By DAVID DERBYSHIRE

Last updated at 09:05 07 December 2007

On a first date in the human world, it would guarantee it was also the last.

But for a female dolphin waiting to be wooed, there is nothing more romantic than a spectacular bouquet...of weed.

Researchers have discovered that the male of the species gathers gifts from his submarine environment to present to a potential mate.

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It could be just a stick, or a lump of debris. But for that really special one, only algae will do.

The findings come from a three-year study in the Brazilian Amazon. Hundreds of males were observed carrying objects in their snouts, and there was convincing evidence that the practice was linked to mating rituals.

Dr Tony Martin, of the British Antarctic Survey in Cambridge, said: "My colleagues were sceptical when I first suggested the idea, but now I think the evidence is overwhelming."

With experts from the National Institute of Amazonian Research in Brazil, he studied 6,026 groups, or pods, of dolphins between 2003 and 2006 in the flooded rainforest reserve of Mamiraua.

Of these, 221 groups included at least one dolphin carrying an object such as weed, a stick or a lump of clay.

These groups usually contained a mixture of adult females and that the objects were mostly being carried by the males.

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The males were far more aggressive to each other in groups where

objects were being carried - a clue that the behaviour was linked to sex.

Dr Martin said that if the behaviour was play, females and juveniles would also do it.

Fascinatingly, DNA tests on adults and calves revealed that the males who carried the most objects were the most successful fathers.

Dr Martin told New Scientist magazine that the carrying of objects is a sign of culture - skills and behaviour learned from previous generations, rather than passed down in genes.

Only a few species of animal have culture and most are apes and monkeys.

A second study of bottlenosedolphins in Australia offered further evidence of cultural behaviour.

Some of the creatures were found to break off pieces of marine sponge and hold them over their snouts to protect themselves when probing the ocean floor.