Page 98 - Zoo Animal Learning and Training

70 Box A1 Animal Vision

VetBooks.ir Similar to visual acuity is accommodation, that some animals, such as geckos and hawk-
moths, have special highly efficient cones to
this is the eye’s ability to change its focal
length so that objects at varying distances are
2006). In order to perceive colour the brain
brought into focus. This is done by changing allow colour vision at night (Kelber and Roth
the shape of the lens, and also to some extent compares the outputs of different classes of
the shape of the eye itself. Thus as a general cones, each maximally responsive to differ-
rule, larger eyes have a greater range of ent wavelengths of light. Humans have three
accommodation than smaller ones. Large cone types, and are thus referred to as tri-
eyes provide long focal lengths and space for chromats. As a general rule, animals with
a larger lens which in combination gives an fewer cone types can distinguish fewer
ability for higher resolution over a greater colours. Our colour vision, and that of Old
range of distances (Land and Nilsson 2012). World monkeys and apes, is amongst the
Differences in accommodation mean differ- best found in mammals, since most mam-
ences in the shortest distance at which an mals are dichromats and have two classes of
object can be brought into clear focus. Thus cones and as such cannot distinguish as many
humans with their greater accommodation colours as we can (Jacobs 1993). However
can focus on an object just 7 cm away, whereas birds, reptiles and amphibians typically have
for a dog anything closer than 33–50 cm will four types of cone which allows them to per-
be blurred (Miller and Murphy 1995). ceive a wider array of colours than we can.
Colour is perhaps one of the most obvious Often the more colourful a species is, the
aspects of the world around us, yet few ani- better its colour vision. However there are
mals see it in the way we do. Colour vision some exceptions. Cephalopods, well known
typically requires cone cells, a special type of for their colour‐based displays and camou-
receptor cell in the retina. Cones normally flage, are completely colour blind (Marshall
require higher levels of light to function and Messenger 1996) (Figure A1.1). Some
which is why we cannot see colour well in species can use properties of light that we
dim light. However there are some excep- cannot detect, such as ultraviolet wave-
tions, for example it has recently been shown lengths, the near infrared, or polarisation.

Figure A1.1 This broadclub cuttlefish, Sepia latimanis, like other cuttlefish has the ability to change colour yet
is colour blind, so body colour itself is unlikely to be the visual signal which this species uses to communicate
with one another. Source: Jeroen Stevens.