I don't know if this is intentional or not, so I'm documenting it.
Animals gain experience after level 10, but even after hitting 101 points (or much much higher) don't actually level.
On the other hand, early Subdue Animal on a scout gives some silly powerful units for the era. Especially on marathon maps. The first barbs are 1 and 2 str, but Tigers, Lions, and Bears (oh my) have 3 and 4 str. Plus these units, especially the spider, are sickly useful in torturing your opponents by sniping off scouts, workers, etc.
A nearly foolproof way to capture a spider with a scout: Combat I, Subdue, Woodsman, Guerilla(only 17 exp total), and find a spider next to a wooded hill. Congrats on your new spider next turn. Early in 0.20 the poison meant going to a friendly city to heal, now you can heal on your own.
Unfortunately, its the only way to get caged animals, effectively forever. Unless you rush for the ritual to convert barbarians then about the time you get them is the time barbarians disappear. Also, the ability to get caged bears without a carnival was given because they're the strongest animals - except now spiders are, and their cage doesn't even give happiness. Just silk.