Fig. 4. Changes in heart rate with development in homozygous bre mutant animals raised at 25°C, 28°C or 31°C. (A-C) Chamber-specific heart rate (i.e. atrial rate and ventricular rates) in the 2:1 rhythm, and heart rate at the 1:1 rhythm for the three different temperatures. For comparison, heart rate of wild-type animals is included. (D) Comparison of heart rate of mutants in the 1:1 rhythm at the three different temperatures in order to demonstrate the inverse temperature sensitivity of heart rate during the 1:1 rhythm. N-values for the separate stages and groups as well as a statistical comparison of the various stages and groups are listed in Tables 1 and 2.