Fiddler crabs are masters of the dating world

Published: 15 March 2017 at 00:01

A male banana fiddler crab waving

New study shows that courtship display indicates physical fitness and size of home

A new study shows that the elaborate courtship displays of the male fiddler crab help to advertise both their physical fitness and the size of their home.

The research, led by Dr Sophie Mowles of Anglia Ruskin University and published in the journal Royal Society Open Science, studied the behaviour of the male banana fiddler crab (Uca mjoebergi).

Sexual selection due to female choice has led to the evolution of courtship displays that appear to communicate a male’s quality as a mate. Male banana fiddler crabs initially attempt to attract a female by waving their brightly coloured major claw in the air.

As the female approaches they switch to producing a drumming signal, which is transmitted through the ground as a series of rapid vibrations. Drumming conveys information about both a male’s stamina and the size of his home – the study found that the frequency of drumming was positively related to the size of the male’s burrow.

Male fiddler crabs that wave and drum their claws rapidly during courtship were shown to have high levels of fitness. And the act of waving and drumming was found to be physically costly for the crabs. To prove this, the researchers tested the sprint speed of crabs on a special racetrack. Males that had been induced to court had a significantly poorer performance in sprint trials than non-courting control males.

The researchers believe the physical investment required to drum and wave allows females to select the fittest mates. The advantage of drumming as well as waving is that males can still provide information from inside their burrow when they are no longer visible. This could also benefit females because it reduces their risk of being coerced into mating when they enter a male’s burrow to assess its volume.

Dr Mowles, Lecturer in Animal and Environmental Biology at Anglia Ruskin University, said:

“Males frequently produce elaborate displays to attract females. These are often thought to demonstrate the quality of his genes, or his fitness, through a display of effort.  Here, we have demonstrated that a male’s level of physical fitness is indeed communicated through the vigour of his display. 

“Male fiddler crabs that wave and drum their claws rapidly during courtship were found to have high physical fitness, despite the fatiguing nature of this display. Therefore, by choosing a vigorously displaying male, a female can ensure she selects a good quality mate who can offer her good genes for her offspring.

“Energetically costly ‘signals of stamina’ reflect the ability of the male to perform other demanding activities such as foraging and sprinting to avoid predators. These are attributes a choosy female should want to pass on to her offspring. The ability to perform a demanding display may also indicate that the male is in good condition, preventing females from mating with diseased or parasitised males.”