The relationship between heart rate and rate of oxygen consumption in Galapagos marine iguanas (Amblyrhynchus cristatus) at two different temperatures

To enable the use of heart rate (fH) for estimating field metabolic rate (FMR) in free-ranging Galapagos marine iguanas Amblyrhynchus cristatus, we determined the relationships between f(H) and mass-specific rate of oxygen consumption (s(V) over dot O-2) in seven iguanas before and during exercise on a treadmill and during the post-exercise period. The experiments were conducted at 27 and 35degreesC, which are the temperatures that represent the lowest and highest average body temperatures of these animals in the field during summer. There were linear and significant relationships between f(H) and s(V) over dot O-2, at both temperatures (r(2)=0.86 and 0.91 at 27degreesC and 36degreesC, respectively). The slopes of the two regression lines did not differ, but there were significant differences in their intercepts. Thus, while heart rate can be used to predict FMR, the effects of temperature on the intercept of the regression must be taken into account when converting f(H) to s(V) over dot O-2. On the basis of our data, this can be achieved by applying the following formula: s(V) over dot O-2 = 0.0113f(H) - 0.2983Q(10)(T-b-27)/10. The increase in s(V) over dot O-2, with elevated body temperature results from an increase in f(H), with no significant change in mass-specific oxygen pulse (sO(2) pulse; cardiac stroke volume times the difference in oxygen content between arterial and mixed venous blood). However, during exercise at both temperatures, increases in f(H) are insufficient to provide all of the additional O-2 required and there are also significant increases in the sO(2) pulses. This creates the situation whereby the same fH at the two temperatures can represent different values of s(V) over dot O-2.