The production and exchange of sperm whale coda vocalizations

Abstract

The sperm whale (Physeter macrocephalus) is a highly social animal that exchanges patterns of clicks ('codas') amongst members of long-term social units. Due to the difficulty of assigning codas to individual whales, little was known of the coda production of individuals or the exchange of vocalizations between them.

To address this problem, I developed a method for assigning recorded codas to whales using differences in the inter-pulse intervals of clicks. Applying this method to recordings of two social units - one studied off Dominica, the other off the Galapagos Islands - revealed that sperm whales produced codas at regular 3-5 s intervals. Furthermore, whales synchronized their vocal output, resulting in duet-like sequences of overlap exchanges. I suggest that these exchanges affirm social relationships between unit members.

The coda repertoires of both studied units were highly redundant, indicating little potential for syntactic communication. Moreover, whales in both units tended to repeat one particular coda type, both in the sequences of individuals and in overlap exchanges. Given that units preferentially associate with other units with similar coda repertoires (i.e. from the same acoustic clan), I suggest that the repetition of a particular coda type functions to bond unit members and/or communicate clan affiliation.

Most whales in the Dominican social unit had similar coda repertoires, again suggesting that coda types function to advertize clan identity. However, the repertoires of two whales - a male calf and its mother - differed from those of others in the unit, including each other, suggesting that their repertoires signal individual identity. While adult females in the unit vocalized at approximately equal rates, the calf and a juvenile male vocalized less often. This is likely because they did not engage in overlapping exchanges, interactions that may have a social bonding function unnecessary for males that leave their natal unit.

To examine the distance between vocalizing whales, I developed an acoustic array consisting of remotely-piloted vessels. Deployment of the array in the Sargasso Sea in 2004 revealed that sperm whales engaged in overlapping coda exchanges both with nearby and comparatively distant whales. This again supports a social bonding function for overlapping exchanges. The acoustic array was also used to show that the waveform structures of coda clicks, like echolocation clicks, vary with the recording position relative to the orientation of the whale.

In summary, this thesis suggests that coda production functions to affirm social bonds between unit members (via coda overlapping and matching), advertize clan identity, and advertize individual identity for animals with an increased need to do so.