Bentham is often regarded as an early proponent of animal welfare, and is frequently quoted by animal rights activists for having stated: 'The question is not, Can they reason? nor Can they talk? but, Can they suffer?' (Bentham, Introduction to the Principles of Morals and Legislation, ed. J.H. Burns and H.L.A. Hart, intro F. Rosen (Oxford, 1996), p. 283.
This is, in fact, a partial quotation which is not necessarily representative of Bentham's true position on the issue. Bentham was more than willing to accept the killing of animals for food, or to protect human life, provided that the animals were not made to suffer:
'If the being eaten were all, there is very good reason why we should be suffered to eat such of them [animals] as we like to eat: we are the better for it, and they are never the worse. They have none of the long-protracted anticipations of future misery which we have. The death they suffer in our hands commonly is, and always may be, a speedier, and by that means, a less painful one, than that which would await them in the inevitable course of nature. If the being killed were all, there is very good reason why we should be suffered to kill such as molest us; we should be the worse for their living, and they are never the worse for being dead. But is there any reason why we should be suffered to torment them? Not any that I can see.'
Nor did Bentham object, per se, to medical experiments on animals. In a letter to the editor of the Morning Chronicle, dated 9 March 1825, he wrote:
I never have seen, nor ever can see, any objection to the putting of dogs and other inferior animals to pain, in the way of medical experiment, when that experiment has a determinate object, beneficial to mankind, accompanied with a fair prospect of the accomplishment of it. But I have a decided and insuperable objection to the putting of them to pain without any such view. To my apprehension, every act by which, without prospect of preponderant good, pain is knowingly and willingly produced in any being whatsoever, is an act of cruelty; and, like other bad habits, the more the correspondent habit is indulged in, the stronger it grows, and the more frequently productive of its bad fruit. I am unable to comprehend how it should be, that to him to whom it is a matter of amusement to see a dog or a horse suffer, it should not be matter of like amusement to see a man suffer; seeing, as I do, how much more morality as well as intelligence, an adult quadruped of those and many other species has in him, than any biped has for some months after he has been brought into existence; nor does it appear to me how it should be, that a person to whom the production of pain, either in the one or in the other instance, is a source of amusement, would scruple to give himself that amusement when he could do so under an assurance of impunity.
Some of the manuscripts on which Bentham wrote about the treatment of animals can be found here on Transcribe Bentham.
Also see manuscript, in which Bentham recalls a childhood event which may have helped shape his thinking on the issue.