In the waters off the northern coast of New Zealand swims a group of one of the world's most poorly understood cetaceans. Named for their resemblance to their better-known cousins, false killer whales dwell in warm tropical and temperate seas across the globe. But humans usually find them only when they become stranded.

Because false killer whales are so elusive, scientists have only a basic understanding of their social lives. Past studies of individuals near Hawaii and Costa Rica have found that false killer whales are social animals that can maintain friendships—swimming, hunting and cavorting—for years.

They also form relationships that cross species boundaries. In a new study researchers tracked the movements and interactions of New Zealand's false killer whales from a few dozen sightings spread over 17 years. On the rare occasions that the animals were spotted, they were often accompanied by common bottlenose dolphins. Using photographs to identify individuals by their distinctively notched dorsal fins, the researchers found that social pairings between individuals of the two species span both time and space. Some of the interspecies pairings lasted more than five years, with pairs spotted together at locations up to 650 kilometers apart. The study was published online in Marine Mammal Science.

Some benefits of interspecies groupings may be purely practical. For starters, the fish that both mammals eat tend to be found together, “with the dolphins preying on kahawai and the whales preying on the much larger kingfish,” says lead study author Jochen Zaeschmar, a graduate student at Massey University in New Zealand. The two species probably also benefit from working together to detect and avoid predators.

But Zaeschmar also found evidence of social contact between the two species, such as two animals touching as they swam side by side. “The fact that interactions between individual members of each species were observed regularly over the course of five years is an important finding,” says Justin Gregg, a research associate at the Dolphin Communication Project. It means that false killer whales and bottlenose dolphins choose to spend time with specific members of the other species rather than randomly mixing or engaging in brief opportunistic encounters.

False killer whales are not the only creatures with diverse friends. “We observe giant moray eels and coral groupers—two distantly related species—foraging in a truly mutualistic and cooperative fashion,” Gregg says. So perhaps it should come as no surprise that the marine mammals, “with their complex social behavior, are capable of engaging in equally as sophisticated mixed-species interactions,” he adds.