Blog #400 It’s All in the Eyes—Predator or Prey


Eyes of a domestic cat (Pinterest) and Osprey (Osprey Tales), lion (istock) and 

goat (Pinterest), human (B.Martell), and gecko (Fineart America).


THE PREY


Big-horned sheep and white-tailed deer, courtesy of flicker.



Grazing prey animals have horizontally elongated pupils and laterally placed eyes. Horizontally slit pupils have an expanded field of view because more light is let in from the front, back, and sides. The orientation also helps limit the amount of dazzling light from the sun so the animal can see the ground better. The animal can rotate so that its pupils are aligned with the ground whether its head is upright or looking down. Grazing animals’ eyes can rotate by 50 degrees or more in each eye, a range 10 times greater than human eyes. Horizontally elongated pupils create sharp images of horizontal contours ahead and behind, creating a horizontally panoramic view essential for detecting predators from various directions. Once a predator is detected, the animal needs to see where they are running and jumping on uneven terrain while keeping a look out for the predator from the corner of its eyes. 


The optimal pupil shape for terrestrial prey is horizontally elongated. Such a pupil improves image quality for horizontal contours in front of and behind the animal and thereby helps solve the fundamental problem of guiding rapid locomotion in a forward direction despite lateral eye placement. It also facilitates a horizontally panoramic view for detecting predators approaching along the ground. 


THE PREDATORS


Female Osprey eating a fish, courtesy of Karel and Cindy Sedlacek.


Ambush predators with front-facing eyes and vertically elongated pupils create astigmatic depth of field such that images of vertical contours nearer or farther than the distance to which the eye is focused are sharp, whereas images of horizontal contours at different distances are blurred. Vertical slit-shaped pupils allow for more musculature in the eye and a greater range in the amount of light entering the eye than round pupils. For example, the vertical slits of geckos and domestic cats go through a 135- and 300-fold change in the area between constricted and dilated states, while human circular pupils undergo only a 15-fold change.


However, vertical pupils are not ubiquitous among ambush predators. Vertical pupils are associated with predators that are close to the ground—small animals with shoulder heights less than 16.5 inches—such as domestic cats. Big cats like tigers and lions have round pupils similar to humans and dogs. Vertical pupils appear to maximize the ability of small animals to judge the distances of prey. Depth-of-field cues based upon blur are more effective for estimating distances for short animals than tall ones.

Most diurnal predators, like Ospreys and all birds, have circular pupils. The only birds known to have a slit pupil (and it is vertically elongated) are black skimmers. The primary foraging method for the black skimmer is to fly close to the water surface with its lower beak in the water, snapping shut when it contacts prey. The black skimmer is crepuscular (active primarily during the twilight period) or nocturnal. This niche is visually somewhat similar to the ones encountered by short terrestrial predators that tend to have vertical-slit pupils. 


Evolution and Niches


Transitions in pupil shape have occurred multiple times within and between evolutionary lineages and are typically associated with specific ecological niches. Circular pupils are correlated with diurnal activity and active foraging, vertically elongated pupils are associated with nocturnal activity and ambush foraging, and horizontal pupils indicate prey. The number of times pupil shape has changed in these families implies that the shape of the eye’s aperture has evolved in response to the environment, and not because of emergence in a few common ancestors. 


While this study looked only at terrestrial species, the authors plan to look at aquatic species in the future. That might get complicated since at least one side-eyed sea creature—the octopus—is both predator and prey. 


Eyes to the sky!

Candace


Candace E. Cornell 

Friends of Salt Point, Lansing, NY

Cayuga Lake Osprey Network

  



EYES TO THE SKY!


WATCH!

Salt Point Osprey Nest Cam


READ!

On Osprey Time: Ospreys of the Finger Lakes


VISIT!

Cayuga Lake Osprey Trail



HELP PROTECT OSPREYS:

  • Eggs will be hatching soon. Stay 300 feet away from nesting sites during the breeding season. If the Osprey vocalizes, you are too close! Back Off Immediately.

  • Carry binoculars to view wildlife from afar.

  • Help keep local waters clean, healthy, and safe.

  • Recycle used fishing lines, twine, and nets, which can kill Osprey.

  • Join the Cayuga Osprey Network and volunteer to help monitor Osprey nests: cec222@gmail.com.



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