Complex sentence: The only way to tell if a rabbit is single mane or double mane is when they are first born, past that many things contribute to how much mane they actually end up having including chewing on the mane by themselves or others and matts.
Split into simpler sentences: The only way to tell if a rabbit is single mane or double mane is when it is first born. Past that many things contribute to how much mane each individual actually ends up having including chewing on the mane by themselves or others and matts.