Quick Dips

Sorting out Seals

A 100-year-old debate over the family tree of seals, sea lions and walruses may have been resolved by pioneering research carried out by biochemist Dr Gina Lento at Victoria University.

For her PhD thesis, Lento developed new techniques to construct an evolutionary tree showing that true seals, the "eared seals" (sea lions and fur seals) and walruses are all descended from a single land-dwelling ancestor. This new evidence counters a long-standing argument, based on the appearance and behaviour patterns of these pinnipeds, that true seals are descended from one ancestor, and walruses and eared seals from another.

Lento's work offers solid evidence that the pinnipeds are related to land animals like dogs, bears, raccoons and ferrets, but did not pinpoint a single one of the terrestrial ancestors as the closest relative of the pinnipeds.

"There has been considerable debate on the evolutionary origin of the pinnipeds, but this debate has been based on non-molecular techniques and had reached an impasse," she says.

Lento examined mitochondrial DNA sequences from more than 120 different animals, including the three pinniped families and representatives of other carnivores (cats, dogs, bears, raccoons, ferrets, otters, and the red panda), surveying three gene regions from each animal. She developed her own graphical method for displaying the taxonomic information contained in DNA sequences, and this is already known in the trade as "Lentoplots".

Her evolutionary reconstruction supports the idea of a common ancestor for the three pinniped families, though the relationship of the walrus lineage remains partly unresolved.

Lento's work also involved examining the population structures of the Hooker's sea lion and the New Zealand fur seal, which were terribly depleted by sealing last century.

"There are now about 66,000 New Zealand fur seals -- but one captain's log records taking 60,000 of them for their fur on one voyage to the Antipodes Islands in 1823," says Lento. "Most colonies on the sub-Antarctic islands were completely exterminated. The exploitation was similar with the Hooker's sea lions; no one knows what the original populations of these two species were before the sealers came."

The total population of Hooker's sea lions has remained stable at 11,000 for more than a decade -- a worry for conservationists, who would like to see the population rising. New Zealand fur seal numbers have apparently been increasing in recent years but it's not known whether the new colonies are a result of true growth or simply a relocation of the present population.

Lento undertook genetic studies to check relationships between animals, and to find ways of establishing which breeding colony particular animals belonged to. The results indicate a worrying degree of similarity among the Hooker's sea lions, almost to the point where all the individuals are clones of one another -- a healthy population requires genetic diversity. There are three main breeding rookeries, all in the Auckland Islands, with migration between them helping maintain the genetic similarity.

New Zealand fur seals show some great genetic differences. Among the whole population there are individuals of two genetically distinct classes whose genetic differences are almost as large as that generally found between two different species; individuals of each class are virtually identical in appearance and are indistinguishable to field biologists.

A surprise came when Lento compared the Antarctic and Sub-Antarctic fur seals, which have mostly non-overlapping ranges and major physiological differences, and are considered separate species. "But genetically they are indistinguishable," says Lento.

This study indicates priorities for conservation. Knowing that there are two different populations of New Zealand fur seal means attention has to be given to conserving both of them.