Fluctuating temperatures and ectotherm growth: distinguishing non-linear and time-dependent effects

Abstract

Most terrestrial ectotherms experience diurnal and seasonal variation in temperature. Because thermal performance curves are non-linear, mean performance can differ in fluctuating and constant thermal environments. However, time-dependent effects—effects of the order and duration of exposure to temperature—can also influence mean performance. We quantified the non-linear and time-dependent effects of diurnally fluctuating temperatures for larval growth rates in the Tobacco Hornworm, Manduca sexta L., with four main results. First, the shape of the thermal performance curve for growth rate depended on the duration of exposure: e.g. optimal temperature and thermal breadth were greater for growth rates measured over short (24h during the last instar) compared with long (the entire period of larval growth) time periods. Second, larvae reared in diurnally fluctuating temperatures had significantly higher optimal temperatures and maximal growth rates than larvae reared in constant temperatures. Third, we quantified mean growth rates for larvae maintained at three mean temperatures (20°C, 25°C, 30°C) and three diurnal temperature ranges (+0°C, +5°C, +10°C). Diurnal fluctuations had opposite effects on mean growth rates at low vs high mean temperature. Fourth, we used short-term and long-term thermal performance curves to predict the non-linear effects of fluctuating temperatures for mean growth rates, and compared these to our experimental results. Both short- and long-term curves yielded poor predictions of mean growth rate at higher mean temperatures with fluctuations. Our results suggest caution in using constant temperature studies to model the consequences of variable thermal environments.