The energy costs of male aggression and 'waving' in a fiddler crab Uca lactea perplexa were examined by measuring blood glucose and lactate concentrations under natural conditions because determining the costs of a given behavior is critical in the framework of game theory. The glucose level showed the greatest variation at the beginning of the crabs' activity period, and this may imply a variation in 'male quality.' Blood lactate level increased significantly from the early to the middle part of their activity period. The temporal change in behavior indicated that the increase in lactate was due to vigorous waving, and 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 their conspecifics. 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. Is this true for the other animal species?