16 Chapter 2shows that they possess two types of colour-sensitive cones that respond to light
within the blue and green light spectrum, which means that they are likely to see
yellow and blue as primary colours and their combination colours (Loop et al., 1987).
Behavioural research indicates, however, that cats’ colour perception may be muted
or that they are ‘behaviourally colour blind’ because they seem much more able to
distinguish between differences in shape, pattern and brightness than between colours
(Bradshaw et al., 2012).
Binocular vision
Like us, cats have forward facing eyes that work together giving them stereoscopic or
3D vision (DeAngelis, 2000). This is achieved by each eye focusing on the same visual
target and producing its own 2D image. These images overlap with each other result-
ing in 3D vision. Stereoscopic vision is a great advantage for a predator because it
allows for accurate judgment of depth and distance.
Some Siamese cats have a genetic misrouting of retinal ganglion cells resulting in
an inability to develop full stereoscopic vision (Bacon et al., 1999). However, other
than the development of an adaptational squint or ‘cross-eyes’, these cats appear
otherwise unaffected and many are perfectly able to hunt successfully. Cross-eyed
Siamese were quite common at one time, but are not seen so often now as more
breeders have become aware of the genetic influence and choose not to breed from
affected cats.
Field of vision
The field of vison is the area that can be seen when the eyes are fixed in one position.
It is made up of the binocular visual field, where the two eyes work together to pro-
vide a stereoscopic image, and the peripheral field which is outside of the central gaze
and is non-binocular. Cats have about the same binocular field of vision as humans,
about 90–100°. But their lateral peripheral visual field is wider, giving them a total of
around 200° compared to around 180° in humans.
Hearing
Cats have one of the widest ranges of hearing among mammals, extending from 48 Hz
to 85 kHz (Heffner and Heffner, 1985), although it is generally accepted that the
useful upper limit is more likely to be around 60 kHz because sounds above this
frequency would have to be fairly loud for a cat to be able to hear them (Bradshaw
et al., 2012). Even so, it is still quite exceptional when you consider that the average
human hearing range is approximately 20 Hz to 20 kHz.
The hearing range of most mammals encompasses that necessary to hear species-
specific auditory communications plus, if a predatory species, the sounds made by
prey. Hearing is one of the cat’s main methods of prey detection; it is therefore not
surprising that their hearing range enables them to hear the very high-pitched ‘ultra-
sound’ signalling made by small rodents and other small prey species. But their hearing