Dolphins are clever and sociable, but they also have a dark side that will make your hair stand on end

Reputation: Dolphins are intelligent, friendly mammals that like to play tricks.

Reality: All of the above is true, but dolphins also go in for sexual harassment, incest and infanticide. Bad dolphins.

Dolphins are smart. Anyone that has seen them performing tricks will know this.

For the doubters amongst you, there is an extraordinary number of studies exploring their cognition. Most of them focus on the most common and well-known kind of dolphin: bottlenose dolphins.

Captive dolphins can remember the whistles for many years, sometimes decades

In a classic study published in 1984, researchers trained a female bottlenose dolphin called Akeakamai to mimic sounds generated by a computer. The electronic sounds, and Akeakamai's responses, are remarkably similar.

Then the biologists began to link these sounds to objects like a hoop, pipe, Frisbee or ball. Akeakamai was quick to figure out the connection and make the vocalization appropriate to each object. In essence, she had learned a new vocabulary.

Wild dolphins achieve similar feats. Each dolphin has its own signature whistle, which acts like a name.

When researchers produce synthetic versions of these calls, the dolphins respond to playbacks as if they know who is calling.

They do not just behave like Flipper

They also remember each other. A 2013 study revealed that captive dolphins can remember the whistles for many years, sometimes decades. In one case, a female called Allie at Brookfield Zoo clearly responded to playbacks of Bermuda-based Bailey, even though they had not seen each other for over 20 years.

Even more impressingly, in 2001 two bottlenose dolphins at the New York Aquarium passed the "mirror test".

After researchers drew patterns and shapes on the animals' skin using "a non-toxic temporary black ink Entre marker", the dolphins quickly swam over to a mirror and spent long periods studying themselves. This suggests they can recognise themselves, at least to some degree, something only a handful of species can do.

All this brainy brilliance may have contributed to the rise of dolphin worship within the New Age movement and beyond. But studies have also revealed another, darker side to dolphins. They do not just behave like Flipper.

"They are very intelligent, but just like humans can be nasty and conniving," says Richard Connor of the University of Massachusetts at Dartmouth and co-director of The Dolphin Research Alliance.

When it comes to the breeding season, there is fierce competition over females. In the 1980s, Connor and his colleagues were the first to document male bottlenose dolphins aggressively "herding" fertile females in Australia's Shark Bay.

The females frequently "bolted", but only managed to escape the males once in every four attempts

"A herding event begins when two or three males capture a female," they wrote in a 1992 paper. The males do this by rushing at her. "In one capture the chasing, displays, and aggression continued for 85 minutes and covered 7km."

With further observations, it became clear that these associations between males were incredibly fluid. Small teams of males were usually part of larger "super-alliances" of up to 14 males.

It is also clear that the females are not particularly willing participants. "Male aggression toward a consort included chasing, hitting with the tail, head-jerks, charging, biting, or slamming bodily into the female," Connor and colleagues wrote in their 1992 paper.

The females frequently "bolted", but only managed to escape the males once in every four attempts. "Over the course of the year, a female will be herded by lots of different alliances over many different months," wrote Connor and colleagues.

The female's determined efforts to escape these controlling males may result from another sinister truth about dolphins.

During 1996 and 1997, 37 young bottlenose dolphins washed up on beaches in Virginia. Superficially, there appeared to be nothing wrong with them, but necropsy revealed evidence of "severe blunt-force trauma".

She does not want to have her movements controlled

The injuries were mainly to the head and chest, "and multiple rib fractures, lung lacerations, and soft tissue contusions were prominent," according to a study published in 2002.

There was lots of evidence that adult dolphins were responsible for the deaths. In particular, one of the researchers saw "several behavioral events benignly dubbed 'calf tossing' in the near shore waters of Virginia Beach".

Calf tossing sounds like a fun game, but it could also be a way for adult males to bash unrelated baby dolphins to death, so that their mothers will come back into oestrus. In 2013, researchers saw male dolphins attack a newborn calf, although in this case the calf seems to have escaped.

If infanticide is indeed a real and present danger in dolphin society, a female would do well to mate with lots of different males from different alliances, says Connor. That way, the males would not know if they were the father of her calf and would be less likely to kill it. "She does not want to have her movements controlled," he says.

The rate of inbreeding in this population is higher than expected by chance alone

There is one more surprise lurking in dolphins' mating behaviour. A 2004 paternity analysis of the Shark Bay population revealed that they occasionally practice incest.

One male, known as BJA, become a father in 1978 – only to reproduce with his own daughter in 1993, 15 years later.

"We have seen males herd their mothers as part of a trio," says Connor. In line with this, a study published in 2010 confirmed that the rate of inbreeding in this population is higher than expected by chance alone.

And you thought sharks were disturbing?