Ectotherms often grow to a smaller adult body size when reared in warmer conditions. Typically, this is because development rate increases faster than growth rate with warming, causing individuals to mature earlier but at a smaller size. Yet, few studies have examined how these temperature-size responses are established over the entire life cycle, from egg to adult.
Using high resolution experimental data for pelagic copepods, we find that the strength of the temperature-size response fluctuates considerably between life stages, with the strongest responses occurring mid-way through the life cycle. This indicates that differences in the thermal sensitivity of growth and development rates are not fixed across life stages. Our analysis reveals that these fluctuations are caused by rapid shifts in the temperature dependence of growth rate, but not development rate. At times, growth rate actually increased faster than development rate with warming, leading to a weakening of the temperature-size response in some life stages. Further examples from the literature, including data for insects and crustaceans, appear to support these conclusions more broadly.
These new findings have important implications for understanding how temperature-size responses are produced, challenging conventional assumptions regarding the thermal sensitivity of life history rates. Whereas equiproportional development (i.e. shared temperature dependence of development rate across life stages) is commonly assumed for arthropods, a similar concept should not be assumed for growth rate.
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Horne, C. R., Hirst, A. G., Atkinson, D., Almeda, R., & Kiørboe, T. (2019). Rapid shifts in the thermal sensitivity of growth but not development rate causes temperature-size response variability during ontogeny in arthropods. Oikos. doi:10.1111/oik.06016