European toads compete for receptive females. A male may try to pull off another male already mounted on a female. The male in the act instantly gives a croak of objection.
The lower the croak, the tougher the toad. A higher-pitched usurper immediately knows the score and hops away. Only if the interloping toad can get down with a lower croak does further hostility ensue.
Amphibians grow their entire lives. Their sounds lower with girth. Hence, toad croak pitch is itself an honest signal, as it is entirely a matter of biophysics. A toad can’t fake his croak.