Male Amazon river dolphins make themselves attractive to the opposite sex by carrying a branch or similar piece of flotsam in their mouth as an attention-getter, the first such behavior documented in aquatic mammals, researchers discovered.
Such behavior has previously been noted only among land animals and only seen in humans and chimpanzees.
These new findings help show that humans are "not as different from other animals as some might like to think," said researcher Anthony Martin, a behavioral ecologist and population biologist at the University of St. Andrews in Scotland.
Amazon river dolphins, also known as botos or pink river dolphins (Inia geoffrensis), live mostly off fish in the Amazon River basin, with the occasional turtle or crab. The botos had often seemed to play with items such as sticks or lumps of hard clay, thrashing them against the surface of the water or tossing them with flicks of their heads.
Martin and his colleagues carried out hundreds of subsequent observations of dolphins. They found the overwhelming majority of those who carried items were adult males, which are larger and pinker than females.
"It's particularly interesting that the complexity of this behavior in these dolphins is considerably greater than that in chimps," Martin said. "Chimp males break off branches, thrash them around and make a lot of noise to show off how macho they are — bit like blokes with big motorbikes and Ferraris, I guess. Botos, however, are much more subtle, and often use their objects in what appears to be a ritualistic way.
"This species has a mythical reputation for enchanting and seducing women in Amazon communities, and you could believe that they really are enchanting their own females with this object-carrying behavior," Martin added.
Aggression among males — such as biting or striking another dolphin with the head or tail — was strongly linked with object-carrying, and perhaps was linked to access to females, the scientists added.