Biologist Richard Dawkins - who has spoken about animal sentience on a number of occasions - has claimed non-human animals may be capable of experiencing more intense levels of pain than humans.
The scientist made the assertion in a video published by Big Think.
In the video, Dawkins - who has been called a hypocrite for eating meat - says: "There's every reason to think that mammals at least, and probably many more, can suffer perhaps as much as we can.
"If you think about what pain is for biologically speaking, pain is a warning to the animal, 'don't do that again'.
"If the animal does something which results in pain, that is a kind of ritual death - it is telling the animal, 'if you do that again you might die and fail to reproduce'. That's why natural selection has built the capacity to feel pain into our nervous systems.
"You could say since pain is there to warn the animal not to do that again... an animal that is a slow learner or less intelligent might need more intense pain in order to deter [them] from doing it again, than a human who is intelligent enough to learn quickly.
"So it's possible non-human animals are capable of feeling more intense pain than we are."
He also talks about the 'un-evolutionary' way we treat animals, saying: "There's quite a lot in [my book] Science in the Soul about the ethics of the way we treat non-human animals. I say non-human because we are of course animals, we are not plants, we're not fungi, we're not bacteria - we are animals.
"There is a double standard in our ethics at present which builds a wall around our own species Homo Sapiens which is rather un-evolutionary, if you think about the fact that we are close cousins of chimpanzees, if you think about the fact we are descended from a common ancestor that lived only about six or seven million years ago.
"If you want to erect a moral wall around our species, and say, for example, that a human embryo, even a very beginning human embryo (long before it develops a nervous system) is somehow worthy of more moral consideration than an adult chimpanzee, then that is a rather un-evolutionary viewpoint."