Fig. 2.1 shows a gray tree frog, Dryophytes versicolor. [Figure 2.1] Males of this species make a very distinctive mating call. Some male gray tree frogs were kept in the laboratory at different temperatures. Their mating calls were recorded. Each call consists of a number of pulses. The calls are repeated at intervals. Fig. 2.2 shows four calls recorded at 20°C. Each vertical line represents a single pulse. [Figure 2.2] A scientist wanted to find out if there was a relationship between body temperature and the interval between calls. The scientist put each frog in a temperature-controlled environment. The scientist took the body temperature of each frog before recording the mating calls.