Latin: "After coitus every animal is sad." Borne out by practical experience and observation.

This nodeshell rescued by Trained Monkeys, Inc., Archaic Linguistics Division

Can also be exressed as Omne animal triste post coitum, which is how the young novice Adso describes his feelings after being seduced by the beautiful peasant girl in the novel The Name of the Rose by Umberto Eco.

I laughed out loud when I read this part of the book and then showed it to a friend who is fluid in latin. He remarded that the sentence sounded wrong but that it could possibly be some kind of vulgar latin.

The correct spelling would be:

omne animal triste post coitum

why triste and not tristis, you ask me. Well, the adjective is tristis, triste (a Latin second class adjective). This means that its forms are:

Masculine tristis
Feminine  tristis
Neuter    triste

And it happens that animal is a neuter noun of the third declension (animal, animalis). Since animal is neuter, the adjective that modifies it must take the neuter gender: triste

As for the source, I have seen it attributed to Ovid and Aristotle - but I can't find it in the Perseus Project.

Notice also that Latin is quite free with word order: the sentence could well be "omne animal triste post coitum" or "triste omne post coitum animal". There could as well be an est nearly anywhere.

thanks to Gritchka and mbk for various reasons

Quite as interesting is the full quote: Triste est omne animal post coitum, praeter mulierem gallumque; "except the woman and the cock (= rooster)".

This is, I am now led to understand, by Galen (c. 130-201), the great physician to the emperors; though until I did a Web search just now to find the exact work, I had thought it was by Aristotle (who would of course have been read in Latin translation for most of history); and I have also seen it given to Ovid. But Kinsey goes for Galen.