Atlantic bottlenose and spotted dolphins are cooperating in unique mixed-species groups that are mostly platonic, but sometimes cross-species sex is involved

Swimming in sync (Image: David Fleetham/Naturepl.com)

IT’S a social network like no other. The dolphins of the Bahamas forage and play together and forge alliances – even though they belong to two distinct species. They’re not the only example of mixed-species dolphin groups, but this level of interaction is unprecedented.

“These interactions likely evolved to allow the species to share space and resources and maintain a stable community,” says Cindy Elliser at Pacific Mammal Research in Anacortes, Washington. She previously worked with Denise Herzing at the Wild Dolphin Project, which has been studying social ties between Atlantic bottlenose (Tursiops truncatus) and spotted (Stenella frontalis) dolphins in the Bahamas for 30 years.


The observations suggest that the two species spend about 15 per cent of their time together, says Elliser. More than half of those encounters involve “friendly” behaviours. “They play and forage together. They even babysit,” she says. On at least two occasions adult female spotted dolphins appeared to be caring for baby bottlenose dolphins for short periods. Researchers have also seen three spotted and two bottlenose dolphins, all pregnant, swimming together.

Elliser says the reasons for these specific behaviours remain a mystery, but it is not simply down to a few individuals with a confused sense of their own identity. “I think that the friendly behaviours are an important part of maintaining relationships.”

“They play, forage and even babysit together. It’s an important part of maintaining relationships”

The males, too, form alliances to fight common foes. In one encounter, for example, two males from each species banded together to confront and chase off an intruder – another male bottlenose dolphin.

We knew that male dolphins routinely form alliances with others of their species. For example, male bottlenoses from Shark Bay, Australia, form large “third-order” groups made up of gangs that are, in turn, made up of two or more duos or trios. But the cross-species dolphin alliances in the Bahamas are unique so far, says Elliser.

Humans are the only other animal known to form such complicated nested alliances, says Richard Connor at the University of Massachusetts, Dartmouth – although ours go beyond three orders. We cooperate at the family, community, national and international level, for instance. Early human species probably interacted with each other, too (see “Neanderthals as neighbours“).

It is probably no coincidence that dolphins and humans share this sophisticated cognitive ability to cooperate, Connor says, since these are the two animals with the largest brain for their body size. He also says there may be real social complexity in the Bahamas encounters, particularly given that so many of them seem to be friendly. This only adds to the evidence that dolphin social ties are particularly sophisticated.

Not all interactions between the two species are friendly, though. Bottlenose males are about twice as long as spotted males and sometimes exploit this to force their way into groups of spotted dolphins and mate with females. Elliser and Herzing found that male spotted dolphins can fend them off, but only by cooperating in very large groups (Marine Mammal Science, doi.org/583).

This cross-species mating may be a way for juvenile male bottlenoses to learn to compete with large males of their own species, Elliser suggests. It may also explain why a larger proportion of spotted rather than bottlenose dolphins routinely engage in the mixed species encounters: the smaller spotted dolphins have to work harder to keep the interactions even.

It’s still too early to say we fully understand the motives behind encounters between dolphin species, but we might have more chances to study them as climate change forces them to share the same waters.

“These types of interactions in social animals may become more common,” says Elliser.

Neanderthals as neighbours It isn’t just species of dolphin that enter into complex social interactions. For thousands of years our ancestors lived alongside, and even interbred with, Neanderthals. “What were those meetings like?” asks Simon Underdown from Oxford Brookes University, UK. Some were probably productive, but fossil evidence shows others were violent, he says. “The mixing of Neanderthals and humans in the Middle East and Europe would have resulted in not only the sharing of ideas, resources and sexual partners, but also violence, rape and destruction.” David Frayer from the University of Kansas thinks Neanderthals may even have been able to speak. So why did they disappear? “My guess is that Neanderthal populations were so small that they just got swamped by the new groups moving in,” he says. Mico Tatalovic

This article appeared in print under the headline “Two species with a yen to live as one”