Fiddler crabs are notorious wavers. On missions to attract a female, many male fiddler crabs wave their outsized claw – the other is more diminutive – in the hope of attracting the right attention. But male European fiddler crabs (Afruca tangeri) add another dimension to their attractive repertoire. They beat the ground with their larger claw and, as a female nears, they also bend their legs to drop their carapace onto the ground – pounding the sand with their entire body – before combining the manoeuvres simultaneously. But it wasn't clear how these relatively small crabs incorporate the vibrations produced in the sand by their actions into their courtship rituals, or how noise generated by the sea and wind might interfere with the male's percussive serenades. Tom Mulder, Ellen Morley and Thomas Miller, from the University of Oxford, UK, set off for the beaches of Portugal's Ria Formosa National Park, armed with GoPro cameras and geophones (microphones that pick up vibrations in the ground), to find out more about the crustaceans’ percussive courtship.
After staking out 50 burrows, it soon became apparent that the presence of a female was essential to encourage the male's percussive performances. The trio – supported by Safi Darden and her team from the University of Exeter, UK – then provided each male with a captive female audience and, over a 2-month period, they recorded more than 8000 courtship activities as part of the crabs' displays, including the vibrations produced in the sand when they pounded the sand with their claws and bodies. In addition, the team recorded the vibrations in the sand produced during the final stage of the males’ courtship performance, when they retreated into their burrow beneath the sand in a crescendo of drumming.
Back in Oxford, the team analysed the intensity of the vibrations, and realised that the loudness of the vibrations carried to attentive females through the sand depended on what the crab was up to, with the loudest vibrations produced when it beat the sand simultaneously with its claw and body. Also, the percussive beats were not tuned to a particular tone, although the team suspects that could be an advantage in the noisy seashore environment, where wind and waves could drown out certain tones. And the males with the largest claws definitely made the biggest noise, allowing them to stand out from the crowd. So European fiddler crabs put on percussive displays to attract a mate, and their smashing serenades are probably a great way for small crustaceans to get heard above the seashore cacophony.
