Evol Ecol Res 5: 43-51 (2003)     Full PDF if your library subscribes.

Thermal time: body size, food quality and the 10°C rule

Eric L. Charnov* and James F. Gillooly

Department of Biology, The University of New Mexico, Albuquerque, NM 87131-1091, USA

Author to whom all correspondence should be addressed.


Developmental rates of ectotherms (y) are often linearly related to temperature (Tc in °C) within some biologically relevant range of temperatures as y = (1/S)(Tc − Tb), where Tb is the estimated temperature at zero development, and the thermal constant S is the development time multiplied by the temperature above Tb (i.e. degree days above Tb). Among similar species, it has been widely shown that S and Tb are negatively related across environments, that S is positively related to body size, and that Tb is independent of body size but increases with mean environmental temperature. Here we present a model that predicts quantitatively each of these relationships by showing that the developmental rate equation (y) is a linear approximation to a universal exponential function (in Kelvin) reflecting the underlying biochemical kinetics of metabolism. The model combines the effects of body size and temperature on individual growth to explain the majority of variation in development rates among a broad assortment of aquatic ectotherms (fish, amphibians, zooplankton) at different life stages. Specifically, the model predicts that body size enters as (mass)1/4, and that Tb is about 10°C below the mean developmental temperature for ectotherms in nature (‘the 10°C rule’). We conclude by explaining how differences in food type would affect the model.

Keywords: degree days, growth, temperature, threshold temperature.

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        © 2003 Eric L. Charnov. All EER articles are copyrighted by their authors. All authors endorse, permit and license Evolutionary Ecology Ltd. to grant its subscribing institutions/libraries the copying privileges specified below without additional consideration or payment to them or to Evolutionary Ecology, Ltd. These endorsements, in writing, are on file in the office of Evolutionary Ecology, Ltd. Consult authors for permission to use any portion of their work in derivative works, compilations or to distribute their work in any commercial manner.

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