In crustaceans, claws are good examples of multi-functional organs used by males and females during foraging, agonistic interactions, competition for mates, mates handling. In crayfish, claws are used for prey capture and manipulation, defense against predators, inter- and intra-specific interactions, and in reproductive activities. Claws are the main tools (armaments) of crayfish. Claw function can be subdivided into mechanical and display functions. Crayfish are sexually dimorphic. Male claws are bigger than female claws. Because both females and males posses them, claws are not exclusively secondary sexual ornaments but also function as important tools used for locomotion, food gathering, defense, and as weapons during intra- and interspecific agonistic interactions.

Allometry is the relationship between the size of an organ or structure and the size of the whole organism. Size is a fundamental character of any organism and many behavioral and physiological attributes vary with body size. Big structures (armaments or ornaments) may confer competitive ability to the owners of such structures. If large crayfish with larger claws are usually the winners of agonistic encounters, consequently, they could reproduce more often. Claws are the main tools that crayfish use in foraging, agonistic interactions, and mating. Juvenile male Procambarus clarkii show positive claw allometry and juvenile females show claw isometry. At adult stage, males show positive allometry and females show negative allometry. Males with larger claws are dominant over male conspecifics with smaller claws and similar body size. Big-clawed females are dominant over small-clawed females. Having bigger claws and bigger body size increases the likelihood of mating for males. Males could not subdue females when the females are bigger. Together, male-male competition and mating advantages appear to have selected for enlarged claws.

Can a crayfish female without claws be still able to mate with a big clawed male?