Energetics of free-ranging large herbivores: when should costs affect foraging behaviour?

Published relationships were used to build a mathematical model that predic ts the daily net energy balance of free-ranging domestic sheep (Ovis aries L.) grazing in the U.K. hills. Net energy balance was predicted for a plaus ible range of environmental conditions. The behaviour of the model suggeste d the following predictions. Locomotion will be a relatively unimportant en ergetic cost. Ambient temperature and rainfall alone will rarely affect ene rgy expenditure, whereas wind will greatly increase energetic costs in wint er. These are further increased, but to a relatively small extent, by any c oncurrent rainfall. Predictions of foraging behaviour based on maximisation of energy intake alone are likely to significantly overestimate dry matter intake from climatically exposed vegetation in winter. Where shelter is av ailable, such models will also overestimate total intake in winter by not t aking account of sheltering behaviour. This effect will be most pronounced when forage is of low digestibility or availability, wind speeds are high, or the level of coat insulation is low. Foraging models based instead on ma ximisation of net energy balance are likely to greatly improve predictions of the impact of large herbivores on vegetation and the mechanisms driving their population dynamics.