Manta rays usually like to swim on their own, but that doesn't mean these creatures are complete loners. Despite their reputation as solitary fish, a five-year-long study in Indonesia suggests manta rays are more social than we give them credit for.
Reef manta rays (Mobula alfredi) are known to congregate at feeding and cleaning sites, but researchers have never been sure exactly why. It could be these fish are merely drawn to the same locations, where food or protection is guaranteed.
But there is another explanation, too: perhaps it's because they're drawn to each other.
"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," says marine biologist Rob Perryman from Macquarie University in Australia.
Tracking more than 500 groups of manta rays at Indonesia's Raja Ampat Marine Park, Perryman and his colleagues have now described their secret social structures for the first time.
Not only do manta rays appear to strike up close social bonds, the team says they actively choose to group with preferred social partners.
"They're actively choosing to group with another ray they're basically friends with, rather than coincidentally bumping into them," Perryman explained to the ABC.
Recording 3,400 encounters with nearly 600 different mantas, the researchers found two distinct communities of rays living in the same area. One of the groups was mostly made up of females, while the other group was a mix of females, males, and juveniles.
These two groups were not exactly close-knit - at least not in the way that communities of dolphins or whales are - but there were some stronger relationships among the many weaker ones.
At cleaning stations, where manta rays are groomed by cleaner wrasse and other small fish, researchers kept noticing the same groups. Using unique spots on their bellies to identify specific individuals, the team started logging where and with whom each manta ray spent their time.
"Many individuals were observed multiple times at a single cleaning station, but infrequently or not at all at others, indicating strong site preferences," the authors write.
"In general, female rays were more likely to be encountered at cleaning stations than males, while mature males were more likely to be encountered at feeding areas."
At cleaning stations, the researchers noticed strong social bonds lasting over several weeks or months. These were particularly apparent among female manta rays and in mixed sex relations, although the juvenile males also seemed to stick together, albeit to a lesser extent.
When it came to adult male relationships, there was only weak evidence for short-term bonds.
The findings have the researchers thinking that maybe these cleaning stations are handy meet-up points for socialising with others.
"Enabling social behaviour may be a primary cause of manta ray visitations to cleaning stations, that act as 'social gathering points'," the authors suggest.
"Hierarchical social organisation in these locations could allow mature females to group with preferred social partners and simultaneously avoid unwanted mating attempts by mature males."
But are these manta rays actively seeking out their friends, or are they merely creating acquaintances at the sites they like to frequent? Perryman and his colleagues have shown that these close social bonds are not random, but that doesn't mean they aren't opportunistic.
The social relationships highlighted in this study seem to be fickle and short-lived compared to dolphins and whales, and that's only based on what they're doing at cleaning and feeding sites.
Further research will need to be done before we can say any more about the secret lives of manta rays, but this isn't just about human curiosity. Finding out how manta rays interact with one another is crucial if we want to protect these already vulnerable creatures from extinction.
"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."
The findings were published in Behavioral Ecology and Sociobiology.