By reptile standards, alligators are positively chatty: according to Hilary Bierman and colleagues, ‘They are the most vocal of the non-avian reptiles’. The team adds that the animals are known to be able to pinpoint the source of sounds with accuracy, but it wasn't clear exactly how they did it. ‘Different vertebrate lineages have evolved external and/or internal anatomical adaptations to enhance these [auditory] cues, such as pinnae and interaural canals’, says Bierman, from the University of Maryland College Park, USA. However, alligators lack the external structures and it wasn't clear whether they had interaural canals linking their ears. Collaborating with a group of international scientists from the University of Massachusetts-Lowell, USA, the University of Colorado School of Medicine, USA, and the University of Southern Denmark, Bierman investigated the mechanisms that alligators use to locate sounds (p.1094).
First, the team tested how sound travelled around the animals' heads to see whether they somehow channel sound to help them locate the origin, listening for the minute time and volume differences between the sound arriving at the two ears, but the team found no evidence that the animal's body alters sound transmission sufficiently for the animal to be able to detect the difference. And when the team measured the alligators' brainstem responses to sounds, they were too fast for the animals to be sensing the time difference between a sound arriving at the two ears.
Next, the team looked for internal structures in the alligators' heads that might propagate sound between the two eardrums. ‘Acoustical coupling produces directional responses at the tympanum’, says Bierman. She explains that sound reaches both sides of the eardrum – travelling externally to reach the outer side and through head structures to the internal side – to amplify the vibration at some frequencies when the head is aligned with the sound. This maximises the pressure differences on the two sides of the eardrum, magnifying the time difference between the sound arriving at the eardrum via two different paths to allow the animal to pinpoint the source. Viewing slices through the heads of young alligators, the team could clearly see two channels linking the two middle ears that could transmit sound between the two eardrums. They also measured differences in the alligator's brainstem responses to sounds depending on their location. And when the team looked at the eardrum's vibration, they could see that it was amplified at certain frequencies, as they would expect if alligators use the pressure difference at the eardrum for orientation.
Assembling all of the evidence together, Bierman and colleagues suggest that the reptiles rely on magnified time difference at the eardrum to locate noises. They also suspect that this is the mechanism that the archosaur ancestors of modern crocodilians and birds used to pinpoint sounds.