Table 1 Measurements of scleral brightness and highest irido-scleral contrast (HC; both measured in greyscale intensity) in selected domesticated (= Dom) and wild mammals, as well as published data on their species-specific axial ocular diameters. In case ocular diameter measurements were not available, values for similarly sized related species were used (anoa, European rabbit) or the corresponding measurements of the domesticated (polecat, European wildcat, Przwewalski’s horse, wild sheep) or wild (domestic yak) counterpart were adopted. Note that there is an ongoing debate as to whether and to which extent Przewalski’s horses are feral descendants of an ancient domesticated lineage [32]. Data on adult ocular diameter measurements derive from: a—species mean values from [30], b— [33], c— [34], d— [35], e— [36], f— [37]. *value corresponds to Sylvilagus audobonii, **value corresponds to Hippotragus niger, a bovid of similar body mass, ***value corresponds to Bos bison

Domestic rabbit (Oryctolagus cuniculus f. domestica)

European rabbit (Oryctolagus cuniculus)

15

16

204.97

184.75

184.53

168.44

18.1^a

14.9^b*

Ferret (Mustela furo)

Polecat (Mustela putorius & Mustela eversmanni)

15

15

107.9

124.6

113.07

132.67

7.5^a

7.5^a

Dog (Canis familiaris)

Wolf (Canis lupus)

25

25

171.86

142.38

122.2

84.28

20.1^a

22.6^a

House cat (Felis catus)

European wildcat (Felis silvestris)

16

15

161.44

169.30

49.94

56.93

21.9^a

21.9^a

Domestic donkey (Equus asinus)

Zebra (Equus grevyi & Equus quagga)

21

15

81.24

50.93

62.38

27.80

39.0^b

42.5^a

Domestic horse (Equus ferus caballus)

Przewalski’s horse (Equus ferus przewalskii)*

18

15

93.06

72.67

79.06

51.93

38.7^a

38.7^a

Llama (Lama glama)

Guanaco (Lama guanicoe)

16

15

68.41

58.27

42.56

57

35^a

36^a

Domestic pig (Sus domesticus)

Wild boar (Sus scrofa)

16

15

169.19

75.67

120.50

47.73

23.9^c

24.8^a

Domestic water buffalo (Bubalus bubalis)

Anoa (Bubalus depressicornis & Bubalus quarlesi)

15

15

56.27

57.97

55.27

55.07

33.4^d

30^a**

Taurine cattle (Bos taurus)

European bison (Bos bonasus)

19

16

91.95

99.31

93.05

111.88

30.8^a

36.8^e***

Domestic yak (Bos grunniens)

American bison (Bos bison)

15

20

85.07

90.13

95.2

99.35

36.8^e***

36.8^e

Domestic sheep (Ovis aries)

Wild sheep (Ovis ammon & Ovis vignei)

17

15

126.06

78

82.59

53.73

26.1^a

26.1^a

Domestic goat (Capra hircus)

Wild goat (Capra falconeri, Capra ibex, & Capra nubiana)

18

15

96.36

72.53

52.33

62.07

30.1^f

35.0^b