The two populations of humpback whales in the southern Indian Ocean have complex, and distinct, songs, sharing just one of the so-called song themes.
Humpback whales living on different sides of the southern Indian Ocean bellow very different songs, suggesting the behemoths don't mingle much, or at least they aren't freely sharing their musical material, a new study finds.
The results, published in the January issue of the journal Marine Mammal Science, give scientists more information about how culture in the form of these songs spreads among these whales that can reach lengths of 50 feet (15 meters).
Among humpback whales (Megaptera novaeangliae), males are the typical crooners, singing their tunes at a population's winter breeding grounds, along migration routes and at summer feeding grounds. The songs consist of complex arrangements of so-called themes, which are mixes of wails, moans and shrieks that get repeated in cycles lasting up to 30 minutes, according to the researchers.
Like disc jockeys fading out one song while starting another, humpback whales also transition between song themes; these transitional "phrases" combine bits from both the preceding and subsequent themes. (A past mathematical analysis revealed just how complex these songs are, using grammatical rules to string together hours-long melodies.)
While past research has suggested humpbacks sharing the same ocean basin also share similar songs, the new study suggests that's not the case for the two populations on both sides of the southern Indian Ocean.
The scientists used hydrophones to record humpback whale songs from 19 individuals in two spots along the coast of Madagascar and three areas along Western Australia during the 2006 breeding season. The team captured more than 20 hours of whale song (either whole song cycles or fragments of songs). Their analysis revealed a total of 11 different themes in both regions, with just one theme shared by both populations. [Video of the Whale Songs]
"Songs from Madagascar and Western Australia only shared one similar theme, the rest of the themes were completely different," said lead author Anita Murray, who conducted the research while a graduate student at Columbia University and the Wildlife Conservation Society.
Murray and colleagues say the reason for the song anomaly is a mystery.
"It could be the influence of singing whales from other ocean basins, such as the South Pacific or Atlantic, indicating an exchange of individuals between oceans which is unique to the Southern Hemisphere," said Murray, who is currently a doctoral student at the University of Queensland in Australia.
The researchers note the findings are limited by the fact that they are based on just one breeding season, and further research is needed to elucidate the reasons behind the whales' distinct songs.
As for why the two populations share one song, the researchers speculate that "some number of males from Madagascar and Western Australia could potentially be in acoustic contact during the feeding season and during this contact cultural transmission in song content would occur," they write in the journal article.