By Anna-Marie Lever
Science and Nature reporter, BBC News
Bottlenose dolphins call to their newborns to ensure they can identify mum
Female bottlenose dolphins whistle 10 times more often than usual after giving birth in order to help newborns recognise who is "mum".
The findings by a US team appear in the journal Marine Mammal Science.
These "signature whistles" are unique to each animal, allowing them to be used for identification.
Bottlenose dolphins are highly social; in their first weeks, calves encounter many adult females that they could potentially mistake for their mothers.
"The most obvious explanation for the increase in maternal signature whistle production is the need for the mother to be in contact with her calf," zoologist Dr Deborah Fripp from Dallas Zoo suggested.
"However, the decrease in signature whistle production of [dolphin] mother Lotty after three weeks does not fit this idea, especially as calves actually wander further from their mothers as they get older."
Instead, Dr Fripp said a likely purpose of this whistling enables a process called imprinting, whereby the calf learns to recognise its mother.
"Bottlenose dolphins can swim at birth and are highly social. In other species, these traits are associated with imprinting. A calf can easily get separated from its mother and find itself among many other dolphins."
In some bird species, the critical period for imprinting is as short as a few hours. In some mammals, it is the first few weeks of life.
Imprinting may also help stop females from stealing newborns from other mothers. This behaviour has been reported before in bottlenose dolphins (Tursiops truncatus), and newborn Lotus was stolen on day one, though subsequently returned.
"Theft incidents almost always occur in the first day of the calf's life. Perhaps this is because after a calf has imprinted on its mother, such theft is more difficult," Dr Fripp explained.
Although dolphins can whistle at birth, they are not born with their unique signature whistle.
"Dolphin mothers do not teach their babies how to whistle, so the increase in whistle production at birth is not for this," Dr Fripp said.
She added: "Calves' whistles are almost never similar to their mothers'. Interestingly, female calf whistles are more similar to those animals in their environment which they are not interacting with than to those of animals they know."
Dr Fripp investigated maternal whistle use in four captive dolphins at Kolmardens Djurpark in Kolmarden, Sweden. The females were named Nephele, Vicky, Delphi and Lotty.
The four females each had their own calf. Unfortunately, all but the last calf born - Lotus, the son of Lotty - died within two weeks of life.
"It is sad that the calves died. The infant mortality rate was high, but this year is not representative," said Dr Fripp.
"Infant mortality rate in the wild is not known - if a calf is born and dies within a week it probably won't be recorded. In captivity it is."
As soon as Lotus was born, Vicky stole him and took him to the surface. Lotus remained with Vicky until day six when he was removed from the pool for a day of medical treatment. On his return, he was reclaimed by rightful mother Lotty.
Dr Fripp added: "Unfortunately, with only one calf surviving to week three, and a calf with an unusual first week at that, the evidence to show a return to normal levels of maternal signature whistle production is not particularly strong. Future work is needed to examine this."
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