May 18, 2009
Researchers Report Pack Of Dolphin’s Trying To Kill A Calf
Researchers have reported witnessing adult tucuxi dolphins trying to kill a newborn calf of their own species in what is known as the first record of these dolphins attempting infanticide, BBC News reported.
While it is often noted in various mammal species, infanticide is rarely recorded among cetaceans, the group of animals that includes whales and dolphins.
Experts say the behavior has only been reported two other times in bottlenose dolphins, yet now they believe it may be much more common than they originally thought.
Tucuxi dolphins (Sotalia guianensis) make their homes in either the freshwater of the Amazonian basin or off the coast of Brazil to Nicaragua.
While the adult male marine tucuxis has been observed showing aggression toward other males during the breeding season, researchers have never known them to be violent towards younger members of their species.
However, university researchers surveying the dolphins in Sepetiba Bay, Brazil noted exactly that behavior when they saw a group of six adult tucuxi dolphins approach a mother with her newborn calf.
The male dolphins separated the baby from its mother by hitting her body with their flukes and ramming her. Although the female attempted to avoid the interlopers, the males herded her away from her offspring.
The researchers reported that the female dolphin frequently exposed her belly at the water surface, in what was determined as either a passive behavior or a signal that she would be sexually receptive to the males.
Two of the adult males frequently rammed the calf and held it underwater while others kept the female away.
The calf was tossed into the air and again pushed underwater, where afterwards it became disorientated and showed difficulty swimming.
The researchers reported seeing the mother a few days later, yet no evidence of the calf's survival was found.
The researchers wrote in the journal Marine Mammal Science that they believe the injuries the newborn calf received from the encounter were fatal.
Mariana Nery of the Southern University of Chile in Valdivia said it was the first time they had seen that type of aggression in that species of dolphin.
"It is difficult to say if they really attempted to kill the calf or if it was playful behavior that went too far. But they clearly looked to separate the calf from the mother," she said.
Nery and colleague Sheila Simao of the Federal Rural University of Rio de Janeiro, Brazil, believe the incident adds to the growing evidence that wild dolphins may indeed practice infanticide.
Males of other animal species often kill babies they did not father in order to induce the female to become sexually receptive to them.
Nery said female dolphins become sexually receptive within a few days of losing a calf.
Therefore, she believes that fact combined with the sexual interest shown by the group in the mother suggests they killed the calf for similar reasons.
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