Seasonal changes in the preferred body temperature, cardiovascular, and respiratory responses to hypoxia in the toad, Bufo paracnemis

Estivation is accompanied by a reduction of oxygen consumption in amphibians during drought. We tested the hypothesis that, during the dry season, the toad Bufo paracnemis selects a lower preferred body temperature (T-b), and would be less sensitive to hypoxia, than during its active period. Therefore, during winter (dry season in Sao Paulo state, Brazil) and summer, we measured the effects of hypoxia (7% inspired O-2) on preferred T-b. Additionally, pulmonary ventilation, heart rate, blood pressure, and oxygen consumption were also measured in toads at 15 and 25 degreesC. Blood gases were measured at 25 degreesC. Oxygen consumption was significantly higher during summer in toads at 25 degreesC. Under normoxia, preferred T-b was higher during summer than during winter, and hypoxia caused a drop in preferred Tb during both seasons. In both seasons, toads at 15 degreesC showed reduced pulmonary ventilation, heart rate, and blood pressure, and hypoxia had no effect. At 25 degreesC during summer only, hypoxia caused an increase in ventilation. Season had no effect on blood gases. We conclude that B. paracnemis displays an endogenous seasonal pattern of thermoregulation and control of ventilation. The decreased preferred Tb and the physiological responses to hypoxia may be beneficial to toads encountering drought and when food is not available. J. Exp. Zool. 289:359-365, 2001. (C) 2001 Wiley-Liss, Inc.