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ing to their vocalizations. In many species of seals that live in polar waters, both Arctic and Antarctic, males produce complex songs that can be a dominant part of the acoustic soundscape during the breeding season. Bearded seals in the Arctic (http://www.dosits.org/files/dosits/bseal1.mp3) and Weddell seals in the Antarctic (http://www.dosits.org/ files/dosits/wed222.mp3) produce trilling songs and har- bor seals in temperate latitudes produce underwater roars that function in defense of territories (http://www.dosits. org/files/dosits/hbseal.mp3).
Another important function of vocalizations among seals in- volves parental behavior. Seal pups are often born on crowd- ed beaches, and in many species, the mother leaves the pup so that she can forage at sea to gain nutrition for lactation. Here when the mother returns to the beach, the mother and pup face the problem of identifying one another. In many of the species tested, females make a pup attraction call that the pup learns to use to identify its mother and mothers learn to identify vocalizations of their pup (Trillmich, 1981). The vocal identification system along with visual and olfactory cues can help a mother and pup to reunite, even in a large beach with many mother-pup combinations.
Individual Identification in Dolphins
This problem of individual identification is just as impor- tant for cetaceans where strongly bonded individuals may routinely separate out of sight of one another. Unlike seals where the pup may wean after just a few months, young bot- tlenose dolphins typically suckle for 3-5 yr, and suckling has been reported in pilot and sperm whales older than 10 yr of age (Kasuya and Marsh, 1984). A newborn calf bottlenose dolphin (from now on, I am using “dolphin” for “bottlenose dolphin”) will swim very close to the mother, but by 1 yr of age, mother and calf frequently separate out of sight of one another for many minutes at a time. Dolphins maintain contact by using an individually distinctive signature whistle (Janik and Sayigh, 2013; http://acousticstoday.org/whistle). When wild dolphins separate, they are more likely to pro- duce signature whistles, and they increase their whistle rate at the maximum separation and as they reunite.
Dolphin whistles are tonal signals where the frequency is modulated from about 2-20 kHz, usually in a complex and distinctive pattern. Female dolphins in the wild develop a distinctive and stereotyped whistle by 1 yr of age, and this whistle is stable for decades. Males also develop a stereo- typed whistle by 1 yr of age, but as they mature and leave their mother, many will form an alliance with another adult
Figure 5. Examples of signature whistles of a bottlenose dolphin whose whistle was imitated (top row), of imitations of this signature whistle (middle row), and the signature whistle of the dolphin mak- ing the imitation (bottom row). The number in the upper left of each imitation is the average similarity score given by human judges on a scale of 1 (not similar) to 5 (very similar). From King et al. (2013).
male. As the alliance forms, the whistles of the alliance part- ners tend to converge and become more similar (Watwood et al. 2004), although they still retain individually distinctive features.
Dolphins do not just produce their own signature whistle, but they also produce a variety of other whistles. The func- tion of most of these is poorly defined, but there is a distinc- tive pattern where dolphins that share a strong bond, either male alliance partners or mother-calf pairs, may occasion- ally imitate one another’s signature whistle (King et al., 2013; Figure 5). After a dolphin produces its own signature whis- tle, it is more likely to whistle back to playback of a whistle that matches its signature whistle than to control whistles. These results suggest not only that dolphins use their signa- ture whistles to maintain contact, but they can also initiate an interaction by imitating the signature whistle of a partner.
This pattern of dolphins copying individually distinctive sig- nature whistles appears to involve the skill of vocal learn- ing described for humpback song, but it is difficult to prove with just observations of natural calls. Richards et al. (1984) trained bottlenose dolphins in the laboratory to imitate syn- thetic whistle-like signals generated by a computer. Their demonstration of imitation of a broad range of these arbi- trary signals demonstrated clear abilities for vocal learning of whistle sounds in dolphins.
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