Deer

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Field-of-view:

If you have ever seen a herd of deer with their heads down foraging in a field, you will notice that at any given time, at least one animal will have its head up and alert, scanning for potential predators. Although there are fewer large carnivorous predators today than in the past, deer evolved to be a wary prey-species. Their stalk-like necks loft their heads above vegetation and their widely-set eyes take in almost 310 degrees of vision.

Depth-of-field:

Image: A deer eye, showing characteristic horizontal pupil. Photo by Gabilla Temesvári, usage under Creative Commons.

Like many ungulates, the pupils of deer are horizontal slits. In addition to reducing glare from above on sunny days, horizontal pupils do not block peripheral vision when constricted (a problem with round pupils). Thus, even on sunny days with a fully constricted iris, a deer can see with perfect peripheral vision across the horizon. 

Advantages of a Horizontal Pupil

Recent theoretical studies have uncovered another advantage to horizontal pupils (Held et al., 2012). Due to the pupil orientation, the focal plane is not parallel to the retina as it is in humans and other animals with round pupils. Instead, the focal plane is more horizontal, like a tilt-shift lens. This means that deer have better depth perception across the plane of the ground--a handy advantage for sensing predators like big cats or wolves approaching across a field!

Image: Figure 3 from Held et al. 2003: “Slit Pupils and Astigmatic Depth of Field,” (A) The retinal image generated by an eye with a vertical-slit pupil. The vertical and horizontal dimensions of the pupil used for rendering are 5.5 and 1.1 mm, respectively. The eye is focused on the left-most cross at a distance of 20 cm. The other crosses are positioned at distances of 40 and 60 cm. The horizontal limbs of the more distant crosses are more blurred than the vertical limbs.
(B) The retinal image generated by a natural scene. The pupil has the same aspect ratio as in (A) but has been magnified by a factor of 10 to make the blur more noticeable in this small image. The eye is focused at a distance of 200 cm. Note that the horizontal contours of distant objects are more blurred than the vertical contours. The 3D model was designed by Guillermo M. Leal Llaguno of Evolucion and rendered using the Physically Based Rendering Toolkit (PBRT).

Focal field:

Unlike human and avian fovea, the fovea in deer is elongated horizontally and is far less dense than humans (D’Angelo et al., 2008). This is a trade-off between narrow, high acuity vision and wide, low-acuity vision.

Spectral sensitivity:

Have you ever considered how funny in looks when deer hunters don layer after layer of detailed camouflage clothing, but wear a bright orange cap? To trichromats like us, the contrast is appalling. Deer, on the other hand, are dichromats. They can perceive only low frequency light in green to red range and high frequency light in the blue and ultraviolet range, but colors we perceive as yellows and oranges are indistinguishable to deer.

References:

Held, R. T., Cooper, E. A., and Banks, M. S. (2012). Blur and disparity are complementary cues to depth. Curr. Biol. 22, 426–431.

D’Angelo, G. J., Glasser, A., Wendt, M., Williams, G. A., Osborn, D. A., Gallagher, G. R., et al. (2008). Visual specialization of an herbivore prey species, the white-tailed deer. Can. J. Zool. 86, 735–743.