Changes in blood glucose and lactate levels of male fiddler crabs: effects of aggression and claw waving

Determining the costs of a given behaviour is critical in the framework of game theory. We estimated the energy costs of male aggression and claw waving in a fiddler crab Uca lactea perplexa, by examining blood glucose and lactate levels under natural conditions. Male fiddler crabs have a single enlarged claw used in courtship waving and fighting. The greatest variation in glucose level was found at the beginning of the activity period, implying variation in ‘male quality’. Blood lactate level increased significantly from the early to the middle part of the crabs' activity period. The associated temporal change in behaviour indicated that the increase in lactate was due to vigorous waving. This was confirmed by an enclosure experiment using opaque and transparent fences: the waving frequency was not affected by enclosures with transparent fences but was strikingly lower in cages with opaque fences in which the males could not see and wave at females. The lactate level was significantly higher in transparent fence treatments than in opaque fence treatments. Since the transparent fence prevented aggressive contact between males, the higher lactate concentration in control males than in males enclosed with the transparent fence indicated that aggressive interactions were also costly. In contrast to some vocal displays in songbirds, our results showed that the waving display of the fiddler crab, as well as agonistic behaviour, is energetically costly.