Scientists have discovered a pre-historic mainland species of sea lion thought to have been wiped out by Polynesian settlers and replaced by the modern New Zealand sea lion.
A team of Otago University biologists believe the pre-historic species, which once dominated South Island shores, became extinct as recently as 600 years ago before a lineage previously limited to the waters of the cold subantarctic took their place.
The discovery, published today in the international journal Proceedings of the Royal Society B: Biological Sciences, was made during a Marsden-funded study which aimed to investigate changes in the New Zealand sea lion population since human settlement.
Professor Jon Waters, who led the study, said it was suspected there may have been an extinction-replacement event in the New Zealand sea lion, something which had already been established in yellow-eyed penguins.
The "aha moment" was when the team noted a distinct difference in DNA sequences from the first genetic material, drawn from prehistoric bones, he said.
"We thought, wow. At that moment, we knew we were on to something - it was like chalk and cheese."
Zoology PhD student Catherine Collins, who carried out the research, said the team was startled to identify the previously unknown lineage.
She said it was estimated the mainland sea-lions became extinct between 1300 and 1500AD, soon after Polynesian settlement, as remains found in middens, where waste was deposited, suggest they were hunted extensively.
"The extinction apparently created an opportunity for the subantarctic lineage to colonise New Zealand's mainland."
Professor Waters said modern New Zealand sea lions could have recolonised the mainland within a century of their predecessors being wiped out.
"Our findings demonstrate that our current sea-lion population is not a declining remnant of an original mainland population, but rather represents a new arrival from the subantarctic," he said.
"Competition between the two lineages may have previously prevented the subantarctic lineage from expanding northwards to the mainland of New Zealand."
The team had not been able to establish whether the two species were physically different, but based on the distinct environmental differences in their habitat, there was reason to assume so.
"My gut feeling is they would have been different, but it's pretty speculative at the moment," he said.
Professor Waters anticipated that future study of the prehistoric remains would lead to a greater understanding of its biology.
The findings come at a time when the New Zealand sea lion is under renewed threat in their main subantarctic breeding grounds, with a fast-tracked threat management plan aimed at what is driving a sharp decline.
A recent count revealed the number of pups at their main breeding ground, the Auckland Islands, was just 1575 - a figure 18 per cent lower than last year.
It was the third-lowest rate since monitoring began in the mid-1990s.
The modern New Zealand sea lion once numbered in the hundreds of thousands, and their habitat extended throughout New Zealand.
However, in the 19th century, the species was decimated for its blubber and skins, and in 1997, it was declared a threatened species under the Marine Mammals Protection Act.
The New Zealand sea lion breeds mainly on the Auckland Islands (70 per cent of the species) and Campbell Island (30 per cent), with small numbers found on Stewart Island.
While the Campbell Island population is faring well, the Auckland Islands colony has been declining rapidly for a number of years.