Researchers from the British Antarctic Survey found that populations of humpback whales living in the Southern Hemisphere, North Pacific and North Atlantic are more genetically distinct than originally thought.
Based on the results, published in Proceedings of the Royal Society B, the team is calling for these three groups to be considered different subspecies.
Humpback Whales: Great Travellers
Humpback whales are great travellers, swimming huge distances between their winter breeding (low latitude) and summer feeding (high latitude) grounds every year. However, although they undertake the longest migration any mammal is willing to attempt, different populations of humpbacks very rarely cross paths with each other.
“We knew from earlier analyses that humpback whale populations showed strong maternal fidelity to breeding and feeding grounds within oceans, but we did not have a good understanding of the evolutionary relationships among oceans,” leading author Jennifer Jackson from the British Antarctic Survey told Decoded Science.
To find the reasons for this isolation between “neighbours,” the team compiled one of the largest and most comprehensive genetic databases ever assembled for living populations of whales and dolphins. After collecting samples from free humpback whales across the world, researchers analysed two types of DNA: mitochondrial DNA, which is solely inherited from the mother; and nuclear DNA, inherited from both parents.
Crucially, this way it was possible to determine not only the general patterns of migration, but also look more closely at how females in particular moved throughout their evolution.
Whale Migration Paths That (almost) Never Cross … But What’s Keeping Them Apart?
Researchers know most humpback whales prefer the comfort of their own ocean of birth, and rarely move to another breeding area. It turns out this relative isolation over millions of years has led to independent evolutionary pathways, leading the authors to suggest that humpbacks from the three global oceans should be considered different subspecies.
“Information from both the maternally inherited mitochondrial DNA and the bi-parentally inherited nuclear DNA confirmed the isolation of the oceanic populations, consistent with a subspecies ranking of these differences,” said Jackson.
Curiously, in the absence of a real physical barrier to prevent such movements between populations, it seems the warm waters around the equator represent a “do not cross” line for humpbacks, although scientists don’t clearly understand the reasons behind this phenomenon.
For example, “in some parts of the world, the oceanographic conditions are such that the warmest waters occur a few degrees north of the true equator rather than being at the equator itself,” said Jackson. Echoing these conditions, “in these places the distribution of humpback breeding and calving grounds is slightly north of the equator.”
To make any transition between populations even harder, the separation over millions of years has led to a drift in the breeding and feeding cycles between the two hemispheres, which are now about six months from each other. After such a long time, explains Jackson, whales “may be biologically programmed to those cycles and moving between hemispheres and mating successfully could require them to synchronise.”
Occasional Migration Changes
Despite the North/South divide, the genetic analysis supports occasional cases of migration and it’s not unheard of humpbacks originally from one breeding ground turn up on a completely different area a few years later. For example, in 2010, Peter Stevick and his team spotted a Brazilian humpback whale turning up in breeding grounds near Madagascar.
“It probably happens when humpbacks are foraging in the Southern Ocean, when they can end up travelling rather long distances. When the summer ends it is possible that they just follow another humpback migration stream heading north to a different breeding ground from the one they came from,” said Jackson. “When you think about the distances they travel, it is clear that a whale could physically swim across the equator and make it to high latitude feeding grounds in the opposite hemisphere, but it seems that this happens very infrequently.”
Less evident in the results, but a second possibility involves whales occasionally turning up early or late to their breeding grounds. If this happens on the low latitude grounds (such as off the coast of Costa Rica) which are “time-shared” with whales from the other hemisphere, there’s a chance of a fleeting romantic moment between whales coming very early from one hemisphere and whales leaving very late from the other hemisphere. This way, explained Jackson, “the gene flow could occur without actual physical migration of whales between the hemispheres.”
Humpback Whale Migration: When Everything Was Covered in Ice….
Interestingly, this study also allowed an insight into the humpback’s migration patterns during the periods when much of the northern waters were covered with ice, over 20,000 years ago. In contrast with Antarctic minke whales for example, who spend a lot of time under ice, humpbacks tend to shy away from strongly ice-covered areas.
For this reason Jackson suggested that, while some whales may have been able to find ice-free areas in the Northern Hemisphere (such as the Norwegian sea) and remain isolated, most had no choice but travel down looking for the next meal, inevitably mingling with the southern populations. Once the ice retracted, the northern animals bid farewell to the warm waters and re-colonised their old breeding grounds, but not without leaving genetic clues captured by Jackson’s team, witness to this temporary forced intermixing.
Over 20 Years Studying Humpback Whales
This study builds on more than 20 years of previous genetic analyses of humpback whales, but this has hardly scratched the surface. In the future, Jackson’s plans involve using genetic data from whales to understand parameters like abundance, bottleneck sizes and connectivity between populations. “As we get more and more DNA sequences using new genomic technology, our capacity to understand these parameters increases proportionately,” she concluded.