If you don't know what I'm talking about, coqui frogs are several related species of frogs native to Puerto Rico that are named for their distinctive (and loud) calls. This facet of their biology has prompted much vigorous debate ever since they were accidentally released in the state of Hawaii, with many people finding their calls obnoxious at night (they are quite loud) and working to kill as many as they can, and just as many people defending them and working to protect them.
I tend to like them, myself. They fill much the same role as background crickets (if slightly louder), but are much more melodious than insects, to my ear. They also remind me of the spring peepers back in Northern California.
Oh. I guess I didn't really talk about fugues, did I? Simply put, fugues are a complex musical technique, considered by many musicologists to be the most complex of contrapuntal forms (unsurprisingly, the Baroque era was their golden age, especially in the hands of such masters as Handel and Bach).
Oversimplifying greatly, a fugue has as part of its integral nature a repetition of a musical phrase over and over in different voices and keys, often in a staggered fashion, with a whole lot of other, more complicated things happening as well. If you take the coqui frog's two-note co-QUI! as a phrase, they could be said to be playing a very simple, rather extended, and somewhat limited fugue all night long, to a gentle and pervasive backdrop of crickets, accompanied by the odd cadenza-like trill of a gecko, or two male coqui's engaging in a “singing duel”. That is all.