le the aforementioned paper makes no mention of cone (colour-sensing) cells on the retina of Erinaceus europaeus, however, in his 1965 Hedgehogs: A Comprehensive Study (cited in Nigel Reeve???s, Hedgehogs), Konrad Herter notes that about 4% of the hog???s retina are cone cells -- humans and other primate are closer to 25% -- and writes of how he was able to train his subjects to distinguish yellow