J. therm. Biol. Vol. 16, No. 6, pp. 33%343, 1991 0306-4565/91 $3.1)0+ 0.00 Printed in Great Britain. All rights reserved Copyright © 1991 Pergamon Press plc THERMAL RESPONSES OF THE FRESH WATER TURTLE MAUREMYS CASPICA TO STEP-FUNCTION CHANGES IN THE AMBIENT TEMPERATURE T. PAGES,J. F. FUSTERand L. PALACIOS Unidad de Fisiologia, Departamento de Bioquimica y Fisiologia, Universidad de Barcelona, Avenida Diagonal, 645 E-08071 Barcelona, Spain (Received 18 August 1990; accepted in revisedform 3 August 1991) Abstract--1. The turtle Mauremys caspica cools significantly faster than it heats in air. The heating/cooling ratio is 0.49. 2. The variation of body temperature in relation to time-course in response to a step-function change of environmental temperature, fitted to a second-order system improves that of a first-order system. 3. The gradient between ambient temperature (Ta) and equilibrium body temperature (Tb) increases significantly and progressively when ambient temperature rises over 25°C. 4. At 40°C thermoregnlatory hyperventilation was detected, implying an increase in air convection requirement (ventilation relative to 02 consumption, l;'E/l?o2). Key Word Index: Turtles; heating and cooling rates; thermoregulation; ventilation; Mauremys caspica; reptile INTRODUCTION Turtles are ectotherms and as such rely primarily on heat from the environment to achieve and maintain body temperature within the intervals necessary for their daily activities. The primary means of thermo- regulation in turtles, as in all ectotherms, is be- havioural, and they simply avoid extremely hot environments. When denied behavioural means, or when behavioural means are not sufficient to combat heat stress in the environment, some turtles can use physiological mechanisms of control to keep body temperatures (T b) below lethal limits for short periods of time. Turtles raise their Tb through the absorption of solar radiation by basking in water or air (Boyer, 1965; Spotila et al., 1984). Basking is a complex behavioural pattern affected by nutritional status, acclimation, season and sex of the individual (Hammond et al., 1988). When ambient temperature (Ta) is below the preferred Tb the turtles can seek a step change in Ta by means of basking behaviour or water immersion. Some studies carried out in turtles on heat transfer relationships show different strategies for thermal control, such as behavioural mechanisms (Sturbaum and Riedesel, 1974; Meek, 1984; Jading et al., 1984, 1989) and physiological thermoregulatory systems (Weathers, 1970; Weathers and White, 1971; Cloudsley-Thompson, 1974) that differ in their thermal dynamics from a model in which heat trans- fer is based on thermal conductivity. Sturbaum (1982) has reviewed the main thermoregulatory responses in turtles, with a description of the different physiologi- cal mechanisms used to avoid critical temperatures and maintain body temperature within their thermo- preferendum. This author suggested a role for respir- atory responses to hyperthermia. Bartholomew (1982) and Turner (1982) agree with this approach and also suggest a possible control of body tempera- ture by means of vasomotor responses with adjust- ments of blood flow within the body. Most of the studies on thermoregulatory responses in turtles have been carried out on North American species. It would also be interesting to analyse ther- mal responses in a turtle from the Mediterranean area, Mauremys caspica. Few studies have been car- ried out on this species, and in particular, studies on physiological thermoregulatory mechanisms in this species have not been reported. In this study, we determined the rate of change of body temperature following exposure of animals to step changes of ambient temperature. The step changes simulate ther- mal alterations which the animal experiences under natural conditions in basking. Analysis of the dy- namic characteristics of these processes may allow us to predict other patterns of thermal variation and indicates that a ventilatory response to hyperthermia has a regulatory role. MATERIALS AND METHODS Twenty-one fresh water turtles, M. caspica, with body mass between 75 and 600 g, were used. The animals were caught in Laguna de Adra (Almerla, southeastern Spain). Before experimental studies began, the animals were maintained for 2 weeks under laboratory con- ditions at 20-25°C, without food but with water ad libitum. Body mass was determined before acclim- ation. Experimental ambient temperatures were achieved by means of two thermostatic chambers. During the heating phase, the chamber was con- trolled at 30 _+ I°C to represent a physiological tem- perature and until 47 ± 2°C for extreme temperature 337 TB 16/6----C