You are eligible for a visa on arrival good for exactly 30 days. This visa can be extended for another 30. In theory you're not supposed to do any work (including voluntary unpaid one) unless you have a work visa. In reality I have never heard any foreigner got into trouble for unpaid work.
I just thought of something: if you pay them a token amount (like $5) for the chance to do things at the facility, you would actually be safe. It's completely legal for tourists to spend money for the opportunity to handle animals.