Full Idea
Pleasure is the chief good, because all animals from the moment of their birth are delighted with pleasure and offended by pain by their natural instinct, without the employment of reason.
Gist of Idea
Pleasure is the chief good because it is the most natural, especially for animals
Source
report of Epicurus (fragments/reports [c.289 BCE]) by Diogenes Laertius - Lives of Eminent Philosophers 10.29
Book Reference
Diogenes Laertius: 'Diogenes Laertius', ed/tr. Yonge,C.D. [Henry G. Bohn 1853], p.473
A Reaction
The highest pleasure of predators is likely to be the killing of weaker animals. What all animals do isn't much of a criterion for the natural chief good. They also breathe.