Since being sentient is common to both animals and men, that should at least give the former the right not to be needlessly mistreated by the latter.
Gist of Idea
Both men and animals are sentient, which should give the latter the right not to be mistreated
Jean-Jacques Rousseau (Discourse on the Origin of Inequality , Pref)
This is why utilitarianism led to the founding of the RSPCA in Britain. There is a disturbing picture of people smashing up animals for fun, if they can only persuade themselves that the animals are not sentient. I've heard fishermen claim that.
Rousseau,Jean-Jacques: 'The Basic Political Writings', ed/tr. Cress,Donald A. [Hackett 1987], p.36