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Biophysical, physiological and behavioral approaches to reptilian thermal biology have been combined in a synoptic model, using data derived from a study of the snake Natrix maura in a Spanish river and in the laboratory. The model investigates the relations between the costs and benefits of physiological performance, body temperature, the season of activity, and the thermal benefits of physiological performance, body temperature, the season of activity, and the thermal environment. The performance variale chosen was aerobic metabolism. The cost associated with high aerobic metabolism at low body temperatures is the energy requirement for maintenance; the benefit is an increased activity season.

The cost of maintenance was more sensitive to differences in the level and temperature sensitivity of metabolism than was the benefit of the time for activity. The pattern of performance and body temperature obesrved in N. maura was close to that predicted for maximum efficiency, in terms of the activity season produced per unit of annual maintenance cost. It is noted, however, that animals will be selected for net benefit, and not for efficiency as such.

A definition of fitness for the study of the physiological ecology of performance was developed, to take account of the level of performance available during activity, and of selection for net benefit. This definition is: the energy intake above maintenance costs x the probability of survival. It is related to general ecological fitness by considering lifetime strategies of survival and reproduction, and the energy equivalence of offspring. These strategies are the subject of the physiological ecology of allocation, which is thus complementary to that of performance.

The following are also discussed: the role of performance in interactions with predators and prey; other possible evolutionary determinants of body temperature; the limitations of different approachs to thermal biology; suitable systems for further study.