Observations on the thermal selection of the pantanal caiman (Caiman crocodilus yacare) hatchlings (Crocodylia: Alligatoridae)

Abstract

Three options of water temperatures in a thermal gradient that changed from 22 to 35°C were offered to Pantanal caiman (Caiman croocodilus yacare) hatchlings. Animals from two nests were submitted to temperatures during 19 days (Nest 1 - 20 caimans caught in the nature hours after hatching), corresponding to 1 to 7 observations periods, and 13 days (Nest 2 - 20 animals obtained by artificial incubation) relative to observation period 8. Caimans remain fasting during experimental period. Hatchlings were allocated in a enclosure containing three styrofoam boxes (120 L) disposed side by side enclosed by wire netting and filled with water to the top. Thus, the animals were free to access the boxes, which temperatures were changed at the end of each observation period to avoid position effects. Air temperature was keep close to 22°C by air conditioning and water temperature was controlled by thermostats. The trial was divided in three phases determined by elimination of the less frequented temperature and its substitution by another temperature. In a first phase of the experiment that contained 1, 2 and 3 observation periods, the thermostats were regulated to 22°C (BoxD), 26°C (BxC) and 32°C (BxA). In phase 2 to 26°C, 29.5°C (BxB) and 32°C, including 4, 5 and 6 periods. In the third phase that included 7 and 8 periods, the thermostats were adjusted to 29.5°C, 32°C and 35°C (BxE). The number of caimans in each box was recorded two to three times a day (8:00, 14:00 and 17:00h) before pointed water temperature. Mean water temperatures (+ standard deviation) and respective caiman frequency means by box/temperature were: Phase 1 (BxD/22.3 ± 1.2°C - 10.3%; BxC/27.0 ± 2.4°C - 32.0% and BxA/31.2 ± 1.1°C - 57.6%); Phase 2 (BxC/26.6 ± 2.8°C - 13.3%; BxB/29.3 ± 2.2°C - 27.7% and BxA/31.3 ± 1.2°C - 59.0%) and Phase 3 (BxB/29.5 ± 3.6°C - 23.7%; BxA/31.0 ± 2.2°C - 31.9 and BxE/34.0 ± 2.2°C - 44.4%). Observing that in the phases 1 and 2 the box with hottest water (CxA) was more frequented. In the phase 3, the difference between higher and smaller frequency straiten to 20.7%, in relation to almost 46.5% in phases 1 and 2, indicating that water temperatures between 29.5 and 35°C resulted in body temperatures more next to the comfort zone, in the experimental conditions.