There's a few interesting posts about critters on this thread in the official forums:
Andrea Di Stefano wrote:
The critters right now lower the boredom and provide some xp when Norns interact with them.
The critters themselves don't reproduce at the moment but they do feed (flower, mushrooms, weeds, bugs, pests, etc.) and rest. If their food source is depleted for whatever reason, they die of starvation.
They can also die of old age or if they're hit on by the Norns!
Also, life times are not tweaked yet.