Eye Size and Shape

Figure 5 plots axial eye diameter against head and body length in extant primates and other mammals. Nocturnal primate eyes have larger axial diameters than most similarly sized nonprimate mammals. Tarsiers have longer eyes relative to body size than any other mammals. Axial diameter of the eye scales with negative allometry across all diurnal and nocturnal mammals, except

Table 1. Reduced major axis (RMA) regression statistics

Variables

Activity

n

RMA slope

RMA intercept

r

CD vs. AD

Aves

Nocturnal

129

0.967

-0.100

0.951

Diurnal

166

0.930

-0.174

0.971

Mammals

Nocturnal

174

0.912

0.0194

0.979

Diurnal

103

1.138

-0.332

0.921

Primates

Nocturnal

30

0.765

0.179

0.971

Diurnal

49

0.963

-0.192

0.892

CD vs. HBL

Aves

Nocturnal

146

0.875

-0.799

0.864

Diurnal

193

0.678

-0.590

0.826

Mammals

Nocturnal

174

0.802

-0.941

0.699

Diurnal

103

0.576

-0.480

0.852

Primates

Nocturnal

30

0.417

0.103

0.539

Diurnal

49

0.450

-0.182

0.895

AD vs. HBL

Aves

Nocturnal

129

0.906

-0.726

0.85

Diurnal

166

0.729

-0.446

0.837

Mammals

Nocturnal

197

0.882

-1.059

0.724

Diurnal

107

0.507

-0.132

0.881

Primates

Nocturnal

30

0.546

-0.100

0.603

Diurnal

56

0.467

0.011

0.903

AD = axial diameter; CD = cornea diameter; HBL = head and body length.

AD = axial diameter; CD = cornea diameter; HBL = head and body length.

those at very small body sizes (Kiltie, 2000) and all diurnal and nocturnal primates. However, as noted elsewhere (Ross, 2000), the relationship between these variables across all mammals is not linear, being positively allometric at small body sizes and negatively allometric at large body sizes. Because small primates have relatively larger eyes than other mammals, the primate slopes are significantly less steep than those for all mammals combined.

Figure 6 plots axial diameter of the eye against head and body length in extant primates and birds. Tarsiers fall within the range of nocturnal birds, i.e., Strigiformes (owls), Caprimulgidae (nightjars), and Podargidae (frog-mouths). Most of the diurnal birds lying between the nocturnal bird and diurnal primate distributions are falconiforms (Hall, 2005). The rest of the diurnal birds are parrots, pigeons, and procellariform sea-birds, which fall among and below the primate distributions. Across nocturnal and diurnal birds, eye axial diameter is negatively allometric. However, the 95% confidence limits for the slope across nocturnal birds almost include 1.00 (= 0.99), a slope significantly steeper than that of primates.

D

Nocturnal

primates^* SSmK

>

IP <

.. «* < •

m

• Nocturnal mammal Fossorial mammal Diurnal primate o Diurnal mammal > Crepuscular mammal

Cathemeral primate □ Unknown

Log head and body length (mm)

• Nocturnal primate

• Nocturnal mammal Fossorial mammal Diurnal primate o Diurnal mammal > Crepuscular mammal

Cathemeral primate □ Unknown

Log head and body length (mm)

Figure 5. Bivariate plot of axial diameter (log10) against head and body length (log10) across all mammals. Minimum spanning polygons for nocturnal primates and diurnal primates are added. Nocturnal primates have longer axial lengths for their body size than similarly sized nonprimate mammals.

Figure 5. Bivariate plot of axial diameter (log10) against head and body length (log10) across all mammals. Minimum spanning polygons for nocturnal primates and diurnal primates are added. Nocturnal primates have longer axial lengths for their body size than similarly sized nonprimate mammals.

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