Machine Learning Unlocks Secrets of Crab Seismic Courtship Dances

In a groundbreaking Oxford-led study, scientists tapped into the seismic love language of fiddler crabs—revealing how these crustaceans use ground vibrations, not visuals, to size each other up on the noisy seashore.

Research: Constraints on percussive seismic signals in a noisy environment by European fiddler crabs, Afruca tangeri. Image Credit: aDam Wildlife / Shutterstock

For the first time, a study led by University of Oxford researchers has used geophones to 'listen in' to the fascinating courtship displays of Fiddler crabs. The findings, published in the Journal of Experimental Biology, provide new insights into how the animals communicate effectively on the noisy seashore.

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 signaler's size and 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 8,000 percussive seismic signal recordings, the researchers demonstrated that the male's morphology 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 his 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 signals' length, rhythm, and loudness 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 classify behaviours with up to 70% accuracy automatically. 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 these fiddler crabs call home, and changing the loudness and repetition rate of the signals, although simple, is 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."