Whence Come Obligations To Animals?
The cynical reading of this is that it's a sort of auxillary hypothesis meant to keep our cultural intuitions from being out of sync with our moral theory. Our society accepts a lot of things (e.g. factory farming) that would be violations of any reasonable animal rights theory, yet also condemns many things (e.g. torturing kittens) that would be allowed by a pure Cartesian speciesism, while being internally inconsistent (e.g. our revulsion at eating horses while we chow down on cows). Vague obligations are therefore a useful fudge factor, to be invoked when we encounter practices that we don't like, while those we do like can be excused by declaring the harm "necessary" and pointing out the animal's lack of rights.
A somewhat less cynical, but still problematic, reading is that our obligation to animals is purely conventional, a sort of supererogatory charity. We, as individuals or as a society, can agree to take on certain obligations toward non-rights-holding entities. Those obligations are fully binding after the commitment is made, but they originate in the desires of the obligation-holder (including the desire to be a certain sort of person or culture). Hence no argument of morality or justice can be used to show that we ought to adopt a certain level of obligation.