Early Social Bonds Predict Survival in Male Dolphins
October 15, 2012 – Male dolphins that are socially well connected during the three to six years before they are weaned are more likely to survive to adulthood, according to a new Georgetown study.
The prestigious online journal PLoS ONE today published the study by first author and alumnus Margaret Stanton (G’11) and Georgetown’s renowned animal behaviorist Janet Mann.
Mann, a professor of biology and psychology, has conducted research on bottlenose dolphin calves in Shark Bay, Australia, for more than 25 years.
“Animals such as primates, dolphins and elephants are … socially complex and live a very long time, so it often takes decades to figure out the consequences of a set of behaviors or relationships,” notes Stanton, who obtained her Ph.D. in biology from the university.
She notes that “in humans, having a supportive social network is linked to a number of positive outcomes, including lower stress, higher immune function, and lower rates of mortality from disease.”
Mann explains that from an early age “male calves are attracted to other male calves as play partners, often leaving their mothers to find their young friends.”
They chase, display and leap about for as much as 20 minutes or more, she says.
“In contrast, male calves do not seek juvenile male play partners,” Mann adds. “Interactions between young calves and older juvenile males are not as friendly, and seem to be more about dominance rather than reciprocal role exchange seen between two male calves.”
Targets of Aggression
Mann says the so-called “juvenile period” after the calf stage is particularly difficult for male dolphins.
“Even though both sons and daughters stay in the same area after weaning, sons are no longer under the protection of their mothers,” she explains, “whereas daughters typically continue to have strong bonds with the mother.
“Additionally, males are the likely targets of aggression from both juvenile and adult males.”
The study notes that bottlenose dolphins experience prolonged periods of development. In Shark Bay, calves begin catching fish at four months of age, but are not weaned for several years and don’t reach sexual maturity until they are between 10 and 15 years old.
“Unlike primates, juvenile bottlenose dolphins are not buffered by stable kin groups between weaning and sexual maturity,” the study goes on to explain, “therefore the post-weaning social and ecological challenges facing young bottlenose dolphins may be even greater than those facing young primates.”
But both male and female bottlenose dolphins remain in their natal areas before they are weaned, enabling both sexes to form social relationships early in life that can persist into adulthood.
Female vs. Males
The study also showed that juvenile males have a significantly higher mortality rate in general than juvenile females.
“One reason early social bonds do not predict survival of juvenile females is that most females survive,” Stanton notes. “Daughters tend to adopt the lifestyles of their mothers, including her social network, whereas sons must make their way into an alliance.”
Once formed, these alliances can last for decades as males vie for access to fertile females.
“Social connections really do matter,” Mann says.