The beneficial acclimation hypothesis versus acclimation of specific traits: Physiological change in water-stressed Manduca sexta caterpillars

Do organisms make beneficial physiological adjustments in response to envir onmental change? We examined this question by measuring the effects of shor t-term (12-36 h) and longterm (larval lifetime) hydric stress on the tobacc o hornworm, Manduca sexta. Larvae were reared from the first instar on low- water (69%) or high-water (80%) artificial diets and then transferred early in the fifth instar to the same or opposite diet (2 x 2 design) ( design). Within the subsequent 36 h, we measured 24-h growth rates and three primar y determinants of the water budget: water gain via consumption and water lo ss via evaporation and defecation. Larvae preexposed to low-water diet grew less rapidly on low-water diet than those switched acutely to low-water di et from high-water diet, showing that larvae preexposed to a particular env ironment do not necessarily acclimate beneficially to that environment. Our data on water fluxes to and from larvae, however, strongly suggest that wa ter-stressed larvae did make beneficial physiological adjustments. Larvae r esponded to short-term hydric stress by minimizing rates of water excretion , primarily by increasing rates of rectal water absorption. Larvae responde d to chronic water stress by significantly reducing rates of evaporative wa ter loss; they also showed additional reductions in fecal water excretion, but these decreases were due to lowered consumption and not to further incr eases in rate of rectal water absorption. This mismatch between maladaptive acclimation of organismal performance and beneficial adjustment of suborga nismal traits can be reconciled by recognizing that organismal physiology i s hierarchical: fitness-related performance traits represent the aggregate outcome of numerous, more mechanistic physiological traits. Although chroni c exposure to an environment may depress the aggregate effect of these mech anistic traits on performance, organisms are not precluded from making bene ficial adjustments to individual traits contributing to performance.