In a crayfish fight to the death, the combatant with the larger claws usually has better odds. In fact, many crayfish fights don't even make it to the death. After sizing up each other's claws, called chelae, the crayfish with the smaller chelae will often surrender before the fight even begins. This is a sound strategy: it keeps both parties from getting unnecessarily injured, and the stronger crayfish wins. Unless large chelae are not as strong as they appear. When size-based signals are used to determine the outcome of competition, deception can run rampant. Crayfish may gain an advantage merely by investing more in the size of a chela than in the muscle used to power it. Michael Angilletta Jr of Arizona State University, USA, and Robbie Wilson of the University of Queensland, Australia, set out to find whether chela size was an honest indicator of strength in male crayfish.

The researchers collected wild male crayfish and measured the force produced as each chela clamped down on a sensor composed of metal plates. To be sure that they were measuring actual maximum force production, the researchers measured force production at different temperatures and multiple times for each crayfish.

While the authors found that in general strength increased with chela size, larger chelae exhibited much greater variation in force production. This pattern held across temperatures, so differences in strength among chela are likely to be a product of real physiological differences, instead of the crayfish's motivation at that time.

As a result of this variation in strength, the larger the chela, the more unreliable the chela size as a signal of potential force production. In some cases, larger chelae were weaker than slightly smaller counterparts. These asymmetries of size and strength suggest that deceptive signaling in crayfish is common. Angilletta and Wilson propose two explanations for this phenomenon: either the crayfish benefit tremendously from having deceptive chelae or, alternatively, crayfish pay tremendous costs for attempting to identify their opponents' deceptive chelae.

Cryptic asymmetry between size and performance adds another dimension of uncertainty to the competitive interactions of crayfish. A crayfish may not be able to accurately account for the strength of his competitor, or even know which chela is most dangerous. Further study may elucidate some of the complexities of dominance and the evolution of deceptive signaling in crayfish, to find out why bigger isn't always better.

M. J.
R. S.
Cryptic asymmetry: unreliable signals mask asymmetric performance of crayfish weapons
Biol. Lett.