Testing the adaptive significance of acclimation: A strong inference approach

Acclimation is a common phenotypic response to environmental change. Acclim ation is often thought to enhance performance and thus to be adaptive. This view has recently been formalized as the "Beneficial Acclimation Hypothesi s" and predicts that individuals acclimated to one environment perform bett er in that environment than do individuals acclimated to a different enviro nment. Although Beneficial Acclimation is appealing and widely supported, r ecent studies with E. coli and Drosophila have challenged its general valid ity. Although these challenges could be dismissed as mere exceptions, they encourage a re-evaluation of the adaptive significance of acclimation. Our philosophical approach differs from that of most previous studies of acclim ation, in which the prediction derived from a Beneficial Acclimation perspe ctive (e.g., heat tolerance is positively correlated with acclimation tempe rature) is tested against the null hypothesis ("single hypothesis approach" ). Instead, we follow Huey and Berrigan (1996) in advocating a strong infer ence approach (sensu Platt, 1964), which recognizes that Beneficial Acclima tion is actually one of a set of competing hypotheses that make different p redictions as to how developmental temperature influences the thermal sensi tivity of adults ("developmental acclimation"). Using this perspective, Hue y and Berrigan proposed a factorial experimental design (3 developmental by 3 adult temperatures) designed to discriminate among all competing hypothe ses. We now derive a formal statistical model (ANOVA with orthogonal polyno mial contrasts) for this experimental design and use it to evaluate simulta neously the relative impact of each competing hypothesis. We then apply thi s model to several case studies (Drosophila, Volvox, Trichogramma), and we review also a recent study with E. coli. The influence of Beneficial Acclim ation is supported (albeit often weakly) in most cases. Nevertheless, other hypotheses (especially the Optimal Developmental Temperature Hypothesis) o ften have a greater impact. Even so, however, Beneficial Acclimation usuall y predicts relative performance at extreme test temperatures. We conclude t hat, although rumors of its death are premature, Beneficial Acclimation can not be viewed as the dominant expectation, at least with regard to developm ental temperature acclimation. Moreover, our findings reinforce the view th at a strong inference approach provides a more comprehensive portrait of co mplex biological responses than do single-hypothesis approaches.