Courtship displays have a pivotal role in finding potential mates. This important event often involves visual, auditory, and touch-based cues. Male fiddler crabs are no exception. They drum the ground with their oversized claws and hit the floor with their shells. Now, for the first time ever, a study led by the University of Oxford researchers has eavesdropped on the love song in the sands, listening to the courtship displays of male fiddler crabs using geophones. A geophone is a device that helps detect ground movements caused by seismic waves, in this case, the vibrations caused by the drumming of the ground. The findings of this study have been published in the Journal of Experimental Biology, shedding light on how these crabs communicate through the cacophony of the chaotic seashore.
To investigate how the courtship cues convey information in such a chaotic scenario, researchers from Oxford’s Animal Vibration Lab worked on the courtship behavior of the European fiddler crab (Afruca tangeri), which occupies the Southern mudflats of the Iberian Peninsula. The team used GoPro cameras and geophones to monitor the male crab’s behavior and precisely record seismic vibrations. One of the key questions was to judge how the seismic signals conveyed information about the size of the signaling crab.
The researchers observed four distinct steps during the crab’s courtship display: Subtle waving of the oversized claw, sequential waving, and body drops to produce vibrations, simultaneous motions, and, to top it off, underground drumming display if female crabs approached the male fiddler crab’s burrow. This routine works as a cue for females to get an idea about the size of the males. Male crabs with larger claws produced seismic signals with higher energy and higher amplitude drumming spikes. This serves as a “truth serum”, preventing males from being dishonest about their size, and also sets up a system for females to sense the quality of the male crab and their claw size from a distance. According to lead researcher Tom Muldur, males cannot lie about their physical size.
Researchers also found that although the frequency of the vibrations remained constant, the length, loudness, and rhythm of the signals varied according to the stage of the courtship display. The team designed a machine learning program to classify behaviors like drumming and body drops through seismic displays only. The crabs vary the loudness and repetition of the signals to establish their presence across a chaotic environment.
To conclude, the study is not specific to fiddler crabs. Understanding inter-species and intra-species communication can shed light on conservation, ecology, and biology methods.
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