Maintenance of Temperature of Crocodile Nests (Reptilia, Crocodilidae)

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Nests of Crocodylus porosus maintain warm and stable temperatures. Mean temperature levels are determined by heat generated within the nest and by seasonal variations in ambient temperatures. Daily fluctuations of nest core temperatures are caused by diel variations in heat available to the nest from external sources. Maximum nest core temperatures may occur at night due to the slow transfer of heat from the surface to the centre of the nest. No significant differences in temperature characteristics of nests of different materials were found in this study. Nests of many species of crocodilians have been shown to maintain warm and relatively stable temperatures despite wide ambient fluctuation (Mcllhenny, 1934; Deraniyagala, 1939; Neill, 1946, 1971; Modha, 1967; Joanen, 1969; Medem, 1971; Chabreck, 1973; Staton and Dixon, 1977). Rotting vegetation has been suggested as a major generator of endogenous heat in mound nests (Chabreck, 1973; Tansey, 1973), but it is often difficult to differentiate heat generated within nests from heat generated externally. Hole nests of C. niloticus (cavities under sand beaches) have no apparent sources of internal heat yet maintain temperatures similar to those of mound nests (Modha, 1967a). Daily fluctuations in nest temperatures have been attributed to changes in the insulation of the surface of the nest causing a build up of endogenous heat (C. porosus, Webb et al., 1977) and to fluctuations in external heat available to the nest throughout the day (A. mississippiensis, Chabreck, 1973). Webb et al., (1977) noted that a decline in the temperature of a single nest paralleled a seasonal decline in ambient temperature. Staton and Dixon (1977) have shown that mud nests of Caiman crocodilus crocodilus have lower temperatures than those made of grass (Paspalum fasiculatum), and Joanen and McNease (1971) suggested a similar effect for nests of A. mississippiensis. This paper investigates the temperature of nests of C. porosus with a view to answering the following questions: 1. Does heat produced within the nest contribute significantly to the temperature of the nest? 2. Are nests affected by seasonal variations in ambient temperature? 3. What causes daily fluctuations of nest temperatures? 4. Do nests constructed of different materials differ in their temperature characteristics? METHODS Egg cavity and shaded air temperatures of 11 nests were measured with a Dobros mercury thermometer (limit of reading 0.5?C) at irregular intervals throughout the breeding season (November-April). Readings were taken only from nests younger than two months, and no more than three spot temperatures were taken from any one nest. Five internal and four ambient temperatures were recorded from each of 12 nests at /4, 1/2, or 1 hourly intervals by a multitemperature recorder with nine thermistors (Grant Pty. Ltd. limit of reading *This is a contribution from the joint University of Sydney, Department of Norther Territory Crocodile Program.