Great cormorants (Phalacrocorax carbo) can detect auditory cues while diving

Abstract

In-air hearing in birds has been thoroughly investigated. Sound provides birds with auditory information for species and individual recognition from their complex vocalizations, as well as cues while foraging and for avoiding predators. Some 10% of existing species of birds obtain their food under the water surface. Whether some of these birds make use of acoustic cues while underwater is unknown. An interesting species in this respect is the great cormorant (Phalacrocorax carbo), being one of the most effective marine predators and relying on the aquatic environment for food year round. Here, its underwater hearing abilities were investigated using psychophysics, where the bird learned to detect the presence or absence of a tone while submerged. The greatest sensitivity was found at 2 kHz, with an underwater hearing threshold of 71 dB re 1 μPa rms. The great cormorant is better at hearing underwater than expected, and the hearing thresholds are comparable to seals and toothed whales in the frequency band 1-4 kHz. This opens up the possibility of cormorants and other aquatic birds having special adaptations for underwater hearing and making use of underwater acoustic cues from, e.g., conspecifics, their surroundings, as well as prey and predators.

Amphibious hearing in a diving bird, the great cormorant (Phalacrocorax carbo sinensis)

“…It is unknown whether they use their sense of hearing underwater at all; for instance, to avoid sound-emitting predators, to track down soundemitting prey, to orient relative to ambient underwater sound sources, or perhaps even for underwater sound communication as suggested in a recent study (Thiebault et al, 2019). In-air auditory threshold curves of 8 species of diving birds have been determined using auditory brainstem responses (ABRs) (Crowell et al, 2015) but the underwater hearing ability of diving marine birds has so far been studied in only two species: the long-tailed duck (Clangula hyemalis; Therrien, 2014) performing only shallow dives and the great cormorant (Phalacrocorax carbo; Johansen et al, 2016;Hansen et al, 2017). The last two studies established that one diving cormorant specimen could detect underwater sounds, but the derived data were not sufficient to establish more objective psychophysical hearing thresholds that could be compared between the two media.…”

Gentoo penguins (Pygoscelis papua) react to underwater sounds

“…In addition, noise from bubbles released from the plumage while diving can potentially mask the bird's ability to hear the stimulus, but no bubbles were observed at the time of any of the playbacks. If we assume penguins have similar underwater hearing thresholds as the cormorant (about 70-75 dB re 1 µPa) [21], we expect the reaction threshold found in our study of some 115 dB re 1 µPa to be 45-50 dB above the audiogram. This is surprising low.…”

“…Underwater hearing has so far been studied in two species of marine birds, the long-tailed duck (Clangula hyemalis) and the great cormorant (Phalacrocorax carbo), both of which detect underwater sound [20,21]. Many marine birds actively chase prey such as fish during longer or shorter dives.…”

Avian phenotypic convergence is subject to low genetic constraints based on genomic evidence

“…Foot-propelled diving birds have morphologically and physiologically adapted to aquatic environments. For instance, auditory senses might be crucial for diving birds because dim light in deep water may obscure their vision for hunting; the great cormorant was found to have acute underwater hearing comparable to seals and toothed whales [ 53 ]. A foot-propelled diving ACG, KCNC1 , which may impact auditory neuron functioning in mice, may allow these birds to acutely hear underwater [ 54 ].…”