:::This is a really tricky one.  I agree that starting off with "...is a contentious issue" is certainly not the best/strongest way to start an article, but as soon as we make a definitive statement that non-human animals (in general) feel pain, we are sure to get people challenging this (take a look at the very recent edits on Pain in fish, Pain in crustaceans and Pain in invertebrates).  What about borrowing the lead sentence from Pain and then adding the concern after, e.g. "Pain is a distressing feeling often caused by intense or damaging stimuli; the experience of 'Pain in animals is a contentious/debated subject." DrChrissy