A leucistic sub-Antarctic fur seal: a first for Gough Island and a first for the species
Blog by Christopher Jones, Senior Filed Assistant, RSPB
The birds of Gough Island often get the spotlight, and rightly so given that many are threatened by invasive house mice. However, Gough Island is rich in biodiversity and restoring islands has been shown to benefit entire ecosystems. So whilst the main focus of our work is on the islands seabirds and upcoming restoration work in 2020, a recent unique observation provided the perfect opportunity to provide a short update on another of the islands inhabitants: the sub-Antarctic fur seals!
Gough Island supports the world’s largest breeding population of sub-Antarctic fur seals, with recent estimates at some 300,000 individuals with approximately 60,000 pups born each year that’s about 80% of the global population. They are easily one of the first features anyone notices when arriving at Gough; watching them swim playfully inshore, hearing their loud guttural calls or seeing them scattered along the beaches, they’re hard to miss! The adult males are particularly conspicuous and charismatic with their cream coloured chest contrasted against their dark brown back and a small tuft of hair on the top of their head.
During one of our visits to the closest beach to the base (Seal Beach), for some work with the Northern Rockhopper penguins, we noticed an adult male fur seal unlike any we had seen before. Its coat was a pale blonde colour darkening towards the rear, with normal coloured eyes, exposed skin and flippers. We suspected that this may have been a case of leucism, a genetic condition that causes the partial loss of pigmentation but usually leaves animals with normal colour eyes and body extremities; different to albinism which is the total loss of all pigmentation.
We had known about reports of leucism in the sister species the Antarctic fur seal on Marion, South Shetland and South Georgia islands. In order to confirm our suspicions we contacted seal expert Prof. Marthán Bester, who has conducted research on seals in the Tristan archipelago for the last 40 years. Marthán advised that this case could possibly be confused with greying fur colour caused by old age but is most likely leucism. He also confirmed that leucistic sub-Antarctic fur seals had never been recorded anywhere before now. Given the ongoing long-term monitoring at Gough as well as at several other sub-Antarctic fur seal breeding sites since the 1970's, this condition must be extremely rare in this species. By comparison, South Georgia sustains the world’s largest population of Antarctic fur seals and is known for the comparatively high proportion of leucism; where incidences of leucistic Antarctic fur seal pups have been regularly reported, at a rate of about one in 800 pups.
This is yet another first from Gough Island, and we are sure there is still more to discover!
Link to the paper: https://link.springer.com/article/10.1007%2Fs00300-019-02506-3
The Gough Island Restoration Programme is being carried out by the RSPB in partnership with Tristan da Cunha , BirdLife South Africa and the Department of Environmental Affairs in South Africa, and Island Conservation. The programme is part-funded by the RSPB, the UK Government, the National Fish and Wildlife Foundation and other generous individuals and organisations.
Seal monitoring is carried out by the Mammal Research Institute (University of Pretoria) in collaboration with RSPB.