Goats never use viagra. Despite this, the average male goat is always ready for more. In fact "ready" isn't really a strong enough word for it...

Female goats are a different story. They only do it when they're good and ready, and that is for only a day or two every month. This is why every billy goat fantasizes controlling a herd of at least 500 nanny goats. It's a hectic life for the few billies who succeed in attaining that lofty objective.