Life stage shapes condition-dependent foraging under predation risk in an electric fish.

Low body condition can promote risky foraging, but whether condition dependence differs across life stages remains unclear. Using field and laboratory studies, we tested whether residual body condition (RBC) predicted foraging under moonlight-mediated predation risk in the Amazonian sand knifefish, Gymnorhamphichthys rondoni. This species has a 2-year lifespan and a single late second-year breeding season. Because adults have less time than juveniles to recover condition before breeding, we predicted stronger condition dependence in adults. Among wild fish pooled across life stages, stomach fullness increased overnight on dark (lower-risk) but not bright moonlit (higher-risk) nights, when most stomachs were empty and non-empty stomachs occurred mainly in low-RBC fish. In experiments under laboratory-recreated moonlight, lower-RBC adults were active longer than higher-RBC adults on full-moon nights, as quantified by electric organ discharge rate. RBC did not predict adult activity on new-moon nights or juvenile activity on full- or new-moon nights. A full-moon interaction model showed a significantly stronger RBC-activity relationship in adults than in juveniles. In light-ramping trials, lower-RBC adults remained active at higher illuminance before hiding, whereas most juveniles did not hide. Our results are consistent with the asset-protection prediction of stronger condition-dependent risk-taking in adults facing late-life constraints than in juveniles.