Empirical studies of the reliability of sexual signals are needed to test the prediction that females should prefer males in good condition. We examined whether an attractive sexual signal used by male fiddler crabs, Uca beebei, reliably indicates male condition by measuring male blood glucose and lactate levels in the field. The signal is a mud structure, a ‘pillar’, that attracts females for mating. Blood lactate levels (a measure of energy expenditure) of pillar builders (P males) immediately after they built their pillars did not differ from those of nonbuilders (NP males), suggesting that the cost of building a pillar is small. However, P male lactate levels were significantly higher than those of NP males late in the activity cycle after P males had invested more in vigorous courtship using claw ‘waving’. Whereas P males maintained their glucose levels (a measure of energy availability as the major fuel for ATP production) at around 30 mg/dl throughout their daily activity period, NP males showed significantly lower levels in the middle of the period. These results confirm that pillars reliably signal a male’s condition, as measured by his ability to maintain a blood glucose level necessary for costly courtship, even though the construction of a pillar has minimal energetic cost.
