Male fiddler crabs (Uca species) have one enlarged claw that is used for a waving courtship display. Females prefer males with large claws and fast waves.
Or so it was thought.
Most research on the subject had been conducted under controlled conditions that isolated these traits.
What happens when sex-selected characteristics are not isolated, and environmental variation comes into play?
Fiddler crab vision is specialized to mudflat environments that are flat, clear, and lack topographic complexity.
However, some male fiddler crabs make little mud-mounds upon which they stand to wave and advertise the entrance of their abode to females. This small change in elevation (less than 2 centimetres) has drastic negative effects on a male’s mating success.
Females show a marked aversion to males that signal from atop their tiny castles, despite both large claw size and fast waving frequencies.
It can be assumed that a female’s vision directs her pickiness towards males who are signalling on “her level”.
This kind of variation in female preference helps to maintain genetic diversity in male sexual signals because it means that the most physically capable fiddler crab males are not necessarily getting the girl.
If waving from atop of a mud mound is so unattractive to females, we might ask the logical question – why do some males do it?
It’s possible that higher burrows are deeper and of superior quality such that if a male can lure a female into one she’s more likely to stay.
Higher fiddler crab males may also appear larger and be less susceptible to predators and may, therefore, represent a trade-off between survival and sex.
This post is an excerpt from my book “Wild Sex: The Science Behind Mating in the Animal Kingdom”