Recognising a mother's call is one of the earliest and most important life lessons for a dolphin calf.

They can easily be separated from their mothers in the vast open ocean, so listening out for her call means they could quickly reunite.

But just how early a calf learns its mother's call was not fully understood. In a new study of bottlenose dolphins, researchers listened in on two captive dolphin mothers to understand how mother-calf recognition develops.

They discovered that the mothers rapidly increase their unique "signature whistles" as soon as the calf is born. They whistle up to five times more than usual. But they also saw the mothers do something completely unexpected.

"We were surprised at how strong the results were," says Stephanie King of the University of Western Australia, who led the research at the Dolphin Research Center, a sea pen facility in Florida, US.

The older sister, Pandora, forced the switch

"We call this 'vocal imprinting', where the calf imprints on its mother’s signature whistle," says King.

"It's an important learning phase where the calf learns to recognise its mother, and it's important this happens quickly before mother-calf separations occur."

It was already known that it takes a calf several months to develop its own unique call. "They are not born with a signature whistle, they learn it," says King.

Until they do so, they have to listen closely to find their mothers.

This work, published in the journal Marine Mammal Science, reveals once again how vital sound is to a dolphin's world.

Two weeks after the calves were born a surprising turn of events took place.

It is rare for a female dolphin to steal another's calf

The two mothers, who are also sisters, swapped babies. Or rather, the older sister, Pandora, forced the switch. The other sister, Calusa, attempted to retrieve her calf several times, but was unsuccessful.

This switch may have "reset the imprinting clock", say King and her colleagues, meaning each calf then had to learn its adoptive mother's call.

It is unusual for a female dolphin to steal another's calf but it has previously been documented. It is not clear why two sisters who live close together would do so.

"It may not be as rare as one might think," says co-author Kelly Jaakkola of the Dolphin Research Center. "A female taking another's infant has been seen in many wild animal species."

"Switching between two mothers is unusual," says King. "However, the two mothers are maternal sisters, so each calf was then looked after by its 'aunty'. Evolutionarily speaking there are still fitness benefits in looking after your sister's calf."

When they want to reunite they will whistle

While this behaviour might sound strange, dolphins do have a darker side to them. They have been known to be incestuous, to "rape" and even to kill one another's young. In that light, a little infant-stealing no longer sounds as bad.

Even a year later, the calves had not been swapped back to their original mothers.

It may have been too late. By then the calf would have imprinted on its new mother and now be listening to her call.

Fortunately, there was never any physical altercation between the mothers. Both babies remained healthy and happy. "As they got older, they socialised with both mothers and with each other," says Jaakkola.

In a separate new study in the journal Behavioural Processes, King and colleagues identified just how powerful her signature call is.

Each calf then had to learn its adoptive mother's call

Her team wanted to understand how a mother retrieves her calf as it grows up. To do so, they asked a female dolphin to either fetch an object, such as a ball, or her calf. Only when she went to get her calf did she produce her unique call.

"It fits in with our understanding of signature whistles," says King. "When mothers and calves are separated from one another and want to reunite, they will produce their signature whistle."

Melissa Hogenboom is BBC Earth's feature writer. She is @melissasuzanneh on Twitter.

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