Australian pygmy blue whale gene pool shaped by climate, not whaling

By / 5th of May, 2015

A NEW study of the Australian pygmy blue whale has determined its uniqueness is due to climate change and not whaling, as had been previously believed, and gives hope to a recovery in their numbers.

Australian pygmy blue whales have the smallest gene pool out of all blue whale subspecies. The study used the largest ever genetic dataset compiled to study blue whales and found the small gene pool is the result of changes in climate 20,000 years ago – not whaling.

Dr Catherine Attard of Flinders University authored the paper, published today in Royal Society Publishing’s Biology Letters, with supporting research from Macquarie University, Deakin University and other institutes.

“They’re still at low numbers, but if their low genetic diversity came from whaling, that would lead to inbreeding and not being able to cope with environmental change.”

“We were wondering why pygmy blue whales in Australia have the lowest recorded genetic diversity or population size out of blue whales. It could have been due to whaling in the past, by killing off individuals and losing their genetic diversity,” Dr Attard explains.

“It could be due to natural causes like climate change in the past that reduced the population size, the same way you would expect with whaling, but you could time that to determine when it happened. Or it could just be that the blue whales in Australia always had a small population size.”

There are three recognised subspecies of blue whales: North Atlantic blue whales which primarily occur in the Northern Hemisphere, Antarctic blue whales which are typically found in Antarctica and its surrounds, and pygmy blue whales, that live in the southern Indian Ocean, typically around Australia and Indonesia.

The study compared DNA from 109 blue whales in Australia with 142 blue whales in Antarctica and 46 in Chile in order to determine when a change in genetic diversity occurred.

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“We found that the blue whales in Australia are quite a young population. They only came about around 20,000 years ago. That might sound old, but it’s not in genetic terms. Humans are a young species and we’re two million years old – dinosaurs were around 200 million years ago,” Dr Attard says.

“20,000 years ago, on an evolutionary timescale, is actually quite young.”

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Because of the population’s relative youth, not enough time has passed for it to build up genetic diversity through mutations and other mechanisms. Populations that are small in size can’t carry the same diversity as a large population.

The youth of the population led the research team to question where it originated. By comparing the DNA between subspecies, the researchers were able to trace the genetics back in time – not unlike figuring out the parents of an individual based on their DNA.

“We worked out the most likely thing that happened was some Antarctic blue whales came to Australia 20,000 years ago and ended up staying there, forming a population there. They just kept breeding with others that were hanging about in Australia and feeding.”

The peak of the last glacial period was 20,000 years ago. At that time, the ice surrounding Antarctica would have stretched much further out to sea than it does now.

“Antarctic blue whales would be feeding much further north than they are today. That may have promoted some of them to come to Australia to feed and some could have stayed in Australia – which then developed in to the Australian population we have today, the different subspecies.”

Antarctic blue whales are the biggest subspecies. Over time the pygmy blue whales found in Australia have shortened in length, becoming about five metres shorter than their larger cousins. They also have their own song types and behaviours.

“What it means is that, since low genetic diversity has come about from a natural cause rather than whaling, it means that their low genetic diversity isn’t inhibiting their recovery from whaling.

“They’re still at low numbers, but if their low genetic diversity came from whaling, that would lead to inbreeding and not being able to cope with environmental change.”

It’s important to note that blue whales are still on the endangered species list. Unlike many smaller mammals, they have a very long generation time and lifespan, which means it takes a long time to rebuild population numbers and to acquire genetic diversity.

“In terms of conservation, people need to focus more on potential human impacts on them now, like marine pollution and marine noise to make sure that they recover.”

Key contacts

Dr Catherine Attard Flinders University
61 8 8201 5968