Rights are things that those in power give out, so there is always going to be something assimilationist about rights. Arguments for including the more than human world in our ethical deliberations as rights-bearers have tended to parallel arguments that extend ethical concern outward from those who occupy the moral center. Historically in the US and Europe, for example, we have seen white, Christian men at the center extending rights to non-Christian, non-white men and then women. As the circle of rights holders grows, the ideal is that all of humanity will be included, whatever their race, nationality or gender expression.
But why stop at species? Some scholars and activists have tried to combat what is alternatively termed "speciesism," "humanormativity" and "human exceptionalism" by moving the boundary of the circle beyond the human.
One of the main strategies for expanding the circle is to turn to empirical work designed to show that other animals are really similar to those at the center of the circle and thus deserve rights. To be considered consistent and fair, we are implored to treat like cases alike. If those on the margins of the circle of moral concern can be shown, through ethological and cognitive research, for example, to have some of the qualities that we admire in ourselves and to which we attach value, then we ought to admire and value those qualities in whatever bodies they arise.