Humpback whales can be found all over the world's oceans, despite the heavy beating they took in the era of industrial whaling. They undertake one of the longest annual migrations feeding at high latitudes and breeding near the equator. But there's one peculiar group of humpbacks who live life a bit differently.

Above: A whale nicknamed "Spitfire" swims in the waters off the island of Hallaniyah in southern Oman. Photo copyright Tobias Friedrich, used with permission.

Because humpbacks prefer to migrate along coastlines - perhaps in part to protect their young from predators - they were easy picking for whalers. It's only been in the last few decades, since whaling was banned in most of the world, that populations have begun to recover enough for researchers to understand their genetics. It turns out that the world's humpbacks are not a singular grouping, but sort along migration pathways. That's because the ocean giants have extreme site fidelity, meaning they feed and breed in the same places year after year. That's true for the breeding groups in the North Pacific, North Atlantic, and Southern Oceans.

Advertisement

But there's another group of humpbacks in the Arabian Sea. And it's only recently that scientists have been able to understand just who they are. They think that they form a population of humpbacks separate from every other population in the world.

A whale breaches close to the Southern Omani town of Hasik – an Arabian Sea hotspot favored by breeding whales. Photo copyright Darryl MacDonald, used with permission.

Here's why: for one thing, their breeding cycle is more like that of the Northern Hemisphere populations, which would be odd if they were connected with the rest of the Southern Hemisphere families. They can be heard singing from January through March, like the Northern whales, and calving reaches its peak in March. And speaking of their songs, recordings show that they're highly distinct from songs recorded in Madagascar. Not a single whale that's been photo-identified in Oman has been matched to photo catalogues from humpback research in Madagascar, South Africa, Mozambique, or Zanzibar. That, according to the researchers, suggests "little or no current migration." Other corroborating evidence for their distinction comes from the fact that they carry fewer and smaller barnacles that whales in other places, and they don't typically show scars from cookie cutter shark bites, which are extremely common on other Southern Hemisphere humpbacks.

Advertisement

To see if their genetic data could shed any light on the origin of these odd humpbacks, researchers Cristina Pomilla of the Sackler Institute for Comparative Genomics at the American Museum of Natural History and colleagues extracted nuclear and mitocondrial DNA from 47 whales in the Arabian Sea population, and compared it to DNA collected from the world's other humpback whale populations in other studies.

Their genetic data suggested that the Arabian Sea population - which includes the coastlines of Yemen, Oman, the UAE, Iran, Pakistan, India, and perhaps the Maldives and Sri Lanka - has been distinct from the rest of the world's humpbacks for some 70,000 years.

How were they separated from the rest of their species? The authors think it could be related to climate changes in the late Pleistocene, especially as it affected the Indian Ocean's monsoons. After their breeding season shifted to match the Northern Hemisphere schedule, the asynchronous annual cycles would have kept them genetically distinct from the closest whale populations in the Western Indian Ocean.

A whale called "Saddle" sighted in February 2014 off the Dhofar coast of Oman. Saddle was released from a fishing net by the research team in the year 2000 when he was less than two years old. Photo copyright Tim Collins/WCS, used with permission.

In addition, the researchers verified that there was low genetic diversity in the population due to both recent and historical genetic bottlenecks. The most recent bottleneck "may be due to illegal whaling; during two very short periods in 1965 and 1966 Soviet whalers killed 242 humpback whales in the Arabian Sea (39 of the captured females were also pregnant), a potentially devastating loss for a small population," they say. Modern concerns are lethal entanglement in fishing gear and ship strikes, they add.

It's worrying because while humpbacks are in general classified by the IUCN as a species of "least concern," this particular population is certainly not. "The Arabian Sea humpback whales are the world's most isolated population of this species and definitely the most endangered," said Howard Rosenbaum, Director of the Ocean Giants Program at the Wildlife Conservation Society and a co-author on the study. They might just be the loneliest whales in the world.

[PLoS ONE]