I think it goes something like this: to attract the females, a male wants to have as deep a
voice as possible so he sounds big and tough. Now cold air is denser than hot air, so it would
lower the pitch of his voice (witness the effects of a person inhaling helium for the opposite!).
Therefore the frog tries to find the coldest air he can to ribbet in. Why don't people get
lower-pitched in winter? Because what matters is the air around the vocal cords, which stays
at roughly the same temperature since we are warm-blooded. But a cold-blooded frog uses
its environment to manipulate its body temperature, so can pull of this trick with relative