FRAZIER, M.*; HUEY, R.B.; BERRIGAN, D.: Thermal Dependence of Darwinian Fitness in Insects: Biogeographic Patterns
Latitudinal patterns in the temperature sensitivity of diverse physiological and life history traits are widely documented. However, latitudinal patterns of the thermal dependence of Darwinian fitness have never been described. So we are compiling a database of studies (so far ~ 35 species of insects) that measured fitness (intrinsic rate of increase, r, or net reproductive rate, Ro) at multiple temperatures, and are testing several bio-geographical hypotheses. The hypothesis that tropical species are relatively warm adapted was supported: the optimal temperature for fitness declined significantly with source latitude. The “warmer is better” hypothesis (see TREE 4:131-135), which predicts that species with warm optimal temperatures (hence tropical taxa) should have relatively high intrinsic rates of increase, appears strongly contradicted: although tropical taxa had high net reproductive rates, their intrinsic rates of increase (at their optimal temperature) were actually low, not high. The proximate explanation for this pattern seems to be that tropical taxa have relatively long generation times. Whether the surprisingly low r for tropical species has an adaptive explanation remains to be explored.
