Just like humans, it appears that dolphins prefer to hang out with others who share a common interest.
An international team of researchers made the finding at Shark Bay – a World Heritage Site in Western Australia that’s home to a large population of Indo-Pacific bottlenose dolphins. These dolphins have previously been observed using marine sponges as foraging tools. The dolphins break the sponges off the seafloor, and then wear the marine organisms over their beaks (‘rostrums’) to help them probe the deeper water channels for food.
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This behaviour is well-studied in female dolphins, where it’s far more common than in males. This was thought to be because foraging with sponges is a time-consuming and solitary activity, and so could prevent the male dolphins from socialising with other males – an essential aspect of bottlenose dolphin life.
To find out whether this is the case, the researchers analysed the behaviour of 37 male dolphins – 13 ‘spongers’ and 24 ‘non-spongers’ – in Shark Bay over nine years from 2007 to 2015.
The male spongers spent significantly more time with other spongers, and this was nothing to do with whether they were related. In other words, it was their common interest that brought them together.
This adds a further layer of complexity to what we know about the social lives of male bottlenose dolphins, who form cooperative alliances with other males in order to gain access to females, or to keep rivals away from mating interests. These strong bonds can last for decades.
“Foraging with a sponge was long thought incompatible with the needs of male dolphins in Shark Bay – i.e. to invest time in forming close alliances with other males,” said Dr Simon Allen, co-author of the study and biologist at the University of Bristol. “This study suggests that, like their female counterparts and indeed like humans, male dolphins form social bonds based on shared interests.”