Animals should not have rights because they lack the ability to deliver their duty. According to the social contract theory, individuals were born into the state of nature, where they had to fight each other for survival. People formed government and made rules that limited their rights for the sake of the general stability and happiness.* They could no longer kill or rob others to get what they want because of the limitations, but it also meant that they did not have to worry of murder and theft as much as they used to. The arbitrary duties that people imposed upon themselves were beneficial for the humanity on the whole when all was said and done. People got out of the state of nature and started to cooperate with each other to conquer the nature. Even now, the social contract theory still remains salient. People in any society have been required to follow certain sets of rules to be protected by the law because rights and duties are two sides of the same coin. An English philosopher Roger Scruton pointed this out well in
Cited: http://www.scientificamerican.com/article.cfm?id=should-humans-eat-meat-excerpt http://www.thefederalistpapers.org/wp-content/uploads/2012/12/John-Locke-Thoughts-Concerning-Education.pdf http://www.city-journal.org/html/10_3_urbanities-animal.html http://lawdigitalcommons.bc.edu/cgi/viewcontent.cgi?article=1382&context=ealr http://heartland.org/sites/all/modules/custom/heartland_migration/files/pdfs/15656.pdf http://public.callutheran.edu/~chenxi/phil345_111.pdf http://www.freehealingtools.com/kanzi-an-ape-of-genius