Love songs in the sand: researchers listen in to Fiddler crab courtship

For male fiddler crabs, vibrational signals are a crucial part of their courtship routines -- produced by drumming the ground using their oversized claw, or hitting the floor with their shells. But up to now, it has been unclear how effectively these signals convey information in the sonically chaotic and competitive intertidal regions where fiddler crabs are found.

To investigate this, researchers from the University of Oxford's Animal Vibration Lab assessed the courtship behaviour of the European fiddler crab (Afruca tangeri), which occupies the Southern mudflats of the Iberian Peninsula. For the first time, their novel approach recorded the male crab's behaviour using both GoPro cameras and geophones, which precisely record acoustic vibrations. A key question was how seismic signals are affected by the signaller's size -- and therefore how 'honestly' they convey information about the male's fitness as a mate.

The researchers observed that the male crabs carried out a repetitive, four-step courtship routine: starting with subtly waving their oversized claw, followed by sequential waving and body drops to produce a vibrational signal, then simultaneous motions, and finally an underground drumming display if the female approaches the burrow. At each step, the seismic energy intensifies.

By measuring over 8000 percussive seismic signal recordings, the researchers demonstrated that the morphology of the male was the critical factor in determining the signal features. In particular, males with larger claws produced higher-energy seismic signals, with higher amplitude drumming spikes. This appears to prevent the male from being dishonest about their size, enabling females to assess male quality and claw size from afar.

Lead author of the study Tom Mulder (Department of Biology, University of Oxford) said: "It appears as though the males cannot, or do not, lie about their physical size. Females can rely on the loudness of seismic signals to honestly assess a potential mate's quality, all without needing to see him."

Furthermore, the researchers found that whilst the frequency of the vibrations remained constant, the length, rhythm, and loudness of the signals all varied depending on the courtship behaviour. This meant it was possible to distinguish different behaviours (e.g. body drops vs underground drumming) based on seismic recordings alone. Indeed, the team trained a machine learning programme to automatically classify behaviours with up to 70% accuracy. This opens up the possibility of using machine learning to automatically detect the presence and behaviours of animals based on material-bound vibrations for remote monitoring -- for example, endangered animals in the African savannah, lame livestock, or insect pests on agricultural crops.

Percussive rather than vocal seismic communication tools are advantageous in the landscapes that these fiddler crabs call home and changing the loudness and repetition rate of the signals, although simple, are an effective means for small animals to communicate in these noisy environments.

Corresponding author Dr Beth Mortimer also of the Department of Biology concluded: "Larger claws have the advantage of overcoming seismic noise so that they can signal to females that are further away and females will show more interest in them. However, the advantages are only observed for the percussive signals such as drumming and fortunately for smaller clawed crabs, these are only part of the courtship routine."