Social lives of whales and dolphins shape the spread of infectious diseases

Dolphins, whales and seals are highly social animals. Many live in groups, form long-term relationships, and repeatedly interact with the same individuals over years or even decades. Some dolphins have preferred companions, while others move between groups in fluid, ever-changing social networks.

These social lives bring many benefits, from cooperative foraging to protection against predators. However, our new research, published in the journal Mammal Review, shows they also come with a hidden cost: social connections can shape how infectious diseases spread through marine mammal populations.

By bringing together decades of research from around the world, we unravelled that disease outbreaks in the ocean are shaped not only by how many animals are present, but by who interacts with whom.

Disease outbreaks are hard to predict

Infectious diseases are an increasing threat to wildlife populations globally, and marine mammals are no exception. Dolphins, whales and seals face growing pressures from climate change, pollution, habitat disturbance and human activity. These stressors can weaken their immune systems and make animals more vulnerable to infections.

Yet disease outbreaks in marine mammals are often sudden and difficult to explain. One year a population looks healthy; the next, animals are turning up sick or dead.

Some of the most dramatic examples involve morbilliviruses, highly contagious viruses related to measles, which have caused mass die-offs of dolphins and seals in Europe, North America and Australia.

In other cases, skin diseases such as lobomycosis-like disease have spread through dolphin communities, leading to chronic lesions, weakened health and increased vulnerability to other threats.

Part of the challenge is that the ocean hides much of what is happening. Unlike on land, scientists can’t easily observe every interaction, isolate sick individuals, or intervene early when disease begins to spread.

Another reason outbreaks are hard to predict is that disease doesn’t spread evenly through a population. Just as COVID spread faster in some human communities than others, infections in marine mammals often follow social pathways shaped by relationships and behaviour.

Finding the patterns

To understand these patterns more clearly, we conducted a systematic review, carefully gathering and analysing all available scientific studies that examined links between marine mammal social behaviour and infectious disease spread. Our final dataset included 14 studies that were geographically biased toward North America and Australia.

We looked at research on dolphins, whales and seals that used an analytical tool called social network analysis. This is a way of mapping who interacts with whom in a population, much like mapping connections on social media.

Scientists use photographs, observations and long-term monitoring to identify individual animals and track their associations. From this, they can measure how many companions an animal has, how often it interacts with others, and whether it occupies a central position in the social network.

We then examined how these social patterns related to disease presence, outbreak size and transmission risk.

Watch out for ‘super-spreaders’

One of the clearest findings was that highly connected individuals often play an outsized role in disease spread.

These animals, sometimes called “super-spreaders”, interact with many others and can rapidly pass infections through a population.

In dolphin communities, for example, animals with stronger or more frequent social ties were more likely to be associated with disease. In some cases, outbreaks appeared to hinge on just a few socially central individuals.

At the population level, social structure also mattered. While larger groups can initially facilitate disease transmission through frequent and close contact, the way those groups are organised is crucial.

In some populations, the presence of subgroups (semi-stable sets of animals that spend more time interacting with each other than with others) can act as barriers, slowing the spread of disease between clusters.

In other cases, tightly connected subgroups can allow infections to persist or spread more intensely within the population. As a result, a small but highly connected population may be more vulnerable to disease than a larger population with fewer close interactions.

We also uncovered major gaps in knowledge. Most studies focused on a handful of well-studied species, especially bottlenose dolphins, and were concentrated in North America and Australia.

Many threatened species and regions remain poorly studied, limiting our ability to assess global disease risk.