Amazon river dolphins impress their mate with waterweeds!

Amazon river dolphins use lumps of waterweed or large rocks to impress their mates, much as humans might use flowers or chocolates. Scientists believe the animals hold the objects during displays to woo females or intimidate rivals, the first time aquatic mammals have been seen using objects in social displays.

The lead author of the study, Professor Tony Martin of St Andrews University, said he first realised something interesting was happening while observing the animals in the wild in 2003. "I noticed a dolphin carrying a lump of wood or something in its mouth," he said. "I saw the behaviour again a couple of hours later and noted it down." When it happened for a third time he decided to investigate further.

Trawling back through years of observational data on the animals, his team found 57 instances of the behaviour - nearly all were adult males, suggesting that it was not simply play. The animals pick up sticks, waterweeds, rocks and even in one case a turtle. "I realised then that we were on to something. There was more to this than met the eye," he said.

The animals are extremely difficult to study because the waters they live in are full of silty mud which stops researchers from seeing what goes on under the surface. "It's like trying to look through a keyhole at their behaviour," he said. The observations suggest it is the biggest males which can carry the heaviest objects - and they are the ones which father the most offspring. The research will appear in the journal Biology Letters today.

The Indian Ocean bottlenose dolphin has been seen using marine sponges during foraging, but this is the first time an aquatic mammal has been seen using objects in social display. The behaviour is rare in mammals, but more common in birds.

The dolphins are increasingly threatened by fishermen who kill them illegally for bait. Dolphin numbers are still high, but Martin is concerned: "In the last five years there's been a dramatic decline - more than 50%. It's absolutely frightening."