Sometimes, dolphins swim with another species. A bottlenose dolphin (Tursiops truncatus) seen among spinner dolphins (Stenella longirostris) may have lost a long-term male alliance partner, lost its place in its own group, or joined a larger group for protection. However, only long-term population studies can tell these explanations apart.

A bottlenose dolphin travelling inside a group of spinner dolphins is an uncommon sight, and one of the most revealing things you can witness on the water.

 

At first glance, nothing seems unusual. A group of dolphins moving together, cutting through the blue.

Then you look again. One of them doesn’t belong. Larger, stockier, with a different shape of beak and a different way of moving: a bottlenose dolphin travelling in the middle of a group of spinner dolphins.

Two different species. Two different ways of living. And yet, here they are, together.

This is not common. And that is exactly why it is worth stopping to think about, because the answer changes how you meet every dolphin after it.

When dolphins swim with another species: what are mixed-species groups?

Mixed-species groups occur when individuals of one dolphin species travel, feed or socialise alongside another. Researchers have documented these associations worldwide, and they can last minutes, hours, or in rare cases become a long-term arrangement for a single individual.

Observers have seen dolphins of different species travelling together across the Atlantic, the Pacific, the Mediterranean and the Red Sea. Spinner dolphins, spotted dolphins and bottlenose dolphins are among the species most often observed mixing.

When dolphins swim with another species, however, the reasons resist easy explanation. That is exactly what makes these observations so valuable.

After all, a single dolphin in the wrong group is a question mark. And question marks are where science begins.

Why would a bottlenose dolphin leave its own group?

There are three leading explanations: the loss of a long-term alliance partner, social exclusion from its original group, or a survival strategy in response to predation or environmental pressure. No single sighting can confirm any of them.

1. He may have lost his partner

Male bottlenose dolphins are known for forming strong, durable alliances with other males. These are not casual associations. In populations studied over decades, most famously in Shark Bay, Western Australia, pairs and trios of males cooperate over years, coordinating their movements and navigating a complex social landscape together.

In fact, these bonds are among the most stable relationships documented in the species.

Which means their loss is not trivial.

Consequently, a male who loses his long-term partner finds himself socially exposed in a world built around alliances. For this reason, joining another group, even one of a different species, may be one way to fill that absence.

2. He may have been pushed out

Dolphin societies also operate on codes. Access to a group is not automatic, and it is not permanent.

An individual who fails to respect the social rules of his community, through behaviour, through conflict or through circumstances we cannot observe, may find himself progressively excluded.

In other words, what we read as a dolphin choosing a different group may in fact be a dolphin with very few options left.

3. It may simply be survival

There is also a far more pragmatic possibility.

Spinner dolphins form large, tightly coordinated groups, and numbers offer protection from predators such as sharks: more eyes, more ears, more sonar. In an environment where a solitary animal is far more vulnerable, joining an existing group, regardless of species, may be the most effective way to stay alive.

Similarly, prey scarcity, environmental pressure or increased predation risk could all push an individual toward this kind of adaptation.

Can we know why dolphins swim with another species?

Not from one observation. All three explanations are plausible, and there may be a reason not yet considered. Knowing which one applies requires following the same identified individuals within the same population over many years.

A single encounter is a photograph. It tells you what is, never how it came to be.

Why do long-term dolphin population studies matter?

Long-term studies use photo-identification, recognising individual dolphins by the unique nicks and scars on their dorsal fins, to build life histories across years. This is the only way to reveal whether an unusual behaviour is an isolated event or an early signal of environmental change.

Programmes such as the Sarasota Dolphin Research Program in Florida, running since 1970, and the long-term work in Shark Bay have shown what becomes visible only with time.

Who this dolphin travelled with five years ago. When that association ended. Whether anyone has seen him with this group before, and whether others in his population are showing similar shifts.

Over time, that accumulated record is what turns a curiosity into evidence. It also allows researchers to detect something far more important: how dolphin behaviour is changing in response to a changing environment.

Indeed, shifts in group composition, ranging patterns and association strength are often the earliest signals that an ecosystem is under strain.

Without a long baseline, a behavioural change looks like an anomaly. With one, it looks like a warning.

How does this change the way we watch dolphins?

Understanding why dolphins swim with another species, and what an individual dolphin may have lived through, transforms a generic sighting into a genuine encounter, and makes observers far less likely to crowd, chase or pressure the animal. Understanding is the foundation of ethical dolphin watching, not an optional extra.

The default experience on the water is a generic one. You saw a dolphin. It was beautiful. You took a photo.

But when you know that the animal in front of you may have lost a partner of many years, may be navigating exclusion, or may be adapting to a threat you cannot see, something shifts.

Suddenly, he is no longer a dolphin.

He is that dolphin. With a history. With relationships. With losses.

In short, this is the heart of what respectful dolphin watching actually is. It is not only keeping your distance or following the rules, although those things are non-negotiable. It is the understanding that every individual out there is in the middle of a life you have walked into for ten minutes.

That understanding does two things at once. First, it makes the experience infinitely richer for us. Second, it makes us far less likely to behave in ways that cost them.

You do not crowd, chase or pressure someone whose story you are genuinely curious about.

The more we understand about the dolphins we encounter, their societies, their bonds and their pressures, the less we treat them as scenery, and the more we treat them as what they are: individuals, living complex lives, in a home that belongs to them.

Ultimately, that is where a truly authentic encounter begins.

Read also: Moving as One: what dolphin synchrony teaches us about living together.

Stéphanie Huguenin-Elie

Founder of Dolphin Whisper and advocate for ethical dolphin encounters.

dolphinwhisper.com – Ethical dolphin encounters

Further reading

Sarasota Dolphin Research Program: the world’s longest-running study of a wild dolphin population, since 1970.

Hill-Cousins, S., King, S. L. et al. (2025). Allied male dolphins use synchronous displays to strengthen social bonds in a cooperative context. Movement Ecology, 13, 84.

Quérouil, S. et al. (2008). Why do dolphins form mixed-species associations in the Azores? Ethology.

Frequently asked questions

Do dolphins swim with another species in the wild?

Yes. Researchers have documented associations between dolphin species worldwide. They range from brief encounters while feeding to lasting arrangements where one individual repeatedly travels with a group of another species.

Why do male bottlenose dolphins form alliances?

Male bottlenose dolphins form long-term alliances with other males to cooperate socially and navigate competition within their community. These partnerships can last years and are among the most stable relationships documented in the species.

What is photo-identification in dolphin research?

Photo-identification is a non-invasive research method that identifies individual dolphins by the unique pattern of nicks, notches and scars on their dorsal fins. It allows researchers to follow the same animals across decades without capture or tagging.