This is a graph of body temperature (vertical axis) against ambient temperature (horizontal axis) for endothermic mammals and a lizard in laboratory studies. The plotted points for a lizard fall on a line that is at 45 degrees to the horizontal axis, increasing towards the right. The plot shows that body temperature in the lizard is linearly related to ambient temperature and body temperature tracks the change in ambient temperature. There is no evidence of regulation. Plots for three monotremes and a cat are at a shallower angle to the horizontal axis. The shallower the angle, the better the regulation of body temperature. All four animals show an ability to regulate. The plotted line for the cat is close to being horizontal, indicating a high degree of regulation.