Why the barking?

The title of the film in France is "Canine", which makes more sense than "Dogtooth", as it means both your canine tooth (your dogtooth) and also dog-like.

The father has brought up his children as a group of pet dogs. Yes, they wear clothes and eat at the table, but their understanding of language and their environment is dictated by their "owners" (parents). They have no concept of personal names (only eldest or youngest), are taught to revile cats, are allowed to play with limited toys. Their basic needs are met, but they have no free will, expressing themselves by obedience or random acts of violence. Even the licking is a dog-like expression.

The barking is simply how dogs defend their territory. That's what I think is going on - hope it helps!