Page 104 - The Welfare of Cattle

sense and sensIbILItY 81

providing them with good binocular vision and limited panoramic vision. Due to the relatively nar-
row binocular field of vision characteristic of many grazing species, cattle do not have good depth
perception. This is one reason that cattle balk at shadows. The dark shadow on the ground appears to
be a large hole to cattle as they are unable to accurately recognize its depth, or lack thereof. Notice
when handling cattle that they often will lower and raise their heads at the edge of the shadow trying
to assess the safety for moving forward. It should also be noted that cattle cannot see directly behind
them as is also seen in many other animals. When working with cattle, it is important to be aware of
the blind spot and make sure to not surprise an animal while approaching from directly behind them.
A cow’s pupil is horizontal and oval in shape. People who do not take the time to watch cattle
closely may not have observed this, particularly in dark-eyed cattle as most are. On the contrary,
many nocturnal animals, like cats and prosimians, have vertical, slit pupils; it is thought that slit
pupils are better at contracting to protect the inner eye from too much light, a protection needed in
nocturnal animals as they have highly sensitive retinas (Charman, 1991).
The pupil’s function is to protect the eye from letting in too much light but additionally helps with
visual acuity. Rehkämper, Perrey, Werner, Opfermann-Rüngeler, and Görlach (2000) determined
that their research bulls were better able to discriminate images when the images were presented
vertically rather than horizontally, supposedly due to the shape of cows’ pupils (Rehkämper, Perrey,
Werner, Opfermann-Rüngeler, & Görlach, 2000). Considering that cattle scan the horizon for pred-
ators that would likely have a vertical aspect as they approach from the distance, suggesting that
this difference in visual acuity based on orientation may be an adaptive characteristic of the eye of
grazing animals. Additionally, ungulates have a “visual streak” on the retina, which is an elongated
area of high ganglion cell density (Hebel, 1976; Hughes, 1977). It has been theorized that the visual
streak found on the retina of ungulates is a visual adaptation that enables the prey species to better
identify predators on the horizon (Hughes, 1977). The placement on the head and topography of a
cow’s eye are characteristic of grazing species and essential to survival.

Colors—to See or Not to See

It is often asked if animals, including cattle, can see colors as humans can. To see color, the eye
has to be able to respond to different spectral sensitivities (i.e., different wavelengths of light). Color
vision adds contrast between objects seen and therefore increases the visibility of objects within its
surroundings. In the mammalian eye, there are different types of photoreceptors that have different
photopigments and an associated neural network that can detect changes within this photosensitive
system. Rods and cones are the two types of photoreceptors found in the retina of vertebrates. Rods
assist with vision in dim light and do not perceive color and cones assist with vision in bright light
and do perceive color. Early research suggested that cattle have a high proportion of rods in the
retina (ranging between 3:1 rods:cones to 6:1) (Rochon-Duvigneaud, 1943) which supports cattle’s
behavior pattern, diurnal with crepuscular grazing activity.
Humans have trichromatic color vision, meaning that they have three different types of cone
receptors (S-cone, short wavelength; M-cone, medium wavelength; L-cone, long wavelength)
and each type reacts specifically within a certain spectral range with a peak spectral sensitivity.
Measurements of photopigments in bovine and other ungulate retina have indicated two unique
photopigments with peak sensitives in the short wavelengths (S cone; 439–456 nm) and the medium
to long wavelengths (M/L cone; 537–557 nm), indicating that cattle are dichromats (Jacobs, 1993;
Jacobs, Deegan, & Neitz, 1998). The two bovine cones specifically were shown to have peak
sensitivities at 455 nm (S-cone) and 554 nm (M/L-cone) (Jacobs, Deegan, & Neitz, 1998) (for
reference, the peak sensitivity for human S-, M-, and L-cones are 419 nm, 531 nm, and 538 nm,
respectively; Dartnall et al., 1983). Interestingly, many of the papers discussing dichromatic vision
in cattle and other ungulates do not draw conclusions about what colors cattle can actually perceive.
The photopigment absorption curves developed from measuring spectral sensitivity of cones can