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Box A1

Animal Vision
Andrew Smith

Vision is a key sense for humans, yet for other judge depth (Timney and Keil 1999). The
species the world isn’t always quite as we see relative size of an animal’s binocular field
it. Species may differ in their field of view, depends on where its eyes are located. Our
depth perception, visual acuity (ability to dis- forward facing eyes provide us with 140° of
criminate objects from a distance), colour binocular vision, whereas dogs (Canis lupus
vision and the perception of time. For exam- familiaris) with eyes set slightly to the side of
ple, to distinguish colour an animal’s eye their heads have a larger 240° field of view of
must typically have more than one type of which 30–60% is binocular, and woodcock
cone cell, since colour perception is based on (Scolopax spp.) and other birds with eyes on
comparing the outputs of the different classes the very sides of their heads can have 360°
of cone. As such, those animals with rod‐only visual fields with only 5°of binocularity in the
retinas such deep sea fish (Hunt et al. 2001) horizontal plane (Miller and Murphy 1995;
and deep diving cetaceans, e.g. the sperm Martin 2009). Animals differ in their vertical
whale (Physeter macrocephalus) (Meredith fields of view too, thus those birds with eyes
et al. 2013) see the world in monochrome on the sides of their heads may overlap both
(shades of grey), perceiving no colour. behind and above them, thus providing them
The physical area that an animal can see is with total panoramic vision of the space
referred to as its field of view. Humans, like around their head.
other primates, have forward facing eyes, Animals not only differ in their fields of
and thus have a relatively narrow field of view, but also show variation of vision within
view, ca. 180°, compared to an animal with these fields. A key aspect of vision is the abil-
eyes on the sides of its head, such as the scal- ity to resolve detail (visual acuity). Some ani-
loped hammerhead shark (Sphyrna lewini), mals, such as birds of prey, have a much
which has a full 360° field of vision (McComb greater visual acuity than humans. Whereas a
et al. 2009). The area seen by both eyes simul- person might just be able to discern a 2 mm
taneously is the binocular field of vision, and object from 6 m, it’s estimated an eagle can
that seen by one eye, the monocular field. detect it from 35 m away (Hodos 2012). In
Whilst monocular cues can be used to deter- contrast, the visual acuity of other species is
mine depth (Timney and Keil 1995; Martin poorer than in humans, that of dogs for exam-
2009), experiments show that performance, ple has been estimated at 20–40%. Thus what
for mammals at least, may be better when an we can distinguish from 9 m away, a dog can
animal uses its binocular field of vision to only see at 2 m (Miller and Murphy 1995).

Zoo Animal Learning and Training, First Edition. Edited by Vicky A. Melfi, Nicole R. Dorey, and Samantha J. Ward.
© 2020 John Wiley & Sons Ltd. Published 2020 by John Wiley & Sons Ltd.