I love it when animals do things that we don’t expect, especially when they do things we might have species- centeredly thought were unique to humans, or when they do something that appears to be “out of character.”

So that’s warning enough right away that this story about dolphins isn’t going to be one of the usual anecdotes about them rescuing a human from sharks (though that does apparently happen) or how they (undoubtedly) have complex communication, or have been witnessed using tools.

In fact, the only cliche I can think of about dolphins that this story will allude to is the one about their high intelligence. Because it turns out that some dolphins — the Bottlenose variety, in particular — ain’t quite so bright and cuddly as we thought.

Scientists who autopsy cetaceans that have washed up dead on British beaches have reached a grim conclusion: some mammalian species are being killed by Bottlenose dolphins.

Dead porpoises turn up regularly on beaches around Britain. Sometimes they have starved to death; sometimes they have drowned after being accidentally caught up in fishing nets; sometimes tuberculosis has killed them. But when I looked at the records of the environment ministry, the major cause of death of porpoises last year was listed as “physical trauma (Bottlenose-dolphin attack).”

North-east Scotland and west Wales, where Bottlenose dolphins live side-by-side with Harbour porpoises, are where most of the killings occur (15 out of 56 porpoise deaths last year). But last week the Cornish Wildlife Trust, responsible for wildlife in the county in England’s far southwest, reported that it had recovered a dead Risso’s dolphin from a beach in Cornwall. The animal had the deep bite marks characteristic of a Bottlenose-dolphin attack.

So what’s causing Bottlenose dolphins to kill other cetaceans?

They are not doing so for food, as the victims are not consumed once killed. The conclusion that many people have leapt to is that the Bottlenose dolphins — known to be strongly territorial — are reacting to fish shortages in the same way as a desperate human might. In other words, they are eliminating the competition.

It’s a sensible enough argument, and we hear all the time about how overfished the sea is. But at the moment it’s guesswork.

One dolphin expert I spoke to, Nick Tregenza, who advises the Marine Strandings Network in Cornwall, says he doesn’t think it is food competition that is behind the killings.

“They could,” he says, “be doing it for fun.”

Well, we know they are highly intelligent. They have complex forms of communication — including “signature whistles” that may function as their names — that we are only starting to understand. They are one of the few non-primate species that have been documented using tools (Australian dolphins have been seen wearing sponges over their snouts to protect the sensitive tissue as they dig on the seabed for food). And their sense of fun and games is well known.

Only recently, a dolphin and her calf chased the dive boat I was on in Costa Rica. Seeing them alongside, leaping through the wake of the boat and diving, close enough to actually look them in the eye, I got the uncanny feeling that the mother was showing her calf some interesting land animals that sometimes come to sea.

But this “dolphin killing dolphin” story got me thinking about culture. Also last month, there was a story that wild dolphins in south Australia had been seen “tail-walking” — the sort of party trick taught in captivity, but never before seen spontaneously in the wild.

Biologists said it appeared a “culture” of tail-walking had sprung up in the area, perhaps because 20 years ago, one of the dolphins now living there had spent time in a dolphinarium and might have seen the trick there.

Tregenza said that the Risso’s dolphin found dead last week is not a competitor of Bottlenose dolphins. They eat squid, not the same food as the Bottlenose dolphin. Perhaps the bigger dolphin mistook the Risso’s dolphin for a prey item. Or perhaps, like serial killers, dolphins are simply killing for the fun of it.

This might even explain why the trail of death started in the north — in Scotland — before being discovered further south, in Wales. Now it has become evident further south still, in Cornwall.

“Killing for fun” might have been invented by northern dolphins, and passed on, like a craze.

Of course, this is all speculation at the moment. But it makes me wonder what dolphins living around the coast of Japan — particularly in Wakayama Prefecture — might learn. Any survivors of the annual “drive fisheries” there (oikomi), in which thousands of dolphins are herded into the shallow water of Hatagiri Bay at Taiji and speared to death, might pick up some nasty habits from humans.

Imagine a “teacher” dolphin, like the one that may have started showing wild dolphins how to tail-walk, or the ones who may be influencing Bottlenose dolphins in Britain to kill other cetaceans. If such a teacher witnessed the Taiji dolphin slaughter and survived, could we get a Rambo dolphin seeking revenge?

Perhaps — luckily for us — dolphins don’t have the weapons to threaten mere humans. Yet. Tregenza told me he’d heard a report that dolphins have been picking up rocks from the seabed around the coast and throwing the stones around on the surface. Throwing them at each other?

No. Apparently a fisherman said he saw a stone thrown at a seal.

Last week — in events that Japan’s media almost entirely shielded from the nation’s delicate populace — protesters gathered outside Japanese embassies all over the world to mark the start of the country’s utterly cruel, pointless and illegal (since the meat subsequently sold is way over the government’s permitted mercury levels) annual dolphin-slaughter season. Many Japan Times regulars — though very few Japanese who are not JT readers — will have seen the horrific pictures of the sea turned blood-red by the vicious stabbing. Imagine if dolphins started throwing stones at their human killers? Now there’s an unusual behavior I’d like to see.

The second volume of Natural Selections columns translated into Japanese is published at ¥1,500 by Shinchosha. The title is “Hito wa ima mo shinka shiteru (The Evolving Human: How new biology explains your journey through life).”