Australasian Science Magazine

A pygmy blue whale. Credit: research team

By Catherine Attard, Luciana Möller & Luciano Beheregaray

A DNA study has determined whether the low genetic diversity of Australia’s blue whales was caused by past natural events or recent whaling, and offered hope for their long-term survival.

Hundreds of thousands of blue whales (Balaenoptera musculus) were killed by whalers during the 20th century. They are the largest animal known to have existed, which made them a prime target for whaling.

Blue whales can weigh more than 160 tonnes and reach more than 30 metres long. Although immense in size, they feed only on small shrimp-like crustaceans known as krill. They require large amounts of krill to support their large size.

In winter they migrate to warmer waters closer to the Equator to breed. One population of blue whales feeds on the krill available in Australian waters in the summer and autumn, and migrates northward, likely to Indonesia, to breed in the winter.

The population feeding in Australian waters has the lowest recorded genetic diversity of blue whales in the world (Fig. 1). This population is a member of a subspecies of blue whale, the pygmy blue whale (B. m. brevicauda), which is smaller than the other blue whale subspecies but can still reach 24 metres in length. The amount that their population size decreased due to whaling is uncertain.

The low genetic diversity of blue whales in Australia could be due to reduced population size following whaling or due to natural causes. We conducted a DNA study to determine which of these factors was responsible.

Human and Natural Effects on Genetic Diversity

When populations decrease in size due to human impacts such as whaling, it is more likely that genetic variation within the population will be lost. Genetic diversity declines in small populations because the few individuals that have a unique genetic variation may, by chance, not breed and therefore would not pass variation to the next generation. There is more chance in larger populations that at least one individual with a particular genetic variation will breed and pass on that variation to the next generation.

Genetic variations that are lost may be highly important to the survival and reproduction of individuals in the population. For example, the ability to cope with certain environmental conditions, such as hot weather, may be lost, which means that individuals in the population could perish during future changes in the environment.

In small populations there are also less mates available so inbreeding between relatives may occur. This can leave the next generation with a reduced ability to survive and reproduce if they have inherited deleterious genetic variations that are shared by their parents.

An alternative explanation to low genetic diversity is that a population has had or currently has a naturally small population size. Populations that are small in size due to human impacts or small in size due to natural causes are both expected to have low genetic diversity.

Natural shifts in population size and consequent shifts in genetic diversity have been seen throughout evolutionary history as the Earth’s climate has changed. Some populations are also naturally small because the habitat where they are found can only support a relatively small number of individuals.

Naturally low genetic diversity is part of evolution and is not in itself a cause for concern. Instead, human causes of low genetic diversity are concerning as they may drive the extinction of populations or species.

Therefore, determining the cause of low genetic diversity is necessary for understanding whether humans have genetically threatened a population or species.

The Answer Is in the DNA

Past changes in genetic diversity leave patterns in DNA, so DNA from living individuals can be used to trace the demographic history of a population or species. This means we can use DNA from blue whales feeding in Australia to determine the reason for their low genetic diversity. The possible reasons are a decrease in population size due to whaling, a decrease in population size due to past natural changes, or a naturally small and unvarying population size.

A decrease in population size due to whaling would generally show no or very few new genetic variants, whereas a decrease in population size due to past natural changes would show new variants. This is because more time has passed in the latter case for the population size to recover and for mutations to form new genetic variants. These new genetic variants are recognisable by their rarity and similarity in DNA sequences.

A naturally small and unvarying population size will instead show genetic variants that have relatively dissimilar DNA sequences. This is because the variants have had much time to accumulate mutations and to lose, by chance, the intermediate variants.

How do we obtain DNA from a live whale to assess genetic diversity and find these patterns? From a boat we use biopsy equipment to propel a dart that collects a small amount of skin and blubber. The dart then floats on the water and we can collect it with a net. This is akin to being pricked with a needle, and the blue whales have no or minimal response to the dart.

DNA is extracted from the collected sample and then analysed to determine and assess genetic patterns within it. We found that the blue whales feeding in Australian waters have low genetic diversity due to a natural decrease in population size rather than due to whaling or a naturally small and unvarying population size.

The genetic pattern suggested that the blue whale population decreased in size about 20,000 years ago. This estimation involves statistically rigorous analyses and is based on the concept that more rare genetic variants, and mutational differences in these variants, are expected the longer ago the population decreased in size.

The pattern in the DNA is likely due to the founding of blue whales in Australia around 20,000 years ago by relatively few individuals. These blue whales then increased in population size in Australia and areas where they breed.

Twenty thousand years is a very young evolutionary age for a population. This means that not enough time has passed for these blue whales to substantially increase in genetic diversity by accumulating mutations, so their genetic diversity remains low today relative to other blue whale populations.

We compared the pattern in the Australian blue whale DNA to patterns in the DNA of blue whales elsewhere in the Southern Hemisphere. These were blue whales that feed in Antarctica, which are a subspecies called the Antarctic blue whale (B. m. intermedia), and blue whales that feed in Chile, which may be a novel subspecies. We found remarkable differences among these blue whales.

We found that Antarctic and Chilean blue whales have existed for hundreds of thousands of years. This explains why they have a larger genetic diversity than blue whales from Australia; they have had a long time to accumulate mutations and therefore genetic diversity.

In addition we found that Antarctic blue whales have a larger population size than blue whales from Australia or Chile. This means that Antarctic blue whales can hold, and have held, more genetic diversity than their counterparts elsewhere.

Amazingly, when blue whales feeding in Australia were smallest in their population size, Antarctic blue whales were at their largest population size. The habitat Antarctic blue whales occupied 20,000 years ago must have been able to support much greater numbers of individuals than their habitat can today. Blue whales feeding in Chile instead have apparently had a naturally constant population size over time.

A Natural Explanation

Much of the biodiversity we see today, especially in higher latitudes, has been shaped during recent glacial periods. We are currently in an interglacial period. During glacial periods, the ice sheets at the poles reached further towards the Equator and the sea levels were lower.

For some species, genetic diversity declined or was lost during glacial periods because populations contracted to refugia or became extinct. Genetic diversity subsequently increased during interglacial periods because there were increases in population size as populations expanded from refugia, or populations were founded in habitable areas and then increased in size.

To infer why blue whales came to Australia 20,000 years ago, we looked at what the climate was like at that time. Twenty thousand years ago was towards the end of the last glacial period when ice sheets were at their last maximum extent.

The Antarctic blue whale would have been forced further north at that time than they are today, as they could not inhabit ice-covered waters. Their habitat could have been spatially larger or held more krill than their habitat today, which would explain their larger population size at that time. A few Antarctic blue whales likely ventured as far north as Australia to escape the ice, and then they remained. These were the founders of the Australian blue whales.

By analysing samples from contemporary Australian, Antarctic and Chilean blue whales we were able to identify records in their DNA that attest to this hypothesis. In doing so, we provided evidence that the evolution of blue whales into different populations and subspecies has been moulded by past natural climate change.

Hope for Blue Whales in Australia

We now know that the low genetic diversity of blue whales feeding in Australia is due to natural rather than anthropogenic causes. It gives hope to the long-term survival in Australia of the largest living animal as the population has had this low genetic diversity for thousands of years.

Blue whales are currently protected from whaling but they are still recovering in numbers. Anthropogenic impacts occurring today may yet inhibit their recovery. These include marine noise, pollution and climate change.

If they don’t recover they will have a smaller population size than their natural population size. Therefore they may yet have a human-caused decrease in genetic diversity.

In addition, populations with naturally low genetic diversity are likely to have less capacity to adapt to human-caused changes in the environment. With less genetic variation there is less chance that whales would have a variant that may be beneficial in future conditions, such as previously unexperienced environmental scenarios.

So while this study shows hope for blue whales, continued conservation efforts are required to mitigate the ever-increasing human impacts on their habitat and ensure their long-term survival.

Catherine Attard, Luciana Möller and Luciano Beheregaray are researchers and lecturers at Flinders University. The research has been published in the Royal Society journal Biology Letters (www.tinyurl.com/h569l8j).