I'm attempting to put together a heartbeast, so I'm going through the rules set down in the Bjornaer chapter of Mystery Cults. I'm ending up with a communication of -5 because it is a hoofed beast. Is this communication score relating to how well the beast communicates with humans, or is it assumed that all horses have a -5 to communicate with each other? Perhaps a human would figure an animal doesn't have the vocal capacity of a human, but most beast communication is non-verbal anyway. I'm just trying to understand why the base communication would be so low.
I agree with you on that. I would say that Herd/pack animals have a good communication compared those that operate solo.
I found that the HoH: MC guidelines really don't make much sense. Based on those guidelines, I had hard time seeing how most owls could catch or kill a mouse. Their strength is so low and the mouse has the higher quickness and dodge to get away unless very lucky.
It's between humans and animals. The Domesticated quality adds one to Communication for that reason. I would imagine that all animals have a base Communication within their species or type.
Mark
Only a small proportion of attempts on a prey animal by a predator are successful, in general. Some animals increase the odds by hunting in packs, or using ambush (which is what the owl and cat, and other predators of small, swift prey, rely on). The Ambush Predator quality accounts for this. Furthermore, animals make use of non-combat effects of Brawl, such as grapples and pins. An owl just needs one successful attack on a mouse to pin it to the ground; the mouse then needs to make an opposed Strength check to escape. Sure, the owl is weak (compared to a human, that is), but the mouse is weaker.
However, it cannot be denied that the Ars mechanics does break down when not dealing with humans or creatures close to human size. But then, the same is true for many roleplaying systems which take the view that the vast majority of characters will be human-sized.
Mark
If this is true, then why doesn't a magi's communication score carry over to her heartbeast similar to intelligence? Domesticated animals have experience with humans, so they can get thier thoughts across better. Magi are human, and even if they cannot speak when in animal form, would know how to get a point across better if they needed to.
The combat modifiers seem odd to me as well. A hoof does +1 damage, whereas a club and a kick both get +3. Teeth (+1 Damage) I would relate to a knife (+2), small horns (+2) to a dagger (+3) and large horns (+3) to a short sword (+5), etc.
Sounds like a decent, well reasoned, house rule to me. 
While that is a fair enough idea, remember that there is two sides to the communication. A magus in animal form is still talking to a human. The problem is that while the shape-shifted magus may be smiling, shrugging, and pointing at the tower on the hill, all that the human sees is a bloody great big bear snarling, and doing something threatening with his claws.
The bonus for domesticated animals bonus is not so much that the animal knows how to communicate to the human, but that the human is familiar with the type of animal. So he knows how to interpret the animal's expressions and gestures.