and ITDs. The other two were truncated in range to limit their use to either ILDs or ITDs. The cutoffs in both cases were determined by anatomy. In crocodiles ITDs become less effective above 1000 Hz; ILDs become less effective below 2000 Hz.
To quantify the crocodiles’ ability to locate sound, the researchers used image processing techniques. In the footage from the video camera they virtually drew a line between each crocodile’s ears and then a perpendicular bisector to the tip of its snout. The principal experimental data consisted of the changing trajectory over time of the angle between the bisector and the direction toward the loudspeaker. As a crocodile approached the sound, the angle closed to near zero.
It turned out that the two crocodiles used both ILDs and ITDs to locate sound, but not to the same degree. Although their reactions to hearing the first ILD chirps and the first ITD chirps were just as prompt, the crocodiles typically took several seconds longer to swim to the source of ILD chirps than they did to the source of ITD chirps.
A delay of a few seconds could mean an antelope or other prey eludes capture. Papet and his collaborators conclude that ITDs are likely the predominant cues by which crocodiles locate sound. Their evident use of ILDs as well could arise from their need to locate broadband signals, like the chirps of hatchlings, when low frequencies are muffled by rushing water, rustling trees, and other environmental sources. (L. Papet et al., J. Acoust. Soc. Am. 148, EL 307, 2020.)