Scientists have discovered a pod of sperm whales in the North Atlantic Ocean that seems to have adopted a dolphin with a spinal malformation.
Behavioural ecologists Alexander Wilson and Jens Krause of the Leibniz-Institute of Freshwater Ecology and Inland Fisheries in Berlin found the mixed-species group off the island of Pico in the Azores in 2011, ScienceNOW reports. Over eight days, the scientists spotted the bottlenose dolphin swimming with the whales a further six times, nuzzling against them.
"It really looked like they had accepted the dolphin for whatever reason," said Wilson. "They were being very sociable."
Sperm whales aren't known for creating bonds with other species, National Geographic News reports.
Researchers presented several theories as to why the whales, known as fierce squid hunters, allowed the dolphin to swim with them in an upcoming paper in the journal Aquatic Mammals.
"The researchers could be sure that the bottlenose dolphin was the same one each time because it had a rare spinal curvature that gave its back half an 'S' shape. Although the dolphin seemed otherwise healthy, that probable birth defect could be the key to understanding its attachment to the sperm whale group," ScienceNOW reports. "Very few predators stalk the Azorean waters, so they doubt that it needed the whales for protection. But they speculate that the malformation could have put the animal at a disadvantage among its own kind. Perhaps it couldn't keep up with the other dolphins or had a low social status."
Another theory is that the dolphin fit in with the sperm whales because of the way the whales "babysit" their calves — one adult whale always remains close to the surface to watch the young ones.
"What is likely is that the presence of the calves — which cannot dive very deep or for very long — allowed the dolphin to maintain contact with the group," Wilson said.