Every year, humpback whales migrate between their subtropical winter breeding grounds and their summer feeding grounds in the high latitudes teeming with plankton and fish. Their seasonal migration of several thousand kilometers is the longest of any mammals. Yet, despite the vast distances they travel, their populations don’t cross paths.
Humpback whales (Megaptera novaeangliae) in the North Pacific, North Atlantic, and Southern Hemisphere ocean basins are more isolated than we thought. According to a large, comprehensive genetic dataset, oceanic populations are separate subspecies. The whales rarely cross the warm, equatorial waters.
Scientists have known that the color of their bodies and undersides of their tails (or flukes) can vary based on geography. Humpbacks in the northern oceans tend to be much darker than those in the Southern Hemisphere. Until this study however, researchers didn’t realize that these sorts of subtle differences are actually signs of long-term isolation.
To better understand the histories of worldwide populations, an international team led by Jennifer Jackson from the British Antarctic Survey collected genetic samples from free-swimming whales using a small biopsy dart. They looked at two types of DNA: the mitochondrial DNA from about 2,700 whales and the nuclear DNA from 70 whales. The mtDNA inherited from the mother builds a picture of how females moved across the globe over the last million years; the nuclear DNA, which evolves more slowly, comes from both parents and provides a general pattern of species movements as a whole.
The team found that -- although female whales have crossed from one hemisphere to another at certain times in the last few thousand years -- they generally stay in their ocean of birth. “This isolation means they have been evolving semi-independently for a long time, so the humpbacks in the three global ocean basins should be classified as separate subspecies,” Jackson says in a news release. Each are on independent evolutionary trajectories.
She adds: “This has implications for how we think about their conservation and recovery on a regional scale.” Changes in the ocean leave signatures in the genetic code of marine species, and if one population is devastated, their numbers might not be helped by whales from a different ocean.
The work was published in Proceedings of the Royal Society B this week.
[Via British Antarctic Survey]
Image: DWH Walton