New genetic research has provided compelling evidence that the Arabian Sea humpback whale is a distinct evolutionary lineage, supporting the potential for it to be recognised as a fourth subspecies of humpback whale.

This would be a milestone that has been some 25 years in the making and a journey that has navigated the analysis of DNA from some 2,85 humpback whales sampled across the species’ global range.

Published in Frontiers in Marine Science, the study was led by the Wildlife Conservation Society in close collaboration with the Environment Society of Oman (ESO), the Environment Authority of Oman, and other local and international partners.

Comparing humpback whale DNA with that of the Arabian Sea population, the results revealed high levels of genetic differentiation between those and the three currently recognised humpback whale subspecies of the North Atlantic, North Pacific, and Southern Hemisphere.

The Arabian Sea population showed the lowest genetic diversity of any ocean region examined. 

Together, the findings support potential recognition of the population as Megaptera novaeangliae indica – though formal recognition of a marine mammal subspecies rests with the Society for Marine Mammalogy’s Taxonomy Committee.

Lead author, Dr Rita Amaral, who coordinated the genetic analysis with WCS, said the results settle a long-standing scientific puzzle.

“Arabian Sea humpback whales have long been an enigma to the scientific community. Their ecology is unlike that of any other known humpback whale population, and this new genetic work represents an important milestone in understanding how that uniqueness developed.

“The results clearly indicate the Arabian humpback whales have followed their own evolutionary trajectory for tens of thousands of years and support their recognition as a distinct subspecies.”

The genetic findings reinforce a hypothesis that had long been held by researchers having observed the population’s behaviour. Unlike other humpback whales, Arabian Sea individuals do not undertake long seasonal migrations between high latitude feeding grounds and tropical breeding grounds. Instead, they remain year-round in tropical and subtropical waters, and follow a Northern Hemisphere breeding cycle.

They also sing a distinct song heard nowhere else on Earth. 

The analyses estimate that Arabian Sea whale lineages diverged between roughly 43,000 and 113,000 years ago.

Researchers also examined ancient DNA from the type specimen described as Megaptera indica in 1883 – a whale collected near Basra, Iraq, whose remains are held by the Muséum National d’Histoire naturelle in Paris. Morphological and genetical analyses confirmed the specimen was a humpback whale closely related to today’s Arabian Sea population.

The population’ isolation makes it especially vulnerable. Arabian Sea humpback whales face threats from entanglement in fishing gear, vessel strikes, underwater noise, pollution, and disease. Meanwhile, climate-driven changes to monsoons and ocean upwelling may affect the availability of sardines and other prey.

The paper notes the population is small, with no evidence of growth over the past two decades, and warns it is at risk of extirpation without concerted management of known threats.

“The growing body of research on Arabian Sea humpback whales has given Oman an increasingly clear understanding of both the importance of this population and the pressures it faces,” said Aida Al Jabri from the Environment Authority of Oman.

“This evidence is now helping us move from scientific understanding towards practical conservation action, including regional cooperation through the International Whaling Commission and Oman’s work to explore greater protection for important marine habitats through international mechanisms such as the International Maritime Organisation’s special protection provided by Particularly Sensitive Sea Areas.”

The International Whaling Commission has now recognised the Arabian Sea humpback whales as a discrete breeding and feeding population and has recommended implementation of a Conservation Management Plan.

As the only known humpback whale population in which the entire population both feeds and breeds in the same geographic area, Arabian Sea humpback whales are genetically, demographically, ecologically, and culturally unique. Given their Endangered status, the study warns their disappearance would mean the irreversible loss of a distinctive set of traits offering insights into how baleen whale populations adapt to environmental change.

Dr Howard Rosenbaum, Executive Director of the WCS New York Bight Program, whose Ocean Giants team has contributed extensively to global research on whale and dolphin population structure, said the study strengthens the case for protection.

“Our collaborative efforts through the years, and especially with this most recent publication, further highlights how Arabian Sea humpback whales are genetically distinct from other humpback whales around the world.

“Generating robust science about its unique evolutionary history, isolated nature, and small population size strengthens the case for protecting these humpback whales throughout their range.”

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