While men might try flowers, smart clothes or cars to impress the opposite sex, male Amazon river dolphins carry weed.
Object-carrying has been reported throughout the dolphin鈥檚 range in Brazil, Venezuela and Bolivia, but what had been thought to be play behaviour now appears 鈥� exceptionally among mammals 鈥� to have a sexual function.
Tony Martin of the British Antarctic Survey in Cambridge, UK, and Vera da Silva of the National Institute of Amazonian Research in Amazonas, Brazil, have studied the dolphins for three years in the Brazilian Amazon and are now convinced it is a sexual display. Only humans and chimps do anything remotely similar, says Martin. 鈥淚t鈥檚 so unusual that many of my colleagues were sceptical when I first suggested the idea, but now I think the evidence is overwhelming,鈥� he says.
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Martin and da Silva encountered 6026 groups of dolphins between August 2003 and August 2006 in Mamirau谩, a 225-square-kilometre flooded rainforest reserve. Of these groups, 221 included at least one individual that was carrying an object, such as weed, a stick or a lump of clay.
The researchers found that these groups usually contained adult females, and the carriers were mostly adult males. Aggression between adult males was also 40 times more likely in object-carrying groups. This all points to the behaviour being a form of sexual display, not play, says Martin, who is presenting the work at a conference of the Society for Marine Mammalogy in Cape Town, South Africa, this week. Martins says that if the behaviour was play, he would expect females and juveniles to do it 鈥� and they don鈥檛.
His interpretation is supported by genetic analyses of tissue samples collected from adults and calves suggesting that some of the most frequent object-carriers are among the most successful fathers. The researchers have already analysed 200 adult males and are now comparing DNA from calves against that of the males on their list to identify likely fathers. The results so far are preliminary, Martin stresses, 鈥渂ut I was struck by how many of the most frequent object-carriers were on the list of probable fathers of individual calves,鈥� he says.
鈥淚 was struck by how many of the most frequent object-carriers were on the list of probable fathers of individual calves鈥�
The fact that object-carrying has been reported in isolated populations is remarkable, Martin adds. It means that the behaviour is either ancestral or has evolved independently. 鈥淓ither scenario is fascinating.鈥� And he thinks this is a clear example of culture among dolphins.
This conclusion pitches Martin to the heart of a major debate about animal culture, definitions of which vary. The strict definition says that culture is a cognitively complex skill spread and maintained in a population by social learning rather than by genes or ecology. But only a few species 鈥� almost exclusively primates 鈥� show behaviour that fits this description.
The case to admit dolphins into the culture club is supported by other evidence too. Michael Kr眉tzen of the University of Zurich in Switzerland, and Lars Bejder at Murdoch University in Perth, Australia, have been studying bottlenose dolphins in Shark Bay, Western Australia, where 12 different foraging specialisations have been identified. 鈥淪ponging鈥�, in which an animal breaks off a piece of marine sponge and carries it over its snout, or rostrum, apparently to protect itself while probing the ocean floor, has received most attention because it is the only known example of tool use in dolphins.
Critics have said in the past that genes or ecology could be mostly responsible for this behaviour, disqualifying it as 鈥渃ulture鈥�. Now Kr眉tzen鈥檚 team has studied a different population of dolphins living in an inlet called Useless Loop. Of about 200 animals identified, 32 were spongers, and the researchers obtained skin samples from 12.
DNA analysis showed that they all belong to a single maternal 鈥渟ponging line鈥� 鈥� but a different line to the one identified in the Shark Bay site. Kr眉tzen, also presenting at the South Africa conference, says this is the strongest evidence to date for culture in dolphins. 鈥淚鈥檓 now convinced this behaviour is social learning 鈥� and from that point of view, you can call it a culture,鈥� he says.
