Sometimes animals of the same species look very different – taken side by each, we might not even guess that they belong to the same species at all! While some individuals may be perfectly plain, others may have marvelous markings and kaleidoscopic colours, or perform perplexing displays. When these differences occur between males and females, we call it sexual dimorphism.

What is sexual dimorphism? Dimorphism is the occurrence of two distinct types of individuals in the same species (from the Greek dimorphos – “of two forms”). Sexual dimorphism, then, is the development of differences between males and females of the same species, besides the sexual organs.

Why not examine an intriguing invertebrate example?

Fiddler crabs are cool intertidal crustaceans that can be found in mangrove forests, creeks, sandbars, mudflats and rocky beaches. They live in colonies, with each member occupying and protecting an underground burrow. In addition to being safe havens from fluctuating tides and pesky predators, burrows are love nests and nurseries. While females have two small feeding claws, males have one feeding claw and one prodigious pincer. Male fiddler crabs wave their single colossal claw in a dexterous display to flirt with fine females and negotiate with nosy neighbours.

Research on fiddler crabs in Portugal showed that claw size and waving rate in males were good indicators of burrow depth, and therefore of mate quality. Impressive extremities suggest that male crabs are satisfactory partners so females don’t have to scope out the digs of every eligible bachelor they meet.

Ladies may love males’ large forelimbs, but there are trade-offs to looking mateworthy in the natural world. Is having an absurd appendage a help or a hindrance? On one claw, research suggests that males’ large limbs deter avian predators; captive white ibises were more likely to feast on female crabs, or declawed males, than males that wielded their waving weapons. On the other claw, males feed much more slowly than their female friends. To eat, fiddler crabs scrape the ground with their feeding claws, bringing the sediment up to their mouths where it is sorted by their multiplex mouthparts. Since males have only one feeding claw, they feed at half the rate of their lady lovers – that means they take twice as long to eat the same amount of food.

Male fiddler crabs’ monstrous claws may make lunch a laborious affair, but they also appear to be honest indicators of a good mate. Traits that make mates swoon aren’t necessarily advantageous, however, and females can also be duped by cunning con crabs. Foolhardy female fiddlers, beware!

To read about the link between sexual selection and claw size in fiddler crabs, click here: https://www.int-res.com/articles/meps/189/m189p233.pdf

To learn more about differential avian predation on male and female fiddler crabs, check out this article: https://www.sciencedirect.com/science/article/pii/0003347289900134