At the moment, constitutional law making in most places is by reference only to the best interests of people. This can change, and change is already happening in some places. For example Germany has incorporated animal welfare into their constitution: “Mindful also of its responsibility toward future generations, the state shall protect the natural foundations of life and animals by legislation and, in accordance with law and justice, by executive and judicial action, all within the framework of the constitutional order.” Basic Law of the Republic of Germany, Part. II, Art. 20a for other examples, see Harvard Law School Animal rights program links on the internet.