In many species, males woo females into mating with them. This means they need to find a way to be attractive, which includes everything from elaborate plumage to melodious mating calls. But there's such a thing as too handsome.
As far as neotropical túngara frogs are concerned, the most handsome males are those that make the most appealing calls. These calls are a combination of one long "whine" and a succession of short "chucks." As a general rule, the male that puts on the most chucks is going to be the one most likely to attract his choice of mate.
But what's interesting is that females have an inbuilt limit on how much they can appreciate these calls. While their overall preference is based on the ratio of chucks to whine, at a certain point they can no longer tell the difference between, say, 50 chucks and 55 chucks. There's no point for the males to add any chucks beyond a certain point, and so this places a limit on the adaptation of male mating calls.
This may well speak more generally to why males of other species do not evolve longer tails or more colorful feathers. At a certain point, it's all just overkill, and so there's evolutionary advantage to being ridiculously good-looking. (Emphasis on the quite literal use of "ridiculously" there - being generally attractive is obviously still advantageous in an evolutionary sense.)
While previous studies had pointed to the risk of attracting predators as a possible constraint on overly elaborate mating behaviors, this new research is the first indication that the females of the species actually help guide the evolution of the males.