Orbit orientation and eye morphometrics in giraffes (Giraffa camelopardalis)

Abstract

Giraffe are thought to have excellent vision. We measured eye size, orbit orientation and retina surface area in 27 giraffes of both sexes ranging in age from neonates to mature adults (>10 yrs), to assess how it changes with growth, whether their eye anatomy correlates with their apparently excellent vision and lifestyle, and we have compared our findings with those for other large mammals to assess whether giraffe eye anatomy is unique. We found that giraffe eye volume increases from 33 cm³ at birth to approximately 65 cm³ in adults. The focal (axial) length increases from c. 40 to 48 mm in adults and retina surface area from c. 3000 mm² at birth to 4320 mm² in adults. The orbital axis angle at birth is c. 73° and the horizontal visual field mainly monocular and panoramic. With age the axis angle becomes more acute to c. 50° in adults and the visual field more binocular, changes that occur concurrently with increasing neck length. These results show that the giraffe eye and retinal surface area are larger than in all other ungulates, and their visual fields more binocular, attributes which are consistent with the idea that they have excellent vision.

Key words:

• giraffe
• eye size
• orbit orientation
• visual acuity