Energetic Costs of Agonistic Behavior in Two Herbivorous Damselfishes (Stegastes)

Abstract

Theoretical models and empirical studies suggest that territorial defense is costly to animals. Proposed costs include increased risk of predation and/or injury, decreased time for foraging and mating, and increased energetic demands. Using chamber respirometry, I related oxygen consumption to levels of agonistic behavior in two species of herbivorous damselfishes (Stegastes dorsopunicans and S. planifrons). I then compared frequency of agonistic behavior demonstrated in the laboratory to that observed in the field to make predictions about the energetic cost of agonistic encounters. Both dusky and threespot damselfish exhibited a twofold increase (P < 0.0001) in agonistic behavior, that is, number of forward thrusts/30 min, in laboratory aquaria (duky = 98.7 ? 12.8, threespot = 113.3 + 9.3) compared to that exhibited in the field (duky = 52.5 ? 3.9, ;heespo = 38.8 ? 5.0). Despite the marked increase in agonistic behavior in the presence of a conspecific intruder in the laboratory, there was no concomitant significant increase (Pduky < 0.0001, Preespot < 0.0001) in oxygen consumption for either damselfish species. For dusky damselfish, mean oxygen consumption in the absence of an intruder was 213.1 ? 8.5 jig 02/g bwt/30 min and in the presence of an intruder was 204.8 ? 6.8 jxg 02/g bwt/30 min. Mean oxygen consumption values for the threespot damselfish were 159.5 + 6.5 jig 02/g bwt/30 min (intruder absent) and 154.3 ? 7.4 jig 02/g bwt/30 min (intruder present). The frequency of agonistic encounters observed in the laboratory was significantly higher than that observed under natural conditions. Thus, energy demand, an oft-cited cost of agonistic behavior, appears to be minimal for these two damselfish species.