The Signs of a Sick Species

In the Larry Niven/Jerry Pournelle novel The Mote In God's Eye, humanity meets aliens for the first time, aliens with a horrifying secret. The Moties (as humanity calls them) are sequential hermaphrodites, endlessly changing from one sex to the other. The horrifying aspect is that when female, the Moties must get pregnant or they will die. Population explosions are a way of life for the Moties and the only why to rid yourself of a rapidly growing population is to have wars, lots and lots of nuclear wars.

The true nature of the Moties is first guessed when in a desert, out in the middle of nowhere, the humans find huge buildings holding the making of civilization. The DNA for animals, the seeds for plants and foodstuff, the record of the current civilization. What a sad race that they must protect what they have from themselves, that they must leave clues for those who will follow them. You'd have to be pretty sick as a species to do something like that.