J. Zool., Lond. (1987) 213, 71-80

A. HAILEY AND P. M. C. DAVIES

(With 5 figures in the text)

Natrix maura was active by day and night throughout its annual activity season (March to October), with body temperatures (T_b) between 14-35^oC. Body temperatures during foraging (aquatic, for fish) were constrained by water temperature. Snakes also raised T_b by basking. Basking in the laboratory was analysed by fitting logistic curves to heating rates, and determining half times (HT) as a descriptive statistic. Snakes in the field often basked for much longer than necessary to reach an equilibrium T_b: these basking times often exceeded HT more than ten-fold. This led to the concept of two strategies of thermoregulation, here designated r-thermoregulation and K-thermoregulation.

In r-thermoregulators the heating rate, r, is maximized until the preferred T_b is reached, and then the behaviour for which the high T_b has been achieved is pursued. The advantage of high T_b is short term, usually for increased performance during foraging. This is the commonly studied form of thermoregulation, and is typical of the behaviour of small lizards where it leads to a shuttling pattern of activity. r thermoregulation is usually by basking.

In K thermoregulators an equilibrium T_b, K*, is reached and maintained for long period. K* may be lower than the equilibrium of logistic heating in the conditions, K, if this is too hot. The behaviour of N. maura was often consistent with this pattern; the snakes moved into shade while basking, and at air temperatures above 26^oC most of the body was in shade. Increasing the rate of longer term physiological processes, such as maturation of reproductive products or (as in N. maura) rate of digestion, are the benefit of maintaining K*. K thermoregulation is by basking or thigmothermy.