I think it'd be better to have a breeding area, rather than letting the population decide. You can build a small fenced/walled in area that must have sky above it and some sort of plants growing inside for food source, then you can use a door/gate to keep the animals in. Define the area with a zone marked breeding zone, this zone would work like home or no roof zones, not a growing or stock pile zone. Otherwise it'd disable your ability to grow plants manually inside in case they eat it all. Then again I suppose you can haul food in every so often but anyway.
I'd like the breeding zones because then you can send a guy out to herd some muffalo into the pin then lock it up and keep the population in check that way, maybe even automate a small bit of the hunting for food.
If you use zones like stockpile and growing zone, you can set it to muffalo and set the amount of muffalo before they are allowed to breed. If less than 5 are in the zone, breed till there are 10. For example. Maybe even make it so like horses, they each have their own stable to go into and you can separate the males from the females till breeding is necessary.