Fig. 1

Iris color varies continuously in humans from very light blue to dark brown (upper line, a, b, c, d). Intrapopulational eye color variation is also characteristic of domestic animals (middle line): two adult cats Felis catus (e, f) and two adult domestic Muscovy ducks Cairina moschata domestica (g, h). In wild animals, however, iris color tends to be a fixed trait, with few observed variations due to maturation with age or sexual dichromatism. The bottom line shows color variation in two birds of prey. First a case of variation related to age in Black-winged kites Elanus caeruleus (i: adult; j: juvenile). Second, variation related to sex: Sparrowhawk Accipiter nisus (k: adult male; l: adult female). a: CC BY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0/, Luisangel https://flic.kr/p/4vHKkh). b: CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/, Gorgeous Eyes https://flic.kr/p/7vXh7G). c: CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/, Jean-Simon Asselin https://flic.kr/p/2p4pFU). d: CC BY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0/, Emilio Küffer https://flic.kr/p/agQvbv). e: CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/, Trish Hamme https://flic.kr/p/dzfp8N). f: CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/, Trish Hamme https://flic.kr/p/e8v8ro). k: CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/, Andy Morffew https://flic.kr/p/KjN82N). l: CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/, sighmanb https://flic.kr/p/ecc15G