Australia's bottlenose dolphins have long been known to use sea sponges as a tool for finding a meal—but now researchers may have observed males of another species attempting to woo females with them.
Over a period of 10 years, a team of marine biologists watched male Australian humpback dolphins present large, ornate sponges to females—and on occasion even toss these putative love tokens their way.
“A display to impress a female is not unusual, but using an object in that display is very unusual,” says study leader Simon Allen, a biologist at the University of Western Australia in Perth.
And for a non-human mammal to use an object as part of a courtship display is almost unheard of, he says—which is what makes the new discovery so exciting.
Apes, birds, and cetaceans—a group that includes whales and dolphins—use tools, but the practice is relatively rare and mostly restricted to foraging.
For instance, bottlenose dolphins in Western Australia's Shark Bay will sometimes place sponges onto the tips of their sensitive beaks, which protects them from injury as they scour rocky parts of the seafloor for bottom-dwelling fish.
For the new study, Allen and colleagues observed male humpback dolphins proffering sponges to females on 17 occasions along a 1,000-mile stretch of northwestern Australia. The behaviour occurred in five separate dolphin pods, according to the study, published October 20 in the journal Scientific Reports.
The targeted females were sexually mature, as they were often accompanied by calves of weaning age and thus ready to mate again. Australian humpback dolphins are not monogamous, and both males and females may mate with several partners during the breeding season.
The courtship behaviour was sometimes associated with what the experts refer to as the "banana pose"—when a male arches his back, head, and tail above the water. (See 10 intimate pictures of dolphins.)
“We don’t yet know what he’s doing—whether he’s flexing or sporting an erect penis, we have no idea,” Allen says. “But sometimes he’s just lying there for a while posing near the female, and at other times moving along in the water just behind her.”
The sponges offered by the males were too large to be used as a tool, suggesting it's a symbolic gesture. - PHOTOGRAPH BY JOSH SMITH
It's unknown how the females react to the males' displays, and if they were more likely to mate with the sponge-toting males. (The females definitely did not use the gift sponges for foraging, Allen notes.)
That's why the team wants to observe the sponge courtship underwater, as well as eventually test DNA of offspring to see if gift-giving males fathered more babies.
The discovery is both “fascinating and unexpected,” says Richard Connor, a dolphin expert at the University of Massachusetts Dartmouth who has for decades studied the sponge-using bottlenose dolphins of Shark Bay.
Those male humpback dolphins also “carry sponges and use them in a kind of display toward females is very cool,” he says, noting that it’s not clear whether the males do it to display dominance, or advertise themselves as a suitable mate—or both.
While foraging, dolphins put sea sponges on their noses as a cushion against the rocky seafloor. - PHOTOGRAPH BY ALEX BROWN
Connor adds that “dolphins are generally innovative,” and says he would not be surprised to learn if the marine mammals use other objects as well. (Read about a new species of dolphin discovered in Australia.)
Lindsay Porter, an expert on humpback dolphins at SMRU Consulting in Hong Kong, says the discovery was important for a "species that we know very little of, with regards to their society, social structure, and culture."
The study also "adds to the growing body of literature indicating that all dolphin societies may be far more complex than we ever perceived before,” she says.
In 2013, for example, study co-author Stephanie King and colleagues reported that bottlenose dolphins each have signature whistles to refer to themselves and each other—much like human names.