Okay... So war dogs, at even one spear, get broken easily. Fractured bones, bruises on every body part, all ow.
Grizzly bears react somewhat well, but seem to heal rather slowly. Fractured bones don't happen often, but still, any fractures are bad.
Gorillas, oddly, get beaten up as bad as the dogs. Knocked out teeth, fractures bones, broken bones, even on one spear.
Elephants, however, get only bruises, and heal DURING training. I suggest modding their grazer value to something reasonable. Up until four spears, no fractures at all. Four and five though, ow. Five have a fractured skull, and four had a broken tail. Rhinos fare just as well, no significant changes.
I'd imagine big cats would do good, since they seem to be excellent dodgers. But that defeats the point, doesn't it?
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That was the good news... The bad news: the skill gain is a ridiculous crawl, so slow that, without extreme micro management, this isn't a viable idea. I haven't tried dragons, I bet they'd fare quite well against ten spears, considering I've seen bronze spears glance off their scales.
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The weight of spears DOES cause some good changes. With tower-cap and fungi-wood, fractures got to my elaphants in the trunks and ears commonly. With feather wood, they didn't happen, not even with more spears and time.
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The results of my !!SCIENCE!!? If you have time on your hands, this can help you get slightly stronger animals, but a swimming chamber, as suggested above, may be better. I'll leave that to someone else. For now, onto crushing goblin sieges with a ton of trained animals! This map is great.