Franciscana dolphins are one of the most elusive dolphin species in the world.
They’re quite small — measuring only about five feet at full size — and they spend most of their time living in the often murky coastal waters of Argentina, Uruguay and Brazil. Most of what scientists know about them comes from studying their remains after they’ve been tangled up in fishing nets as bycatch.
So when Marcos César de Oliveira Santos, a marine biologist from the University of São Paulo, noticed them in the blue-green waters by his research station near Ubatuba, Brazil, he was astonished. He started flying drones over the dolphin’s pods, hoping for insights into how these animals live.
That’s when he saw something curious: The males were babysitting, an extremely rare behavior for male mammals.
When looking over the footage, Dr. Santos and his teammates often observed dolphins traveling in groups of three: a mother with her calf, and a smaller dolphin that the researchers presumed was male. (The adult males are noticeably smaller than females; and the researchers also caught attempts at mating or a look at genitals.) These males seemed to be following the mother-calf duo, tagging along on the hunt for food.
Sometimes, after the three dived together, only the calf and the male dolphin would surface. The mother remained below looking for a meal. The male would stay close to the calf until the mother came up for air, and then he often helped guide the baby back to its mother. Almost as if he were babysitting. “It’s amazing,” Dr. Santos said.
Dr. Santos and his team recorded males babysitting calves almost 300 times during their study over the course of a year and a half. They published their findings in the journal Marine Mammal Science.
Male mammals who actively participate in parental care are extremely rare, but there are a few examples. “In monogamous mammals, like gibbons, the males have quite a big role,” said Hal Whitehead, a marine mammal researcher at Dalhousie University in Canada who was not involved in the study. Gibbon dads groom, defend and play with their babies.
Dr. Whitehead has also observed male pilot whales looking after unrelated calves.
Babysitting is relatively common in dolphins and whales, because the calves can’t spend as much time underwater as their mothers. But this job is almost always done by females. So when Dr. Santos presented his initial observations at a conference two years ago, “it was jaw-dropping for everyone in that room,” he recalled.
How would babysitting benefit the males?
In other dolphin species, males might fight each other to get the opportunity to mate or they might compete by producing more sperm. Franciscanas, though, show no signs of aggression and have small testes, suggesting that they use neither of those strategies. The scientists proposed that these dolphins babysat as a different way to ensure their success.
“A potential reason is that they’re trying to impress the females,” Dr. Whitehead said. “‘I’m a nice guy, wouldn’t you like to mate with me?’ kind of thing.”
The biggest mystery is whether these males are related to the calves or not, which would change the interpretation of the behavior. Are they attentive fathers making sure their baby survives and their genes live on, or just optimistic mates-to-be? To answer this, Dr. Santos wants to get DNA samples from the dolphins.
He hopes that there is enough time. Franciscana dolphins are classified as vulnerable to extinction, and their numbers are decreasing, largely because they’re being scooped up in fishing nets.
As the only surviving species in their taxonomic family, these dolphins are perhaps hiding more unexpected behaviors. Researchers now have the first documentation of male babysitting in dolphins, which “we never thought we would describe,” Dr. Santos said. “We are losing the only species of a family that we know almost nothing about.”
