Antarctic fur seal
The Antarctic fur seals’ remarkable homing instinct, which is thought to be the most accurate of any sea mammal, allows the creatures to return to within a single body length of the spot where they were born to give birth to their own pups.
Nearly four million of the sea mammals breed in huge colonies on the virtually featureless beaches of South Georgia every year. After being born, the seals spend five years out at sea feeding before returning to the island to breed.
Using radio tags placed on 335 seals shortly after they were born, researchers at the British Antarctic Survey have discovered that each seal returns to exactly the same location on the beach once they start breeding year after year.
But while typical human Global Positioning Systems (GPS), which use satellites orbiting the earth, can pinpoint a location to an accuracy of around 15 feet, the seals were found to be accurate down to as little as six feet.
Exactly how the seals achieve this feat has left the scientists baffled, but they believe the creatures use a kind of internal compass that helps them find their way across the Southern Ocean to the correct location on the right beach.
Dr Jaume Forcada, a research scientist with the British Antarctic Survey, said: “We don’t know exactly why but it is common among sea birds and other marine mammals to breed in large colonies.
“Antarctic fur seals are among the most site faithful. On average female seals were giving birth to their pups to within 12 metres [36 feet] of where they were pupped themselves.
“Some individuals returned to within one body length of where they had been born.
“It is surprising as these seals can travel really long distances – they go up as far as Uruguay and down as far as the Antarctic peninsula.
“We don’t have any evidence of a navigation system yet, but there must be something really, really strong that brings them back to the same spot.”
Antarctic fur seals, also called Arctocephalus gazella, which were hunted to near extinction in the 1800s but now number in the millions, with around 95% of the population breeding on South Georgia.
The mammals, which grow to around six feet long and eat mainly krill and fish, spend most of the year out at sea hunting before breeding in the summer.
Dr Forcada and his colleague Dr Joe Hoffman, from the University of Bielefeld in Germany, tagged 335 female seals as soon as they were born so they were able to record the location where they were born and compare it to when the seals returned to the beach five years later.
The seals that survived returned to the spot where they were born with unerring accuracy.
The researchers, whose work is published in the scientific journal of Mammalian Biology and Planet Earth Online, found that as the seals got older they also tended to give birth to pups closer to the spot where they were born, which suggests the seals became more accurate in their navigation as they got older.
Male seals are also thought to navigate with a similar degree of accuracy as they also highly faithrul to the sites where they hold territories, but the scientists have yet to study how accurate they are.
Dr Hoffman said: “It’s as if they have some sort of inbuilt GPS system. An accurate internal compass could explain the ability of an animal to return to a particular colony, but it is more difficult to imagine how this could be used on a finer scale.
“In the colony we studied, the small cobblestone beach appears relatively featureless to the human eye. It is possible they could use other cues such as smell.”
Seals are the latest animals to have been found to have an advanced navigation abilities. Sea turtles have been found to use the strength and angle of the Earth’s magnetic field to navigate as do homing pigeons.
Monarch butterflies use light and magnetic sensitive molecules that allow the insects to sense the Earth’s magnetic field when they undertake annual migrations of up to 3,000 miles.
Dr Forcada said the seal’s homing instinct may have developed because it helps ensure that the seals are surrounded by relatives when they give birth.
After giving birth, females move into neighbouring male territories to find a mate, reducing the risk of inbreeding.
He said: “It helps them to create more stable communities in which to give birth and raise pups.
“When the seals are out at sea they tend to be more segregated and individuals tend not to spend time with those around them on the beach.