Non-song social call bouts of migrating humpback whales

Abstract

The use of stereotyped calls within structured bouts has been described for a number of species and may increase the information potential of call repertoires. Humpback whales produce a repertoire of social calls, although little is known about the complexity or function of these calls. In this study, digital acoustic tag recordings were used to investigate social call use within bouts, the use of bouts across different social contexts, and whether particular call type combinations were favored. Call order within bouts was investigated using call transition frequencies and information theory techniques. Call bouts were defined through analysis of inter-call intervals, as any calls within 3.9 s of each other. Bouts were produced significantly more when new whales joined a group compared to groups that did not change membership, and in groups containing multiple adults escorting a female and calf compared to adult only groups. Although social calls tended to be produced in bouts, there were few repeated bout types. However, the order in which most call types were produced within bouts was non-random and dependent on the preceding call type. These bouts appear to be at least partially governed by rules for how individual components are combined.

FIG. 1.

Histogram of SNR measurements from the calls with the highest SNR for a subset of included and excluded bouts. The bimodal distribution centered on 10 dB support the exclusion value of 10 dB for bouts with no calls above 10 dB.

FIG. 2.

Cumulative frequency distribution estimated through the MLM method (a) was a better fit with observed data than the log frequency method (b). The gray line—expected distribution and the black line—observed distribution.

FIG. 3.

Spectrogram of two call bouts determined by the BEC of 3.9 s. The two call bouts are separated by ∼13 s and are therefore two separate bouts. Bout 1 starting at 1.5 s is composed mostly of two of the most common call types in bouts—a yelp is followed by a yap, which is repeated and followed by another yelp and then a wop at 3 s. The two call bouts are labeled in the spectrogram Bout 1 and Bout 2.

FIG. 4.

Spectrograms of (a) purr,roar call bout type and (b) a bout of four bop call types followed by a snort followed by another bop. The calls are labeled above the relevant sound in the spectrograms. Note the difference in time scales.

FIG. 5.

Mean (±standard error of the mean) numbers of (a) all bouts, (b) short bouts, and (c) long bouts produced per 5 min sampling period during stable, joining, and splitting group behavioral states in AA, FC, FCE, 2F2CEE, and FCEE groups. Note: y axes are on different scales.