Lauren Fuge
The beaches of Aotearoa New Zealand were once jam-packed with southern elephant seals, according to new research that reveals how the species has moved over thousands of years.
“At the time of human arrival in New Zealand, you would be hard-pressed to find room on the beaches, with fur seals on the rocky headlands, prehistoric sealions and elephant seals on the sand, and lots of penguins,” says Nic Rawlence, Director of the Otago Palaeogenetics Laboratory and co-author of the study in Global Change Biology.
Today, southern elephant seals (Mirounga leonina) are mostly confined to the bottom tip of South America and the sub-Antarctic Islands, including South Georgia, Macquarie Island, and the Falkland Islands.
But the study showed that they were once spread right across the Southern Ocean – from South Africa to New Zealand to Western Australia.
This population shift is surprisingly rapid – evolutionarily speaking – which suggests that their movements may be a ‘canary in the coal mine’ for understanding the impacts of the swiftly warming Southern Ocean.
The power of palaeogenetics
The researchers wanted to see how the elephant seal population had shifted throughout the last 11,000 years of the Holocene, the geological period since the end of the last ice age. They also wanted to understand how these populations are related.
“We were interested in trying to understand elephant seals in Australia and New Zealand – basically, what the genetic lineages they were attached to,” explains co-author Mark de Bruyn from Griffith University in Australia.
Elephant seals are an interesting species to work with, he notes, because not only were they impacted by massive climatic changes throughout the Holocene, but they have also been the target of both subsistence hunting and industrial sealing.
To understand the history of these apex predators, the team pieced together genetic clues from subfossils (partially preserved skeletal remains). They collaborated with geologists at the University of Maine, who had unearthed elephant seal remains along the Victoria Land coast in Antarctica dating back 8000 years. They also analysed fossils of locally extinct populations in New Zealand and Tasmania, dating back 2-3000 years.
They combined this ancient DNA information with modern genetic data from all living populations, as well as seal remains at archaeological sites across multiple continents.
The different forms of data gave the team multiple strands of information, according to de Bruyn.
“It’s a nice use of genetic and biological data to actually understand geology as well,” he says. “The fossil evidence … gave us really good indication of where the seals lived prior to the end of the Holocene … Then, using the genetics, we were able to tell whether those populations … were closely related or more distantly related.”
They also used climatic data to see how ice sheets have changed over time, which is vital for elephant seals as they need to be able to access both beaches and the ocean for feeding and breeding.
“If an island such as Macquarie is surrounded by ice, it’s pretty hard going for the elephant seals because they aren’t able to access their food source,” de Bruyn says.
Seals on the move
The results showed that the range of southern elephant seals has expanded and contracted over thousands of years, as a result of both climatic changes and human impacts.
“The Ice Ages would have rapidly increased the amount of sea ice surrounding Antarctica, forcing elephant seals to retreat to multiple refugia in South Africa, Australia, New Zealand and South America, before they expanded back out as the climate warmed, including temporarily to the Antarctic mainland,” de Bruyn says.
Remains were found at all of these potential refugia sites, supporting this hypothesis. But the evidence is incomplete. Before the ice retreated, seas were 120 metres lower than they are today; as the ice melted, the seas rose steadily until they stabilised 6000 years ago, undoubtedly drowning evidence of some past seal colonies.
“I think we’ve lost a lot of the evidence, but presumably there’s lots of seal bones lying on the ocean floor somewhere,” de Bruyn says.
The expansion of the elephant seals’ range as the climate naturally warmed wasn’t permanent. Populations then had to contend with another challenge – the arrival of humans.
“Indigenous subsistence hunting and European industrial sealing once again resulted in the contraction of their range, this time to the deep Southern Ocean with their extirpation from Australia and New Zealand,” says de Bruyn.
Seal remains found in middens, as well as artefacts like necklaces made of seal teeth, show that the species was part of both the diet and culture of Indigenous communities in Australia and New Zealand. In some areas, hunting – combined with other environmental changes from human activity – led to the local extinction of populations.
Then the seals suffered another heavy blow. In the 19th century, Europeans began to slaughter elephant seals on an industrial scale as a source of lamp oil. This took the species as a whole to the brink of extinction.
Hunting didn’t stop until the 1960s. They are now protected under Australian law and their numbers appear to be bouncing back – but one of their biggest threats today is climate change.
Southern Ocean futures
Rawlence says that looking at how elephant seals adapted to rapid, severe environmental change in the past can help us understand how they will be impacted in the coming years.
For example, the research shows that although the species is adaptable, their numbers and genetic diversity suffered swift, steep declines when simultaneously confronted with rapid climatic change and human exploitation.
“Their dynamic evolutionary history, plus climate change and human impact, strongly suggests that unless measures are taken to mitigate the effects of human-driven climate change and marine ecosystem deterioration, elephant seals and the Southern Ocean ecosystem are in for a rough ride into the future,” Rawlence says.