Fiddler crabs use seismic vibrations to attract mates

Fiddler crab mating on the bustling mudflats of southern Europe is about sending seismic love signals beneath the surface – quite literally.

In their quest to attract a mate, male crabs produce ground vibrations as part of an elaborate courtship ritual. For the first time, scientists have recorded and analyzed these signals to understand how they communicate in a chaotic and noisy habitat.

Researchers from the University of Oxford have captured the full courtship display of the European fiddler crab (Afruca tangeri), combining video footage and geophone recordings to listen in on the crabs’ seismic conversations.

These crabs inhabit the muddy shores of the Iberian Peninsula, where waves, wind, and competing crabs make for a challenging environment to get a message across.

However, the study reveals that these crustaceans have developed a clever and consistent way to make themselves heard – and felt.

How fiddler crabs use vibrations

Male fiddler crabs are known for their oversized claws, which they use to wave at potential mates and generate ground vibrations. These signals are produced by striking the ground with the claw or shell, and later by drumming from inside their burrows.

The researchers discovered that fiddler crab mating follows a four-step courtship routine, starting with subtle claw waves, followed by body drops, synchronized movements, and ending with underground drumming if a female shows interest.

As each stage progresses, the energy of the vibrations increases. The recordings, captured with GoPro cameras and precise geophones, allowed the team to measure over 8,000 seismic signals during the courtship routines.

Seismic vibrations reveal crab size

The key question was whether these vibrations serve as honest signals of the male’s size and strength. The results suggest they do.

The data showed that larger male crabs with bigger claws produced more powerful seismic signals, especially during drumming. This means the crabs can’t fake their physical traits – what you vibrate is what you are.

“It appears as though the males cannot, or do not, lie about their physical size,” said Tom Mulder from the Department of Biology at the University of Oxford.

“Females can rely on the loudness of seismic signals to honestly assess a potential mate’s quality, all without needing to see him.”

Machine learning meets marine dating

When it comes to fiddler crab mating, the frequency of vibrations stays the same – but other features like rhythm, duration, and loudness change depending on the crab’s demeanor.

This allowed the researchers to classify each action (such as a body drop or an underground drum) by analyzing the vibration patterns.

To test this further, the team used machine learning to teach a computer how to distinguish between different crab behaviors based solely on these recordings.

The program achieved a 70% accuracy rate, showing strong potential for future applications. The technique could lead to the development of non-invasive monitoring systems for a wide range of animals.

From tracking endangered species across the savannah to identifying crop pests or spotting injured livestock, material-bound vibrations could become a new frontier in remote wildlife observation.

Vibrations work for the fiddler crab

When it comes to fiddler crabs, relying on seismic signals instead of sound makes perfect sense. Their noisy, open-air environment is difficult to navigate acoustically, but the ground beneath them offers a stable way to communicate.

By altering the strength and tempo of their vibrations, they can get their messages across without saying a word.

“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,” explained Dr. Beth Mortimer, also from the Department of Biology.

“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.”

So, while size matters when it comes to seismic communication, smaller males can still show off through other parts of the ritual. In the unpredictable world of fiddler crab mating, it’s not just about waving your claw – it’s about how hard you drum.

The full study was published in the journal Journal of Experimental Biology.

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