If a male is in amplexus with a female and another male arrives at the scene, the intruder will attempt to dislodge the mating male by grasping him and pulling him off. This isn't easy and size is important. Only considerably larger males are sometimes successful supplanting a mating male. Frequently, when the intruder contacts the mating pair, the mounted male will give a low frequency croak. Often, the intruder will leave immediately conceding defeat.
Males vary with size and size matters in who wins a contest.
The pitch of croaks is correlated with the size of the croaker. The larger the organ pipe, the lower the frequency produced when air passes over it. Davies and Halliday proposed that intruders judge the size of their opponent by how deep its voice is and therefore assess their own probability of displacing the mating male. Whenever the intruding male touched the mating male, they played a recording of a male croaking. There were for conditions, larger and smaller mating toad and high and low frequency croak.