Conservation management relies on understanding the population structure of vulnerable species and the exchange of individuals among different populations (known as population connectivity). These factors can be difficult to determine in animals such as humpback whales, a marine species that migrate long distances with few barriers to movements. Genetic and photo-identification studies have helped researchers study connectivity among populations of this species, but such techniques require a great amount of effort and time, and often only collect information from a small number of individuals in the population. Acoustic studies are another tool, and one proving to be very useful for researchers studying humpback whales.
Whale songs are made up of complex sequences of vocalisations sung by male whales in a specific pattern. Songs can evolve between years, but at any one time every male in a population usually expresses the same version of the song (although with slight individual flair). Each population has its own unique song, but populations in close enough proximity to hear each other (for example, in the same ocean basin) have more similarities in their songs than distant populations that don’t have the chance to learn each other’s song segments (such as those in different ocean basins). Song similarities among populations may also occur if a whale from one population joins another and males adopt sequences of the immigrant’s song.
Melinda Rekdahl and collaborators studied song exchange among humpback whale populations breeding in Gabon and Madagascar. Males were presumed unlikely to move between the two locations within a single breeding season due to a substantial geographic barrier- the African continent- between them. Any similarity in song would, therefore, suggest the two populations had been close enough to hear each other at other times. After collecting five years of song recordings, the researchers calculated that song similarity varied from ~20% to 100%, depending on the year. Such a high degree of similarity in some years could result from song exchange during periods of population connectivity, such as years when populations were in relatively close proximity in their feeding grounds or along migration pathways. Exchange of whales while feeding or migrating could occur during long migrations along similar pathways to reach find patchy, or even limited, food and would indicate a potential threat to the humpback whale populations. The factors driving population connectivity and song exchange between geographically distinct humpback whale populations will be better understood with a longer study of more individuals, and potentially include comparisons with the songs of even more distant populations.
Rekdahl, M.L., E.C. Garland, G.A. Carvajal, C.D. King, T. Collins, Y. Razafindrakoto & H. Rosenbaum. 2018. Culturally transmitted song exchange between humpback whales (Megaptera novaeangliae) in the southeast Atlantic and southwest Indian Ocean basins. Royal Society Open Science 5: 172305. http://dx.doi.org/10.1098/rsos.172305.
(Title from lyrics of “Whale Song” by Partridge Family.)