It is one of nature's cruellest stories. A helpless newborn is threatened or killed by members of its own species.

It's called infanticide and it occurs in many species, from lions to monkeys. It has rarely been observed in dolphins, but it does happen, despite their cuddly reputation.

For the first time researchers have seen a bottlenose dolphin (Tursiops truncatus) give birth in the wild. This happy moment was immediately followed by an attempt on the baby dolphin's life.

The incident took place in August 2013, and is described in the journal Marine Mammal Science.

Bottlenose dolphins are the most widely dispersed species of dolphin, so it is remarkable that a wild birth had never been seen before.

That it was closely followed by an infanticide attempt was even more surprising, says Robin Perrtree of Savannah State University in Georgia, who witnessed the incident.

Two male dolphins tried to sink the calf about two minutes after it was born, near Tybee Island in Georgia, US.

As Perrtree and colleagues approached a group of dolphins, they saw a female thrashing in the water and then they saw her calf. It was surrounded by a small pool of blood, most likely from the mother's placenta.

"Our waters are very muddy, so we have little visibility," says Perrtree. So they were surprised to see a birth.

"We started recording them as we realized that we had just witnessed a birth, when suddenly a couple of additional animals joined the group and started submerging the calf," says Perrtree.

Each time the two dolphins attempted to sink the calf, its mother pushed it above water, resting it on her head so that it could breathe.

But the pair of males continued to push the calf down for 30 minutes. Perrtree says she was shocked to see them "leaping on top of the newborn".

For the next two and a half hours, the males flanked the mother and her newborn.

Physical attacks subsided but acoustic recordings revealed that they continued to be aggressive underwater. The researchers were too far away to monitor exactly what was going on.

Although she cannot be sure, Perrtree says the attack could have been premeditated.

The males had been sighted hovering near the mother 1.5 hours before the birth. They may have been tracking her in preparation for the infanticide.

"We can't get inside the brain of these animals so it's impossible to say for sure," says Perrtree. "Maybe it was a coincidence, but it definitely raises the question: were they monitoring the female ahead of time?"

You can see a short excerpt of the attack below. Some viewers may find this distressing to watch.

Infanticide is rarely observed in cetaceans, the group that includes dolphins, whales and porpoises.

It has only been witnessed twice in bottlenose dolphins. Submerging the calf in water in order to drown it had not been seen before. In the previous cases, the calf was tossed into the air to cause injury and exhaustion.

The new discovery shows that attacks can take place underwater, which makes them much harder to spot. Infanticide in dolphins may therefore be more common than once believed.

"This is one of many factors that can cause the death of young calves," says Perrtree.

"I don’t think anyone expected the apparent infanticide attempt to happen minutes after birth," says Perrtree.

"All other ones seen in marine mammals have happened days to weeks after birth."

Scientists believe males commit infanticide to free females for mating. If a female dolphin has a young calf to provide for, she will be unavailable for several years.

But if she loses her calf shortly after birth she may be ready to mate again within months.

We do not know what happened to the calf, or its mother. They were observed 24 hours later, but have not been seen since.

This is not in itself surprising. Prior to Perrtree's sighting, the female had not been seen since 2010, a gap of three years.  

She and the calf may well be safe at sea.

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Photographs were collected in accordance with the U.S. Marine Mammal Protection Act on 23 Aug 2013 by R Perrtree under NMFS LOC #14219 issued to Dr Tara Cox