One of the Kilgore Trout's story is called "Rational shit".
Se-Cluger people are rational species, they are perfectionist and always act after think, although they don't have a time travel technology. Each of them usually think about their comfortable houses and happiness. Everything is resolved by discussion, most of them are logical, therefore, there usually no objection to the conclusions. If the problem is not solved, some people just left to the other planets. Killing each other is not a rational solution for them. They can agree that point, that shows they are intelligent. They looked for the perfect rationality, they finally have a technology to change themselves.
One day, they have perfect rationality. Energy of the planet is never wasted, all the disease were solved. The future is planned, they care their descendant. However, they found out they will extinct one day because they are still a life form. The extinction day is far future, however, all the rational Se-Clugers were agreed to continue to live is no sense, and suicided themselves.
The zoo in Tralfamadore also keeps this species, but when they knew their species were gone, they also suicided as a logical conclusion. Tralfamadore people of couse knew this future, but as usual, they did nothing. But they can travel the time, they just back to the past when they want to meet Se-Clugers.
Who cares intelligent and rational?