Over 30 species of cetaceans – or whales and dolphins – have been observed swimming in mixed groups, but because most of such observations are brief and occur only rarely, the majority of these reports are anecdotal and not robustly studied. One such grouping, however, bucks this trend. Off the Bahamas, there are two species of dolphin, the bottlenose and the spotted, that regularly hang out together.

Researchers have been studying the associations of these dolphins and estimate that the two species spend about 15% of their time mixing with each other. This is not an insignificant amount of time, with the animals observed socializing, playing, and fighting with members of the opposite species. This level of interaction between the two species of cetacean is unheard of.

“They have quite a complex relationship,” explained Denise Herzing, from the Wild Dolphin Project, who coauthored a study looking into the association patterns of the species, published in Marine Mammal Science. “Lots of times they cooperate, other times they fight, and we’re not sure what they’re fighting about.” The Wild Dolphin Project has been studying the dolphins in the Bahamas for over 30 years.

It’s unlikely to be about food and could be about territory, but, as Herzing told IFLScience, “we suspect part of the battle is to keep cross-species mating from happening.” They think that the bottlenose dolphins are probably mating with the female spotted dolphins, using their larger size to dominate the male “spotteds.” But why would the bottlenose dolphins want to mate with females from a different species?

“Well, dolphins are very sexual in general,” says Herzing, but it could simply be just because they can. Because it turns out that the male bottlenoses don’t only limit themselves to the females. “They also will dominate the ‘spotteds’ by copulating and mounting with the males, so we have a dominance display between the species with the bottlenose going after the male spotted dolphins.”

In order to counter the larger bottlenose dolphins, the spotteds have their own strategy. The males have synchronization abilities, and are able to faultlessly synchronize their behavior as small organized, coordinated and threatening groups. But that’s what it takes to stop the bottlenose dolphins from having complete dominance.

“But at the same time they know each other, probably as individuals to a certain extent, and can help each other,” explained Herzing. “We have pregnant bottlenose and pregnant spotted hanging out together sometimes, we have interspecies babysitting, which is really interesting. So they’re like neighbors who run into each other occasionally and have figured out a way to live compatibly with each other.”