For a whaler the bowhead whale (Balaena mysticetus) is something close to the perfect prey.
The whale, weighing up to 100 tons, is a slow swimmer, and when it dies, it does not sink to the bottom like most other whales.
Instead, the bowhead whale’s thick layer of blubber acts like a life jacket, keeping the giant afloat on the surface. Precisely the layer of blubber was the primary reason for the intense hunting of bowhead whales in the past.
The fat was boiled into oil, which was an extremely expensive and sought-after commodity in large parts of the world, where it was especially used in lamps. For the same reason, the bowhead whale was hunted so massively over a period of just over 400 years that by the 1920s there were almost no more left.
Not long after, the species was protected, and today the population is on the rise, but the deadly past has left deep traces that may threaten the whale’s future. This is shown by a new comprehensive study that has just been published in the scientific journal Cell.
– We describe how the bowhead whale as a species has lost a large part of its genetic diversity, and how the negative development will only continue, says Eline Lorenzen, professor of molecular natural history at the Globe Institute at the University of Copenhagen and lead author of the new study, to Videnskab.dk.
– In short, this means that the bowhead whale may become far more vulnerable to climate change, diseases and other threats in the future.
Unique time series
Although large, whale bones are hard to find. The skeletons of dead whales typically end up on the sea floor, where they are eaten by fish and microorganisms. In the new study, however, the researchers have had access to a completely unique collection of prehistoric bones from bowhead whales, which in terms of age spans the past 11,000 years.
The bones do not just gossip about the bowhead whale’s past, but can also tell a great deal about the giants’ present and future. The many bones were collected by geologists in the 1980s and 1990s and ended up with Eline Lorenzen and the other authors by chance.
– I canceled a scientific conference in 2017, where I completely coincidentally got into a conversation with a colleague who mentioned this time series of hundreds of bones from bowhead whales collected in high arctic Canada, says Eline Lorenzen.
– I dropped my jaw and had to grab them immediately!
Soon after, she heard about a similar series of bones – also from bowhead whales – collected on Svalbard in northernmost Norway.
– Suddenly we had two independent and comparable time series of the bowhead whale’s history from the eastern to the western Atlantic. So, what are the chances? It’s absolutely crazy! says the professor.
In the new study, the researchers examined the bones according to all the rules of art. They have each been dated using the carbon 14 method, while modern isotope analyzes reveal where the individual whales lived, what they ate and what the ecosystem looked like back then.
Finally, the researchers have mapped the individual whales’ genomes based on fossil DNA in the bones and compared the results with the genome of living bowhead whales.
Overall, the study tells two overall stories:
· The bowhead whale has over time been an extremely robust species that has survived dramatic climate changes over the past several thousand years.
· Today’s bowhead whales have greatly reduced genetic diversity compared to whales that lived before commercial whaling.
Dangerous bottleneck
And what does that mean?
– This means that human predation on these whales has had enormous consequences, which will continue for thousands of years to come, says Eline Lorenzen.
Before the massive impact of humans, the bowhead whale could otherwise withstand most things.
– Our analyzes show that over the past 11,000 years the whales have experienced dramatic climate changes with average temperatures that were two to three degrees higher than today, says the professor.
– They have apparently managed this without major problems, because we see no significant decline in numbers, distribution or genetic diversity.
It, in turn, came with commercial whaling, which took off from about the year 1540. Historical records based on logbooks from whaling ships testify to staggering numbers. The population around Svalbard – officially known as East Greenland-Svalbard-Barents Sea – declined from approximately 52,000 individuals to just a few hundred.
– The decline fits perfectly with what we can read in the genetic data. It’s absolutely crazy, says Eline Lorenzen.
Further west, the Canada-West Greenland population declined from approximately 18,500 to 6,000 individuals.
The development has created a genetic bottleneck that can have a major negative impact on the bowhead whale.
– When the number of individuals in a population falls so drastically, the genetic diversity also shrinks significantly in a few generations, says Eline Lorenzen.
– But that process takes many, many generations and has not been fully expressed yet, because it was only four or five generations ago that whaling ceased.
Genetic diversity is the total pool of genes and other DNA in the genetic material, which a species has to draw on when it must continuously try to adapt to changing environmental conditions, diseases or climate change. In the bowhead whale’s case, that pool has thus become much smaller.
– You can compare it to a Swiss army knife: the greater the genetic variation, the more tools there are in the Swiss army knife, says Eline Lorenzen.
– Today, the bowhead whale has far fewer tools in its Swiss army knife than in the past.
»State of the art«
– It is an incredibly interesting study, says Michael Møller Hansen, professor at the Department of Biology at Aarhus University.
Here he researches genetic variation and its importance for the adaptation and survival of populations and species. He is not involved in the new study, but has read it for Videnskab.dk.
– In terms of research and methodology, it is state of the art (the highest level, ed.), says the professor.
– They combine the best and most modern methods and show very convincingly that the bowhead whale has not been vulnerable to climate change and other environmental factors over the past 11,000 years. The only thing that has really mattered is whaling.
Here, however, the consequences are long-lasting.
– The bowhead whale will lose more and more genetic variation over the coming generations, simply because there are far fewer whales than before. With each generation, variation will be lost until it stabilizes at a lower level, says Michael Møller Hansen.
The projections in the new study run up to the year 3500. Here, the genetic decline is far from over yet.
– It takes tens of thousands of years to rebuild genetic variation, says Eline Lorenzen and points to a major possible challenge for the bowhead whale.
– Historically, the species has adapted to rising temperatures, but it may become more difficult in the coming decades, she says.
The majority of the whales that were killed by whalers came from the more southern parts of the whales’ habitats.
– It is probably the more southerly whales in a given population that have adjusted to higher temperatures, while those further north have not had the same need, says Eline Lorenzen.
– Today, the genetic material from the more southern whales has largely been lost, and this can weaken the species’ ability to adapt.
