On August 23, 2013, Robin Perrtree and her colleagues from Savannah State University became the first scientists in history to witness the birth of a wild bottlenose dolphin. After spending the morning searching for the animals near Tybee Island, Georgia, they spotted a female dolphin thrashing about at the surface, followed by the sudden appearance of a newborn calf together with blood in the water assumed to be from the placenta.
But their celebration was cut short by an unexpected and far less joyful event. Almost immediately after the calf was born, it was attacked by a pair of adult male bottlenose dolphins.
According to a recently published article in the journal Marine Mammal Science detailing the incident, the male dolphins were attempting to drown the newborn. “The immediate impression of the researchers in the field was that this was an infanticide attempt,” suggest Perrtree and her co-authors.
A short video of the incident is available here.
Infanticide is a common behaviour observed throughout the animal kingdom, from insects to lions to humans. Male dolphins have previously been observed attacking calves in Scotland and Florida, and research on dead-stranded calves in Virginia indicates that violent attacks from males of the same species do result in calf deaths.
The attack itself lasted almost half an hour, with the calf surviving largely thanks to its mother’s vigorous efforts to fend off the aggressive males. According to the researchers, "the mother frequently surfaced with the [baby] on her head or back.”
The mother and calf were seen swimming the next day in a group of dolphins that included the two attackers, but the youngster's long-term survival is unknown as neither the mother nor her calf has been seen since 2013.
The researchers involved speculated that the attack on the calf might have been premeditated. The two males had been seen trailing the mother dolphin an hour and a half before. "Maybe it was a coincidence,” suggested Perrtree in an interview with the BBC, “but it definitely raises the question: were they monitoring the female ahead of time?" If so, this might suggest that the males “were tracking the birth in preparation for the infanticide attempt.”
At first glance, it does not make much sense for dolphins to kill members of their own species – especially within their own social group. But this enigma of evolution does have a possible explanation. If a male manages to kill a calf, it will force the female to be fertile sooner, increasing the chances that the infanticidal male could become the father of her next calf.
During the attack on the newborn calf in Georgia, the males were observed with erections, which might indicate that they were attempting to mate with the new mother. While it is possible that the entire incident was simply a mating attempt (with the calf merely caught in the crossfire) and not true infanticide, the researchers suggest that the two might not be mutually exclusive, and that one “would expect them to coincide.”
While dolphins have long been portrayed as friendly and gentle animals in literature and media, this type of violent and possibly infanticidal behaviour confirms what naturalists have long known about them: they are complex social mammals capable of both tenderness and aggression.
The question remains, however, as to how unusual infanticide might be. Much of the attack on the newborn calf reported in this latest incident took place well below the surface. If this submerged calf-drowning technique is the most common method males use to dispatch newborn calves, then it might well be that infanticide, like dolphin birth itself, is commonplace but just rarely observed.
Note: Photographs, video, and data were collected in accordance with the U.S. Marine Mammal Protection Act on 23 Aug 2013 by R. Perrtree under NMFS LOC #14219 issued to Dr Tara Cox.
Top header image: Nancy Magnusson, Flickr