The results of this study indicate that some humpback whale non-song call types in Southeast Alaska are conserved in the acoustic repertoire at the decadal timescale. Twelve of the 16 call types were present in both the 1976 and 2012, demonstrating the persistence of these calls over 36 years. All other call types persisted over multiple decades between the 1997 and 2012. The absence of identical call types in recordings across the four decades does not necessarily indicate that certain call types were absent from the repertoire, since it is unlikely that the non-song calls aggregated for this study comprise the entire vocal repertoire of the entire population at any given time. Limited sampling effort (Table 1) might favor the recording of some call types over others; nonetheless, within the scope of this dataset the overall pattern of long-term conservation of call types within the acoustic repertoire is clear.

Demographic data are consistent with the assertion that stability at this temporal scale indicates call types are conserved across several generations. Given that females mature by the age of 13 and typically reproduce every 1–3 years55,56,57, new whales in our survey regions were born, reached sexual maturity, and gave birth to offspring that subsequently grew to sexual maturity and also gave birth over the 36-year duration of this study58. This is reflected in the dramatic increase in population size from the 1970’s to the 2010’s, which was estimated in 2008 to be more than five times as large as in 197952,53. Demographic studies show that population growth in Southeast Alaska is primarily due to long-term maternally directed site fidelity and birth54,55,59,60 with little evidence of immigration into the feeding areas of the population59. In further support of this point, in Glacier Bay National Park, where humpback whale monitoring efforts have been underway since the 1970’s, almost half of the humpback whales first identified as calves returned to their maternal foraging grounds55. Thus, it is quite likely that throughout the study period multiple generations overlapped spatially and temporally within the survey region and were recorded by our hydrophones.

In this study, some acoustic parameters varied significantly, though non-linearly, across decades. Most vertebrate sounds demonstrate within-call variation, related to individual anatomy, behavioral or environmental context (see Fig. 2 for a visual example from this study), which does not contradict placement into call classes or types19,61. In the eastern Australian humpback whale population call types classified with high degrees of confidence also exhibit fine-scale acoustic variability over time37. In the same population, acoustic parameters of calls also varied with social context, though call type assignment was robust48. Similarly, Deecke et al.61 demonstrated changes in acoustic parameters of killer whale calls, although the overarching call structure remained stable61. Thus, although there were observed differences in acoustic parameters between decades, the persistence of the call types themselves in the acoustic repertoire should be considered stable.

There were some prominent trends in fine-scale acoustic behavior between years. Calls from 1997 were consistently higher and shorter than other time periods and calls of almost all types grew longer from 1997 to 2012. We offer a few possible explanations. Individuals may have adjusted vocal parameters in response to ambient noise conditions; to avoid acoustic masking individuals must either increase their calling amplitude or spectrally or temporally shift their vocalizations62. Alternatively, and equally plausibly, changes in acoustic parameters may reflect differences in social context rather than change over time. According to motivational-structural rules, mammalian vocal sounds encode information about a sender’s motivational state, enabling a receiver to assess the likelihood of certain behaviors occurring63. It has been suggested that motivational state is encoded in humpback whale calls on migratory corridors46. In eastern Australia humpback whales used call types that were higher in frequency at times associated with increased arousal levels (i.e. affiliating groups for mid-level arousal, groups of competing males for high arousal)46,48. In the present study, recordings made in 1997 were typically made in association with large groups of whales engaged in coordinated foraging events. In contrast, despite dedicated observer effort coordinated foraging was not observed in 2007 or 200864,65, and whales in 2012 were typically foraging alone, in low densities, and in some cases were vocalizing in isolation41,48. Coordinated foraging is likely to increase arousal and necessitates fine-scale cooperative interactions between individuals66. This indicates a difference in audience and plausibly a difference in motivational state between datasets in the different decades. High-frequency, short-duration vocalizations that are frequency-modulated are thought to have an ‘appeasing effect’ on receivers63 and have been associated with groups of affiliating humpback whales46. Consistent with this theory, calls from 1997, where groups of animals were engaged in complex coordinated activities, were generally higher, shorter, and more frequency modulated than in decades where social interaction was more limited during recording periods.

Our result, that calls grew generally longer over the duration of this study, corroborates findings in other mysticetes, where call duration increased over time, possibly in response to elevated ambient noise33,58,67. However, statistically significant temporal differences in call features may or may not have practical significance. For example, PC2 values, which related to temporal features, were lower for droplet and teepee calls produced in the 2012 than in the 2000’s, indicating that calls in 2012 were longer in duration; the differences in mean duration, however, were only a fraction of a second. Because these calls are pulsed and often occur in short bouts36,68, a difference in a fraction of a second may reflect a longer duration between call units, or could be an artifact of temporal smearing with distance from the hydrophone. While every effort was made to account for differences in environmental conditions and recording units, the temporal parameters of a call can still be affected by attenuation, reverberation, and other propagation effects. Should these very fine-scale measurements be indicative of true variation in temporal characteristics, they still may not represent enough biological variability to be detectable, or meaningful to a receiver. Additional research into auditory discrimination in humpback whales would be extremely valuable.

In the absence of calibrated ambient sound recordings, demographic information, and fine-scale behavioral sampling, it is not possible to quantify whether motivation, ambient noise conditions, or individual variability contributed most to changes in calling behavior. Future investigation into acoustic variability as a function of social context and ambient noise on feeding grounds would be useful for testing hypotheses about acoustic communication in foraging humpback whales.

The selective pressures that maintain and shape characteristics of communication signals are dictated by social structure13,69. Species like killer whales that live in discrete family units with little or no natal dispersal exhibit temporally stable and stereotyped pod-specific vocalizations70,71,72. Bottlenose dolphin societies characterized as ‘fission-fusion’, with social affiliations ranging from ephemeral to long term, employ stable signature whistles to convey individual identity73,74. We have demonstrated that humpback whales also produce call types that persist across decadal time scales. Given the evidence of long-term affiliation between humpback whales at high latitudes57,75,76 we suggest that some call types may be used communicate identity over time and space. Acoustic identity cues are common across taxa (e.g. northern fur seal (Callorhinus ursinus) mother-pup recognition calls77, king penguin (Aptenodytes patagonicus) contact calls78, Mexican free-tailed bats (Tadarida brasiliensis mexicana) isolation calls79; see Tibbetts & Dale, 2007 for a review80), and – as evidenced by savanna elephants – can be valuable among far ranging social animals15,16. Some of the most commonly documented call types identified in this study, whups and growls, have been frequently identified elsewhere38,39,49, and may act as contact calls34,45. These call types are acoustically variable, and highly persistent over time on both migratory corridors and foraging grounds; this variability may reflect signature information used to confer identity over time. We propose that these call types are good starting points for investigating individual variation and recognition in this species.

Calls may also persist in the vocal repertoire in Southeast Alaska because they are functionally specific to foraging activities in this region. Feeding calls are behaviorally linked to foraging on Pacific herring (Clupea pallasii)41,51,60 and are closely matched to hearing capabilities of this prey, which are most sensitive in the 200–500 Hz range81. Under experimental conditions, playbacks of feeding calls have elicited a “flee and clump” response in Pacific herring, which presumably increases the whales’ foraging efficiency82. Feeding calls are most commonly documented among groups of whales, where they may coordinate individuals; however, they are also produced by solitary animals, and may serve a prey manipulation function in this context41. This would explain both the call stereotypy – feeding calls were correctly classified 98% of the time in this study (Fig. 2, Table 4) – and also differences in call parameters between decades, which are likely related to social context (e.g. solitary foragers versus group foragers) or prey behavior (e.g. school size, school location). Although this call type does not exclusively serve a social function, it is likely to persist within the humpback whale repertoire because it is closely coupled with prey biology and provides a direct benefit to the individual producing it.

Lastly, humpback whale calling behavior in the feeding grounds stands in stark contrast to singing behavior. The composition and structure of song changes within the breeding season resulting in progressive seasonal and inter-annual change21,22,28, whereas this study and the work of Rekdahl et al.37 using 11 years of audio data from the east Australia migratory corridor, make it clear that portions of the call repertoire persist with time37. Importantly, in the east Australian population stable call types were not used as song units in adjacent years37, implying – similar to this study where song was not documented concurrently with calls – that calls function independently of song. Finding this commonality across ocean basins and in contrasting portions of humpback whale migratory range is quite telling. We propose that conservation of call types may be as important to call function as novelty is to song function and encourage future investigations into humpback whale calling behavior to include this hypothesis.