The energetic costs of alternative male reproductive strategies in Xiphophorus nigrensis

The coexistence of alternative male mating strategies depends on the balance between costs and benefits. Here we examine the short-term metabolic costs associated with distinct reproductive strategies in the genetically determined alternative male phenotypes of a northern swordtail, Xiphophorus nigrensis. In this species, large males court females, non-adorned small males chase females, and intermediate males exhibit both courtship and chase behaviors. Using intermittent flow respirometry, we measure oxygen consumption rates and behaviors of each size class in isolation and in the presence of a female. Changes in oxygen consumption between solitary and female presence trials (ΔVO₂) correlated significantly with standard length across all size classes (r = 0.42). Only the large male class exhibited a significant increase in oxygen consumption in female-present trials exhibiting a range of increase from 2 to 200% relative to solitary metabolic rates, but costs of specific courtship displays could not be demonstrated. Sword length explained 54–57% of the variation in oxygen consumption in large male solitary trials and 63–65% in the female-present trials independent of any behavioral correlation with sword length. Our results exhibit similarities to condition-dependent alternative mating systems where the female-favored phenotype has higher energetic costs.