A new look at universals and specificities in the songs of humpback whales.

Whale song

The complex songs produced by humpback whales have been cited as evidence of prodigious memory, innovativeness, sophisticated auditory scene analysis, vocal imitation, and even culture. Researchers believe humpbacks learn their songs culturally because songs appear to change rapidly, consistently, and irreversibly across whales within a population. Here, we present evidence of similarities in song structure both across populations and decades that strongly challenge claims that social learning is the main driver of variations in humpback whale songs over time. Groups of humpback whales that were not in acoustic contact (recorded in Puerto Rico in 1970, Hawaii in 2012, and Colombia in 2013-2019) produced songs in acoustically comparable cycles, suggesting that progression through sound patterns within and across songs is not simply determined by vocal imitation of innovative patterns, but may instead be controlled by production templates that prescribe how singers construct and transform songs over time. Identifying universal constraints on song production is critical to evaluating the role of vocal imitation and cultural transmission in the progressive changes that humpback whales make to their songs and for evaluating the functional relevance of such changes. The current findings illustrate how information theoretic analyses of vocal sequences can potentially obscure key acoustic qualities of signals that may be critical to understanding how vocalizers produce, perceive, and use those sequences. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Songs of humpback whales off Hawaii and Mexico were examined to determine whether they changed similarly in both areas during a breeding season. Songs of 24 individuals were recorded off Kauai, Hawaii and Isla Socorro, Mexico during winter and spring of 1991. Forty-seven acoustic variables describing all levels of song structure were measured for each singer. Similar variables were grouped together into six categories. Mean values for each singer were compared among regions and time periods using two-factor ANOVAs. All but three variables changed between winter and spring in at least one area. Groups of similar variables displayed similar trends. Quantitative characteristics of song elements often changed during the breeding season by the same amount in each area, with little variation within and among individuals. Structures of song patterns often changed differently in each area. Results indicated cultural transmission may have occurred during the season. Alternatively, whales may be predisposed to gradually change features of song independent of cultural influences. Given the low level of variability exhibited by many variables, the latter seems more likely; therefore, temporal change, like song structure, may be governed by a discrete set of rules. Further research on a longer time scale is needed.

The songs of humpback whales (Megaptera novaeagliae), a patterned vocal display usually associated with breeding, serve as an excellent model of large-scale cultural transmission in a nonhuman species. These songs are considered to be particularly complex due to their well-known hierarchical structure and constantly changing nature. Two types of changes occur in the songs of the whales in the South Pacific meta-population: small, progressive evolutionary changes and radical cultural revolutions in which the song’s pattern is entirely replaced during a single breeding season. While patterns of song change and the transmission of these changes among populations have been studied on a broad scale, fine-scale examination of the song’s fundamental components (i.e. unit sequences) is lacking. This thesis aims to investigate fine-scale song structure in order to better understand song learning mechanisms. Firstly, intra-population song structures were examined within the east Australian song over time. Secondly, inter-population learning was assessed during the song’s transmission to the neighbouring population that winters off New Caledonia.Several quantitative methods were developed or applied here to identify song features at the unit sequence level. An acoustic dictionary of the song repertoire was created using a selforganising map, which quantified similarity between units and provided a robust means of classifying units. Metrics for measuring song complexity were modified to address the unique hierarchical structure of humpback whale song and quantify complexity at multiple levels in the song. An analysis of network structure, previously only applied to human language and birdsong, was used to measure the connectivity of units within song arrangements. These methods were applied to the transcription of songs over 13 consecutive years in east Australia and two consecutive years with two song types in New Caledonia, allowing for fine-scale features to be described and quantified. To understand song learning within a single population, fine-scale structural features of the east Australian song were quantified over the same period. The complexity of both theme and unit sequences fluctuated over time, showing a clear relationship with both progressive, evolutionary changes and revolutionary events. Songs became increasingly more complex as the songs evolved through small additions of novel material, but were simplified when cultural revolutions replaced the song pattern. However, these fluctuations were not reflected in underlying network structures or second order entropy of each song. Every song displayed a degree of “small-world” network structure characterised by clusters of highly connected units. Additionally, they contained fine-scale structures such as deterministic transition patterns (i.e. doublet and triplet repetitions), redundant unit usage, and predictable unit arrangements. These features were always present despite marked differences in songs from year to year and are likely to facilitate song learning. How structural features changed during transmission between east Australia and New Caledonia provided insight into inter-population social learning. Each individual theme maintained or increased its complexity during transmission, but the song’s arrangements and the underlying small-world structure remained largely unchanged. Songs were therefore learned across populations with high fidelity and without needing to be simplified. Such precision probably requires a mechanism of song transmission with a high degree of acoustic contact. Recent studies have suggested that such acoustic contact may occur on the shared Antarctic feeding grounds or individual movement between populations.This thesis is the first study to quantify the fine-scale structural and syntactic features of humpback whale song, and does so over multiple years and across two adjacent populations. These results indicate that song learning is probably influenced by a combination of conformity to an underlying structural template and small individual additions of novel material that increase complexity in the song’s arrangement. Widespread rules of song structure should be explored in future studies by examining these fine-scale song features in other populations or ocean basins. Features identified here such as small-world networks and central ‘hub’ units also occur in other complex vocal learning displays, such as birdsong and human language. These commonalities suggest a convergence of certain features in vocal learning across multiple taxa. Given how integral social learning is within human evolution and culture, comparative information on social learning across non-human species is a key factor in further understanding our own cultural evolution.

Temporal and geographical variation in songs of humpback whales, Megaptera novaeangliae: synchronous change in Hawaiian and Mexican breeding assemblages

Recurring patterns in the songs of humpback whales (Megaptera novaeangliae)

Much evidence for non-human culture comes from vocally learned displays, such as the vocal dialects and song displays of birds and cetaceans. While many oscine birds use song complexity to assess male fitness, the role of complexity in humpback whale (Megaptera novaeangliae) song is uncertain owing to population-wide conformity to one song pattern. Although songs change gradually each year, the eastern Australian population also completely replaces their song every few years in cultural 'revolutions'. Revolutions involve learning large amounts of novel material introduced from the Western Australian population. We examined two measures of song structure, complexity and entropy, in the eastern Australian population over 13 consecutive years. These measures aimed to identify the role of complexity and information content in the vocal learning processes of humpback whales. Complexity was quantified at two hierarchical levels: the entire sequence of individual sound 'units' and the stereotyped arrangements of units which comprise a 'theme'. Complexity increased as songs evolved over time but decreased when revolutions occurred. No correlation between complexity and entropy estimates suggests that changes to complexity may represent embellishment to the song which could allow males to stand out amidst population-wide conformity. The consistent reduction in complexity during song revolutions suggests a potential limit to the social learning capacity of novel material in humpback whales.

Cultural processes occur in a wide variety of animal taxa, from insects to cetaceans. The songs of humpback whales are one of the most striking examples of the transmission of a cultural trait and social learning in any nonhuman animal. To understand how songs are learned, we investigate rare cases of song hybridization, where parts of an existing song are spliced with a new one, likely before an individual totally adopts the new song. Song unit sequences were extracted from over 9,300 phrases recorded during two song revolutions across the South Pacific Ocean, allowing fine-scale analysis of composition and sequencing. In hybrid songs the current and new songs were spliced together in two specific ways: (i) singers placed a single hybrid phrase, in which content from both songs were combined, between the two song types when transitioning from one to the other, and/or (ii) singers spliced complete themes from the revolutionary song into the current song. Sequence analysis indicated that both processes were governed by structural similarity rules. Hybrid phrases or theme substitutions occurred at points in the songs where both songs contained “similar sounds arranged in a similar pattern.” Songs appear to be learned as segments (themes/phrase types), akin to birdsong and human language acquisition, and these can be combined in predictable ways if the underlying structural pattern is similar. These snapshots of song change provide insights into the mechanisms underlying song learning in humpback whales, and comparative perspectives on the evolution of human language and culture.

Much evidence for non-human culture comes from vocally learned displays, such as the vocal dialects and song displays of birds and cetaceans. While many oscine birds use song complexity to assess male fitness, the role of complexity in humpback whale (Megaptera novaeangliae) song is uncertain due to population-wide conformity to one song pattern. Although songs change gradually each year, the eastern Australian population also completely replaces their song every few years in cultural ‘revolutions’. Revolutions involve learning large amounts of novel material introduced from the western Australian population. We examined two measures of song structure, complexity and entropy, in the eastern Australian population over 13 consecutive years. These measures aimed to identify the role of complexity and information content in the vocal learning processes of humpback whales. Complexity was quantified at two hierarchical levels: the entire sequence of individual sound ‘units’; and the stereotyped arrangements of units which comprise a ‘theme’. Complexity increased as songs evolved over time but decreased when revolutions occurred. No correlation between complexity and entropy estimates suggests that changes to complexity may represent embellishment to the song which could allow males to stand out amidst population-wide conformity. The consistent reduction in complexity during song revolutions suggests a potential limit to the social learning capacity of novel material in humpback whales.

Singing humpback whales constantly modify their songs over hours, days, months, and years, throughout their adult lives. Intriguingly, humpbacks appear to vary songs in concert, with most singers in a population producing similar songs at any given time. The convergent vocal dynamics of singing humpbacks have convinced many that songs are vocal customs, passed from singer to singer through vocal imitation. This interpretation has recently been challenged, however, by the discovery that singers not in acoustic contact may sing highly similar songs, and also appear to change their songs along similar trajectories. How could singers that cannot hear each other culturally conform? Here, it is argued that the changes humpback whales make to songs are inconsistent with either communal copying or competitive improvisation. Instead, singers apar to be continuously morphing the acoustic properties of songs in predictable ways both within and across songs, even in the absence of cross-population interactions. There is no direct evidence that singing humpback whales learn songs by copying other singers. The fact that groups of singers change songs in similar ways is not evidence of vocal imitation, cultural transmission, or cultural evolution. So called “cultural revolutions” in humpback whale songs, which have been touted as the clearest and most impressive evidence of culture in any nonhuman animal, actually provide evidence against vocal culture in humpback whales. Vocal complexity and convergence can arise through mechanisms other than cultural transmission via vocal imitation, and in the case of humpback whales, genetic predispositions and ecological conditions may be more relevant to determining how singers collectively change songs over time.

Humpback whales (Megaptera novaeangliae) sing long, complex songs which continually change over time. In certain time periods the songs contain repeating patterns which structurally resemble human rhyming, and which, like human rhyming, tend to occur in highly rhythmical contexts. We speculate that humpback whales may use these repetitions as mnemonic devices, much as humans are thought to use rhymes. With this speculation in mind we examine 548 humpback whale songs from 7 years in the eastern North Pacific and 12 years in the western North Atlantic oceans. We find that rhyme‐like structures are most likely to occur in songs containing the most material to be remembered.

Some humpback whales migrate annually from Antarctic feeding grounds to the seas around the Tongan Islands to give birth and mate. The Tongan humpbacks are considered part of Southern Hemisphere Group V that splits during migration, some swimming to Eastern Australia and others to various Polynesian Islands. During this time long complex songs are produced. The song is thought to be a male breeding display and may serve either as an intra-sexual or an inter-sexual signal or both. It is in a constant state of change that occurs every season. Since these changes are directional they cannot be described by drift, and singers incorporate changes as they occur, thus song must be shared through cultural transmission. This investigation describes the cultural changes that occurred in 158 songs recorded from Tongan humpbacks through the 1990s. The rate of change differed within years, some themes were retained for as much as five years and others were lost after only two years. The farther apart the years the less similar are the songs, as in the humpback songs of the Northern Hemisphere. The largest number of changes seems to have occurred in the early 1990s where all themes seemed to have been lost and new ones originated. What initiates these changes remains speculative, but we assess some hypotheses in relation to humpback whale behaviour and cultural transmission in avian song.

Acoustic surveys of vocal animals can have significant advantages over visual surveys, particularly for marine mammals. For acoustic density estimates to be viable, however, the vocal output of the animals surveyed needs to be determined under a range of conditions and shown to be a robust predictor of abundance. In this study, the songs of humpback whales, one of the most vocal and best studied species of marine mammals, were tested as predictors of abundance. Two acoustic metrics, the number of singing whales and amount of songs produced, were compared with the number of whales seen traversing a study site on the eastern coast of Australia over an 18 year period. Although there were predictive relationships between both metrics and numbers of passing whales, these relationships changed significantly as the population grew in size. The proportion of passing whales that sang decreased as the population increased. Singing in humpback whales, therefore, is a poor predictor even of relative abundance and illustrates the caution required when developing acoustic survey techniques particularly when using social vocalizations.

Although the songs of humpback whales have been extensively studied, other vocalizations and percussive sounds, referred to as "social sounds," have received little attention. This study presents the social vocalization repertoire of migrating east Australian humpback whales from a sample of 660 sounds recorded from 61 groups of varying composition, over three years. The social vocalization repertoire of humpback whales was much larger than previously described with a total of 34 separate call types classified aurally and by spectrographic analysis as well as statistically. Of these, 21 call types were the same as units of the song current at the time of recording but used individually instead of as part of the song sequence, while the other 13 calls were stable over the three years of the study and were not part of the song. This study provides a catalog of sounds that can be used as a basis for future studies. It is an essential first step in determining the function, contextual use and cultural transmission of humpback social vocalizations.

A study of the humpback whale song in the Northwestern Hawaiian Islands (NWHI) and the Main Hawaiian Islands (MHI) during the 2009 season suggests that humpback whale song may be more variable than previously suggested. Data from five autonomous acoustic recorders deployed at locations in the NWHI and MHI were analyzed to compare the frequency of occurrence of song units by whales in the island chain. There appears to be a gradient of differences in song units throughout the Hawaiian Island chain, rather than the previously assumed, more discrete differences between breeding populations. Recordings from each site were randomly selected. Song units were classified as one of 23 units and counted to compare between sites. Changes in the frequency of occurrence in a few of the most abundant units suggest a gradual change throughout the island chain. However, this may be confounded by changes that occur throughout the season throughout the ocean basin. Further work examining the amount of variation both between and within humpback whale breeding populations should be conducted.