Robert McNeill (Neill) Alexander, CBE FRS (7 July 1934 – 21 March 2016) was a British zoologist and a leading authority in the field of biomechanics.
Until 1970, he was mainly concerned with fish, investigating the mechanics of swim bladders, tails and fish jaw mechanisms.
Subsequently, he concentrated on the mechanics of terrestrial locomotion, notably walking and running in mammals, particularly on gait selection and its relationship to anatomy and to the structural design of skeletons and muscles.