Scientists spotted the unlikely friends near the Azores Islands, an archipelago about 900 miles off the coast of Portugal.
According to a recent report in Science Magazine, two behavioral ecologists from German marine research firm Leibniz Institute of Freshwater Ecology and Inland Fisheries watched the adult bottlenose dolphin with an S-shaped spine "while it nuzzled and rubbed" against a group of sperm whales that were traveling together. The magazine notes that the sperm whales even "reciprocated" the contact.
The researchers observed this behavior seven times over an eight-day period in 2011, and they believe the relationship between the whales and the dolphin to have been a purely social alliance, per Science Mag. The researchers theorized that the dolphin's scoliosis prevented it from keeping up with its pod, or gave the dolphin "a low social status," prompting it to seek the company of the whales, according to the article.
Rare though inter-species friendships may be, this isn't the first time that dolphins befriending whales has made the news. In 2008 a bottlenose dolphin who was known for playing with humans led two beached pygmy sperm whales back to safety after apparently responding to the whales' calls for help.
In response to the incident, the BBC, citing Justin Gregg of the Dolphin Communication Project, reported that a pygmy sperm whale and dolphin "might have signals in common," which would allow them to communicate with one another. But Gregg stressed that their means of communication would have been very basic, the BBC notes.
Although dolphins are not an endangered species, the sperm whale is one of about 2,000 on the Endangered Species list and is classified as "vulnerable" by the International Union for Conservation of Nature (IUCN), which publishes The Red List of Threatened Species to determine risks of extinction to plants and animals around the world.
Things may get better for the sperm whale in coming years, however. The IUCN points out that, while commercial whaling caused a significant reduction in the global population of the sperm whale in the past, whaling is no longer widespread and its effects are reversible.