Seasonal Changes of Heart Rate-Temperature Relationships in Toads. ∗

Abstract

Summary

Intact hearts of adult male toads(Bufo fowleri), collected locally in May and June (“summer toads”); gave linear heart rate-temperature relationships in the range 14-35°C. Toads from the same collection, tested in December and January (“winter toads”), also gave linear heart rate-temperature relationships, but only between 14 and 23°C. At temperatures between 23 and 35°C the heart rates of the winter toads did not increase when their body temperatures were increased. Seasonal differences in heart rate-temperature relationships had been reported for excised frog hearts and shown to be linked to seasonal changes in thyroid and anterior pituitary activities. The similar seasonal differences in heart rate-temperature relationships shown here in intact toads may also reflect endocrine changes. This working hypothesis remains to be tested directly.