Something killed a lot of sperm whales in the past—and it wasn’t whalers

Sperm whales are a genetic puzzle. The deep-diving, squid-eating giants that inspired Moby Dick are found in every ocean, where they can mate with partners from around the world; as such, they should be quite genetically diverse. Yet, their genetic diversity is actually very low, hinting that something killed a lot of them off in the past. And that something wasn’t whalers.

To reach this conclusion, researchers analyzed mitochondrial genomes (DNA inherited only through the maternal line) from 175 sperm whale samples collected from biopsies of live and dead stranded whales across the globe. Their analysis showed that the current global distribution of sperm whales resulted from a population expansion starting about 100,000 years ago. Sperm whales at that time had apparently been reduced to a small population of about 10,000, when a freezing world caused extensive ice to exclude them from all oceans except the Pacific.

Today’s sperm whales (about 360,000 animals) are all descended from this single population, the team reports online today in Molecular Ecology. They subsequently colonized the Atlantic Ocean multiple times. Whaling has taken another toll, although the full extent is not yet known; it likely depleted some sperm whale populations more than others, the scientists say, noting that collecting information on the population’s overall recovery has proved difficult.

Given today’s warming trends, the habitat of sperm whales may continue to expand, the scientists say. But they caution it’s not clear how climate change will affect the whales’ prey, and urge that protections for large whales remain in place.