What's behind toads' mating behavior?

Male toads appear to be smooth talkers, knowing instinctively how to say the right "words" to woo a mate. Scientists are puzzling over why females respond to calls that, genetically speaking, may amount to sweet nothings.

Research by Purdue University biologist Rick Howard found that female toads seek the deeper calls of older, larger males, and that males may disguise their voices to increase their chances of luring a mate. However, a separate study revealed that, for the offspring, there may be no genetic advantage whether their mother falls for the imitation call or finds the real thing.

When alone, male toads of all sizes will call at a higher frequency and exhibit more variety in the types of pitches they use. If other males are in the area, they lower the pitch of their calls, thereby making themselves seem larger. "It's a bedlam out there in the chorus, because everyone's screaming their heads off. Somehow in this madness, a female has to go around and assess males by listening to the pitch of their calls."

What's not clear is why females prefer the lower-pitched calls in the first...