Men might say it with flowers, but amorous male dolphins prefer to woo a mate with a nice bunch of weeds.

A three-year study of Amazon river dolphins in Brazil has found that males often carry objects such as clumps of weed or sticks in their mouths as they swim, and that the behaviour is almost certainly a kind of sexual display.

While the behaviour had been reported before, it had been assumed to be playful and to have no function.

However, research on 6,026 groups of dolphins has shown that the carriers are usually adult males and that they behave in this way more often when adult females are around.

Aggression among the males in Mamirauá, a 225 sq km (87 sq m) rainforest reserve, was also 40 times more likely in groups in which dolphins were carrying objects. The aggression would be explained by males vying for females.

Of the groups, 221 included at least one individual that was carrying weed, a stick or a lump of clay. The evidence points to these items being used as part of a sexual display to impress females and persuade them to mate.

The research, led by Tony Martin, of the British Antarctic Survey in Cambridge, and Vera da Silva, of the National Institute of Amazonian Research in Amazonas, Brazil, will be presented this week at the Society for Marine Mammology conference in Cape Town, South Africa. Details are also reported today in New Scientist.

The conclusion is supported by genetic comparisons between 200 adult males and DNA from dolphin calves. The early results suggest that males that carry weeds and sticks tend to father more offspring. Dr Martin said: “I was struck by how many of the most frequent object-carriers were on the list of probable fathers of individual calves.”

The findings are particularly significant because only humans and chimpanzees use tools in sexual display in similar fashion. Dr Martin said: “It’s so unusual that many of my colleagues were sceptical when I first suggested the idea, but now I think the evidence is overwhelming.”

This supports the theory that dolphins can develop cultural traditions, by which certain behaviours are learned from the group rather than by instinct. Again, this is peculiar to higher primates and some birds.

The case for dolphin culture is also supported by research led by Michael Krützen, of the University of Zurich in Switzerland, and Lars Bejder, at Murdoch University in Perth, Australia. Their research into bottlenose dolphins in Shark Bay, Western Australia, has identified 12 different foraging specialisations.

One of these is “sponging”, in which a dolphin breaks off a piece of marine sponge to protect its belly as it looks for food on the ocean floor. This is the only known example of tool-use among dolphins.

Dr Krützen’s team has now discovered the same behaviour among a different colony of dolphins in an inlet called Useless Loop. DNA analysis showed that these animals come from a different genetic line to the Shark Bay “spongers”, suggesting that the behaviour has developed twice.

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