Bowhead whales in a genetic bottleneck

The bowhead whale is the oldest living mammal in the world, with an estimated natural lifespan of 268 years. They live in and around the Arctic ocean, feeding off krill. And, like many whales in the 19th and 20th century, had its numbers decimated by commercial whaling, from an estimated 50,000 individuals, down to as low as 4000 individuals. Since the whaling moratorium, numbers have recovered up to around 10,000, but now there are concerns that the lingering effects of intense whaling are rearing their ugly bowhead.

Will - Now I promise that this one wasn't even my suggestion. So Aylwyn, why, if we've stopped wailing and have done since 1986 for the most part, are populations thought to be declining again.

Aylwyn - They may be declining or going up or down for all kinds of reasons. One of the main factors that is thought to affect populations in the wild is climate change and arctic environments are very susceptible to climate change, more so than others. And the concern is about what might happen in the future rather than necessarily what's happened since whaling, since I think it's generally been a good thing for the whales that we've stopped hunting them.

Will - And if that is the case, then if climate change is driving such dramatic shifts in the environment, you're going to need to be able to adapt pretty quickly and that might be a problem for bowhead whales going forward.

Aylwyn - Yeah, that's the thing. And that's the general kind of idea in conservation genetics. And, simply put, that's how different are two individuals typically when taken from the species or from a population. And if they're very similar to each other, that means there's less variance or less versions of a given gene in the population. And if some big change comes along and natural selection can only operate on versions that are already existing, by and large. If there's nothing there, if there's no material for it to work on, to adapt to that new environment or that change, well then there's a danger that the population declines and perhaps even goes extinct.

Will - And Shivani, given that this is in stark contrast to, say, a genetics study on humans where we can resort to stuff like the UK Biobank and have huge swathes of data we can trawl through. How do you go about sequencing a population of wild whales?

Shivani - First of all, you have to choose which whales you want to sequence. So in this case, they had the bowhead whale and the narwhal. These two species were great to sequence because they live in very similar habitats and one kind of underwent reduction in their population size because of a bottleneck event. In the Pliocene era, there was a supernova explosion. So their genetic diversity is fairly low right now, but that's for environmental causes. But on the other hand, the bow whales have low genetic diversity because of hunting. They have similar, essentially diversity levels, but different reasons for why that happened. But they're wild, so that makes it difficult to track them down and sequence. So what they did was they collected tissue samples from Inuit hunters who have been hunting various species of whale for thousands of years, and they took these tissue samples between 1982 and as early as 2020. So you have the tissue sample, you can extract the DNA, it undergoes DNA sequencing and you can compare heterogeneity between the DNA to work out genetic diversity in that animal.

Will - So was there any striking difference in the genetic diversity across this timeline?

Shivani - At present, the genetic diversity in both species is fairly similar, but the main difference is that whales live a very long time. So the narwhals live for a hundred years, but the bow whale lives for 200 years and that makes it very difficult for them to undergo genetic selection for favourable traits. When each generation lives so long and things are changing very quickly, the ice caps are melting, the temperature of their climates are changing rapidly. So that means bowhead whales are less adaptable because they're living essentially twice as long as narwhals. So even though currently the genetic diversity is both quite low, bowhead whales are in more danger essentially. So I think that's helpful for people who are kind of looking into conservation because it gives us an idea of which species need protection and why that's happening. Clearly for the case of the bowhead whales, it's because of this wailing that's been happening in the last few decades,

Will - Given that we stopped wailing in 1986, that's less than two generations of bowhead whales that have managed to be produced since then. It sounds like they're really in quite bad shape, aren't they/

Aylwyn - They certainly are by the looks of it in trouble. I don't know if they're formally classified as endangered.

Will - This is an interesting thing because of the whole bowhead whale, the group of things, there's four different species in total. On average they're least concerned. But I'm really concerned looking at this <laugh>

Aylwyn - <laugh>. Well, it's worth bearing in mind that the actual risk for a species of extinction comes from absolute low numbers. So low numbers of individuals. So there are species that can survive for a very long time and exist with relatively low genetic diversity. And there are others that have much higher genetic diversity. And a good example of that is also one familiar to us humans. Humans also have gone through a bottleneck in the past, say about 50,000 years ago. And as a result, our genetic diversity is relatively low compared to the reality that there are billions of us. We might be worried about human extinction, but not for the kind of reasons that we're worried about with the bowhead whale. So although humans aren't as low in diversity as the bowhead, nevertheless it's not a direct risk in itself just having low diversity. They're not going to sort of inbreed themselves to extinction. If we stop, if we continue to stop wailing and if we manage the population and try to protect them from the risks that they face, then they should recover their genetic diversity and the future should be bright for them. And that's the case for bowhead whales and other endangered species like mountain gorillas, which also have extremely small numbers and extremely low genetic diversity and others all around the world.

Will - Shivani raises an excellent point as well in that a study like this is super useful because it puts another perspective on what we need to conserve. Because if you just took it by raw numbers, you'd want to conserve certain things, but if you take it on risk of genetic diversity collapse, you would have to factor another thing. And maybe you'd realise that there are certain species out there that need more conservation than others.

Aylwyn - Yeah, well that's also where the politics do come in because it's not always clear what you should be working to conserve and what are the natural sort of units of conservation in a given species. Do you try to make sure that the group as a whole is preserved or do you focus within that on subgroups that have their own unique identity, and then how do you identify those? And so some of this particular study was devoted to actually within the narwhal in particular saying, well, actually maybe there are subgroups of the narwhal that have their own identity, their own genetic identity, and maybe we should designate them as separate units. And sometimes when that happens in a species or with a population, that means that there are new resources made available to do that. And perhaps even more money, although there isn't usually a lot of money going around for these kinds of things. So as a result, these things can become very political. And I remember being involved in a study one time where the conservationists were very keen to identify a new species of orangutan because that would change its status within the conservation system and within the government system in Indonesia, and therefore enable them to do a whole bunch of things that they wouldn't previously have been able to do. So there are a lot of different factors that are at play at once when you're trying to identify or trying to describe the genetic diversity of these species.