A five-year study in Indonesian waters has confirmed that wild-roaming reef manta rays (Manta alfredi) form selective bonds with other rays, providing evidence of structured social relationships.

Dwarfing humans who are lucky enough to swim near them, mantas are the largest rays in the ocean, with two recognised species – the reef manta and the giant manta (Manta birostris).

The reef mantas swim at shallow-water feeding and cleaning sites, and their appeal to divers is starting to disrupt their natural behaviour. Combined with climate change, plastic pollution, fishing nets and fisheries that target them for their gills – prized in traditional Chinese medicine – this is threatening the species’ survival.

Robert Perryman from Macquarie University, Australia, and colleagues sought to understand the rays’ social structure to help predict their movements, mating patterns and response to humans to inform conservation and ecotourism.

“Social relationships are something that might be quite easily disturbed by human tourists who have good intentions but may inadvertently disrupt natural social behaviours,” Perryman explains.

Having previously collected data on them at the study’s site, the richly biodiverse habitat at Raja Ampat Marine Park, he had observed that the gregarious mantas exhibited group-like behaviours during feeding and courtship.

But whether that reflected active social preferences or occurred by chance due to feeding aggregations, habitat preferences and seasonal migrations was unclear, and little is known about their social relationships compared to other terrestrial and marine animals.

The undisturbed location provided an ideal opportunity to study the mantas’ social structure and behaviour, with its shallow waters making it easy to observe whole groups of them.

The team gathered data while scuba diving or snorkelling around collection stations or feeding areas.

They individually identified each ray by its pattern of belly spots; like human fingerprints, the spots are unique and remain the same throughout their lives, making them easy to distinguish from each other.

“And they’re very curious and tolerant of humans,” Perryman says, “so it’s usually possible to identify all the individuals within a group during a 60-minute dive.”

From 500 samplings they recorded around 3400 encounters, and by matching each photo to previous sightings they built up a database.

This enabled them to build a social network of relationships between rays that were seen at least 10 times, added to sightings recorded from photographs uploaded to a citizen science website (comprising 10% of the overall data).

Sightings were taken from six sites spanning a 25-kilometre area.

To identify social structures, the researchers analysed whether individual mantas were more likely to be seen together at different times and different locations than would be expected at random, differentiating social from non-social associations.

They found non-random affiliations that segregated the rays into two distinct social communities within a very small area spanning around 15 kilometres. One comprised mostly mature female rays and the other a mix of males, females and juveniles, with little interaction between them.

The females formed longer-term associations, congregating together over weeks and even months. Males tended to form more casual acquaintances, with juveniles and mature males staying away from each other.

Perryman expected to find evidence of social structures. “From their large brains we expect them to be reasonably intelligent animals,” he says.

But he was surprised “by the fine spatial scale that this appears to operate at in the wild” despite being wide-ranging animals, as evidenced by the distinct social groups with strong locational preferences that appeared to be used as meeting points.

By understanding their behaviour, the researchers hope to achieve the right balance between species preservation and ecotourism.

“Knowing how mantas interact is important, particularly in areas where they are susceptible to increasing dive tourism,” says Andrea Marshall, co-founder and principal scientist at the Marine Megafauna Foundation.

“The increasing number of boats and scuba divers around reef mantas in Raja Ampat, particularly at cleaning stations, could break apart their social structures and have impacts on their reproduction.”

Perryman hopes to inform conservation efforts by taking action to avoid disturbing their reproduction and instilling broader respect and compassion for the graceful sea creatures.

The paper is published in the journal Behavioural Ecology and Sociobiology.