Reef manta rays (Mobula alfredi) form social bonds and actively choose their social partners, according to a study by researchers from the Marine Megafauna Foundation, Macquarie University, the University of Papua, and the University of York.
Sharks and rays are often thought to be solitary creatures, but reef manta rays typically form groups at shallow-water feeding and cleaning sites.
Rob Perryman, a researcher with the Marine Megafauna Foundation and PhD student at Macquarie University, and his colleagues studied the structure of more than 500 of these groups over five years, in Indonesia’s Raja Ampat Marine Park, one of the most biodiverse marine habitats on Earth.
They found two distinct but connected communities of rays living together.
These social communities were quite differently structured, one being made up of mostly mature female rays, and the other a mix of males, females and juveniles.
“We still understand very little of how mantas live their lives, but we know they are socially interactive, and these interactions seem important to the structure of their populations,” Perryman said.
“Understanding social relationships can help predict manta ray movements, mating patterns and responses to human impacts. That’s essential for conservation and ecotourism efforts.”
The scientists used social network analysis to show that manta ray communities contain a web of many weak acquaintances, with some stronger, longer-lasting relationships.
Though they do not live in tight-knit social groups, the team noticed that female mantas tend to make long-term bonds with other females, while males did not have many strong connections. This could be due to different reproductive strategies or dispersal patterns.
“Like dolphins, manta rays are intelligent and perform collective behaviors such as foraging and playing,” Perryman said.
“They are curious, often approaching humans, and individuals appear to have different personalities. It turns out that reef manta rays actively choose to group with preferred social partners.”
To identify social structures, the study authors took identification photos of all rays present in each group, and monitored whether individuals were more likely to be seen together than expected if encounters were random.
The locations used by the rays seemed to be important to their social relations.
Manta rays form groups at cleaning stations where they are attended to by cleaner wrasse and other small fish.
Perryman and co-authors observed that certain social groups were regularly seen together at these sites, and so may be using them as social meeting points. Some rays had very strong affinity to certain cleaning stations.
The researchers were surprised to find this given the close proximity of all sites and that mantas are generally mobile and wide-ranging animals.
“The rays left and returned to preferred sites where they formed groups through a ‘fission-fusion’ social process,” they said.
The study was published in the August 2019 issue of the journal Behavioral Ecology and Sociobiology.
Rob Perryman et al. 2019. Social preferences and network structure in a population of reef manta rays. Behavioral Ecology and Sociobiology 73 (8): 114; doi: 10.1007/s00265-019-2720-x