SYDNEY: Australian bottlenose dolphins that use marine sponges to forage for food have been found to socialise in cliques, in the first definitive example of subculture in animals.
The behaviour of the ‘sponger’ dolphins of Shark Bay in Western Australia is the first definitive example of a non-human subculture – where individuals preferentially associate with others of their species who share their behaviours or interests.
Most animal species are thought to learn behaviours as a consequence of membership of social groups bonded by family ties, gender and territory. The workaholic spongers exhibit the opposite of this phenomenon: instead, the foraging behaviour forms the basis of the clique.
“That’s the big difference here,” said Janet Mann, first author of the Nature Communications paper and behavioural ecologist at Georgetown University in Washington D.C.
Dolphins strong candidates for cliquish behaviour
In general, identifying cultural behaviours in animals is difficult.
“I grew up in New York and if I’m in a room with people from New York and people from other parts of the U.S., I might more naturally gravitate to the people from New York – we share a cultural identity,” said Mann.
“[But] how would you recognise this in an animal? It’s really, really hard to do it because you need to figure out what they are identifying [with],” she said.
As a species, dolphins presented as strong candidates for cliquish behaviour. Their societies are human-like: social interactions can involve anything from two to 30 individuals and may be as brief as a few seconds, the dolphin equivalent of firing off an email.
Sponging passed from mother to calf
“I don’t know of any other animal that has quite that kind of intensely fluid society with strong, imbedded social bonds, except for humans,” said Mann.
The dolphin community of Shark Bay was of particular interest. A well-defined subset of the population is known to use sponging as a foraging technique. The dolphin breaks off a basket-shaped sponge and positions it on its rostrum or beak area, using it to probe the sandy seabed for prey.
“They get these fantastic sponges. Some of them are practically as big as their head,” said Mann.
Sponging is passed from mother to calf and once a calf is weaned, the activity is a solitary one. The behaviour is easily observed during boat-based surveys, as the dolphins bring their sponges to the surface when they breathe.
15,000 surveys over 22 years
“With the sponging, it’s so clear who does it and who doesn’t do it that we can test whether the dolphins both recognise and respond to this trait in others,” said Mann.
The researchers performed a mammoth 15,000 surveys over a 22-year period, recording the interactions between known individuals identified by body and dorsal fin markings.
Sponger dolphins are a minority, comprising only 4% of the Shark Bay population. Because of these low numbers and the potential for sponger statistics to be overwhelmed by non-sponger statistics, the researchers restricted their surveys to a 22 square kilometre section of the bay that the spongers – and many non-spongers – were known to occupy.
They used social network analysis to produce a comprehensive picture of dolphin society in the area, detailing direct and indirect links between individuals. In an analogy with Facebook, the analysis not only told the researchers who was friends with who, it showed whether an individual’s friends were also friends with each other – one measure of cliquishness. The researchers also used weighted measures that took into account the amount of contact time between individuals, enabling them to distinguish between strong and weak social bonds.
Strong sponger-to-sponger preferences
The researchers’ analysis revealed that the sponger dolphins had a strong preference for the company of other spongers over non-spongers, forming more bonds and stronger bonds with other spongers. Strikingly, the spongers’ preference to spend time with their own kind was observed despite their relatively small numbers and their comparatively solitary lifestyles. The preferences were found to exist on top of affiliations resulting from family ties, gender and territory.
“Everything showed that they preferentially affiliated with [each other] even though they tend to be less social,” said Mann.
Mann thinks the reason for the spongers’ social preferences boils down to a shared lifestyle that is quite distinct from that of the non-spongers. By associating more closely with other spongers, individuals can share information on sponge and prey location. As known workaholics with similar activity levels, they may also be more inclined to spend time with individuals who share that behaviour.
Hugh Finn, a conservation biologist and dolphin specialist at Murdoch University in Perth, Western Australia, concurs that the study findings are significant, but is unsurprised by them.
“There’s something of a running joke that the dolphins in Shark Bay are the smartest in the world. That’s not true, of course. It just happens to be that the dolphins in Shark Bay have been studied intensively for a long-time,” said Finn.
“Bottlenose dolphins, like humans, are biologically much the same wherever we are. Which is kind of wonderful to think about – what kinds of subcultures might exist among the dolphin communities in your neck of the woods?”
Original paper in Nature Communications
Shark Bay Dolphin Project homepage
Janet Mann’s homepage