Just as men can use fast cars or showy clothes to impress the ladies, so too do male Amazon river dolphins show off stuff to woo the opposite sex.

Among these dolphins, the attention-getter is a male carrying a branch or similar flotsam in its mouth. Such "player" behavior is a first to be documented in aquatic mammals and, among land mammals, it has previously been seen only in chimpanzees and humans, researchers said.

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.

One day scientists noticed that three botos that held objects in their mouths were all adult males. This prompted speculation that such behavior might not be play at all.

Martin and his colleagues found the overwhelming majority of those who carried items were adult males, which are larger and pinker than females.