According to a new study published in the journal PLoS ONE, a small, non-migratory population of humpback whales (Megaptera novaeangliae) in the Arabian Sea is the world’s most genetically distinct humpback whale population, which has remained separate from other populations for about seventy thousand years – remarkable for a species that is typically highly migratory.
Arabian Sea Humpback whale photographed in Dhofar, Southern Oman. Image credit: T. Collins / D. MacDonald.
Known for its haunting songs and acrobatics, the humpback whale holds the record for the world’s longest mammal migration; individuals have been tracked over a distance of more than 9,000 km between polar feeding areas and tropical breeding areas.
“The epic seasonal migrations of humpbacks elsewhere are well known, so the Arabian Sea non-migratory population presents a wonderful and intriguing enigma. They also beg many questions: how and why did the population originate, how does it persist, and how do their behaviors differ from other humpback whales?” said study co-author Dr Tim Collins of Wildlife Conservation Society and the Environment Society of Oman.
Historical records of Arabian Sea humpback whales indicate a distribution extending from Iraq to Iran, Pakistan, India, Oman and Yemen, and possibly the Maldives and Sri Lanka.
To assess the origins of this population, Dr Collins and his colleagues examined nuclear and mitochondrial DNA extracted from tissue samples that were collected as biopsies from 47 individual whales.
The data were then compared to existing data sets from humpback whales in both the Southern Hemisphere and the North Pacific.
All of the sampling was conducted in the Sultanate of Oman, a known hotspot for the animals.
The scientists speculate that the 70,000-year separation might be linked to various glacial episodes in the late Pleistocene and associated shifts in the strength of the Indian Monsoon.
The separation is likely reinforced by breeding cycles that are asynchronous; humpback whales in the Arabian Sea breed on a Northern Hemisphere schedule, whereas the closest whale populations in the Western Indian Ocean breed during a different season.
Other lines of evidence support the genetic data, including an absence of photo-identified individuals from the Arabian Sea appearing with whales in the Western Indian Ocean and vice versa.
Arabian humpback whales also have far fewer barnacle scars than Southern Hemisphere whales, and a total absence of cookie-cutter shark bites.
The study also revealed a comparatively low level of genetic diversity when compared to other humpback populations, as well as the signatures of both distant and recent genetic bottlenecks, events caused by population declines.
“The Arabian Sea humpback whales are the world’s most isolated population of this species and definitely the most endangered,” said senior author Dr Howard Rosenbaum, Director of the Wildlife Conservation Society’s Ocean Giants Program.
The current best estimate of population size is fewer than 100 individuals although this is based solely on the work conducted in Oman.
_____
Pomilla C et al. 2014. The World’s Most Isolated and Distinct Whale Population? Humpback Whales of the Arabian Sea. PLoS ONE 9 (12): e114162; doi: 10.1371/journal.pone.0114162
