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: for example, optimal temperature and thermal breadth were greater for growth rates measured over short (24 h 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, for larvae maintained at three mean temperatures (20, 25 and 30°C) and three diurnal temperature ranges (±0, ±5 and ±10°C), diurnal fluctuations had opposite effects on mean growth rates at low versus high mean temperature. Fourth, both short-and long-term thermal performance curves yielded poor predictions of the non-linear effects of fluctuating temperature on mean growth rates (compared with our experimental results) at higher mean temperatures. Our results suggest caution in using constant temperature studies to model the consequences of variable thermal environments.

Plasticity of upper thermal limits to acute and chronic temperature variation in Manduca sexta larvae

“…Recent studies of growth rate in M. sexta support this interpretation. For example, at a mean temperature of 25°C, diurnal fluctuations (±10°C) during larval development increase maximal growth rates and the optimal temperature for growth for 5th instar larvae, revealing beneficial acclimation to diurnal fluctuations (Kingsolver et al, 2015). By contrast, at a mean temperature of 30°C, increasing diurnal fluctuations during development strongly reduces rates of growth, development and survival (Kingsolver et al, 2015).…”

“…A Manduca larva can experience a wide range of diurnal fluctuations and body temperatures during its larval life (Casey, 1976;Kingsolver et al, 2012;Woods, 2013), and repeated daily exposure to body temperatures above 35-38°C throughout larval development can reduce growth and survival rates (Kingsolver et al, 2015). Manduca sexta larvae also strongly increase HSP70 production in response to acute heat shocks of 38-44°C (Fittinghoff and Riddiford, 1990), and brief (30 min) exposure to temperatures above 44-45°C greatly increases larval mortality (Casey, 1976).…”

The fallacy of the average: on the ubiquity, utility and continuing novelty of Jensen's inequality

“…For example, ecological physiologists and evolutionary biologists have innovatively incorporated the effects of Jensen's inequality into their efforts to model how plants and animals will respond to future changes in Earth's climate (e.g. Ruel and Ayres, 1999;Martin and Huey, 2008;Dillon et al, 2010;Williams et al, 2012;Vasseur et al, 2014;Colinet et al, 2015;Kingsolver et al, 2015;Dowd et al, 2015;Dillon and Woods, 2016;Koussoropolis et al, 2017). By contrast, biologists in other fields are often unaware of the quirks of nonlinear averaging.…”

Foraging in an unsteady world: bumblebee flight performance in field-realistic turbulence

“…In addition to structural and visual complexity [1,2], outdoor environments vary substantially over time, with abiotic conditions (e.g. wind [3], temperature [4] and light, among others) varying over timescales ranging from seconds to seasons. Such environmental complexity can pose significant challenges to flying animals that must move through natural habitats to forage for food [5], capture prey or escape from predators [6], and find mates [7], potentially restricting when and where they can fly, or increasing the energetic cost of flight.…”