The recent history of the sperm whale is practically inseparable from whaling – just take a look at the story of poor old Moby Dick. Over the past 300 years, throughout the era of mass whaling, their numbers were almost totally and utterly decimated. However, a new piece of research suggests that the species was in a lot of trouble long before the invention of harpoons.

A genetic study of these giant deep-divers suggests there could have been a mass die-off of sperm whales some 120,000 years ago when cooling waters cut them off from much of the world’s seas. Around this time, there could have been as little as 10,000 whales solely living in the Pacific.

Even though their numbers have since bounced up to around 360,000 individuals, their genetic diversity remains freakishly low for such a wide-ranging species. You can find sperm whales in many of the world’s seas, from the Mediterranean and the Indian Ocean to the Atlantic and the along the south coast of Australia, but they are most prevalent in the vast swathes of the Pacific Ocean. Here is where all of the world’s sperm whales can trace their heritage back to.

As reported in the journal Molecular Ecology, researchers led by the National Oceanic and Atmospheric Administration collected the mitochondrial DNA from 175 sperm whales across the world, some of which were alive and others of which died in strandings.

Genetic diversity was richest in the Pacific, indicating that they had been here the longest. They only appear to have branched out and grown in population size after the last interglacial period, a window of warmer global temperatures some 125,000 years ago. It then appears that they made multiple migrations out to the Atlantic Ocean at some point between 20,000 and 60,000 years ago.

Sperm whales (Physeter macrocephalus) are the largest of all toothed whales, with some individuals measuring over 16 meters (52 feet) in length and weighing a colossal 40,800 kilograms (45 tons). Their numbers have slightly bounced back in recent decades, but they are still only at a third of their pre-whaling population size. The IUCN Red List still considers them as vulnerable to extinction, although their population reduction is believed to be reversible and relatively well-understood by scientists.

Nevertheless, as this study highlights, their low genetic diversity is still worrying, especially when you consider that our planet's climate is currently undergoing a massive change.

“Although the current warming trends predict the expansion of sperm whale habitat that allows both growth and opportunities for inter-ocean dispersal, we cannot predict how rapid climate change may affect the ecosystems on which sperm whales depend,” the study authors conclude.

"It is highly probable that some sperm whale populations are still endangered or at risk.”

[H/T: Science Magazine]