they probobly could add resources for each animal in bts so even if some other civ managed to get a few of the pens you could trade for some of those animals. also animal pens could provide happiness as long as you have that resource . the way the system works now will allow for that to be done
also i think that all animals should have a spawning mechanisim
gorillas could build a den- but only while standing on bannanas

it probobly would work better if it was any food resource within a jungle
panthers would spawn new panthers but only while standing on jungle.
tigers could spawn a lair only when two tigers are close to eachother.