Eye Structure Enhances Night Vision

Biological Strategy

Vertebrates (Mammals, Fish, Birds, Reptiles)

AskNature Team

Image: Akin / Unsplash / Free non-commercial use

Modify Light/Color

Color in living systems comes from pigments and the interaction of light with surfaces. Color serves many purposes for living systems, such as attracting prey or mates, providing warnings, or protecting through camouflage. To create the effects needed for each purpose, living systems must control the expression or visibility of pigments and the interactions (such as reflectance and refraction) of light. To do so, they have strategies that modify color or light to increase or decrease the color’s position, intensity, opacity, and more. Male hummingbirds, for example, have brightly colored feather patches on their throats; the coloring comes from pigments, structures that refract light, or a combination of the two. When a male hummingbird hovers near a potential mate, it modifies the angle of these feathers to create a bright, colorful display. However, when it needs to mute the colors to reduce conspicuousness, such as to avoid a predator, it modifies the angle again.

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Sense Light (Visible Spectrum) From the Environment

Living systems constantly receive signals from their environment that help them survive. Light (in the visible spectrum) can come from other living systems (such as fireflies) or from non-living sources (such as the sun). Survival often depends on sensing and responding to challenges like low light conditions or light that has been altered in some way. Because basic survival is at stake, living systems must excel at meeting those challenges. A well-known phenomenon is how water bends light. A stork trying to catch a fish underwater can compensate for this bending effect so that when it strikes at the fish, it has a good chance of catching it.

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Vertebrates (Mammals, Fish, Birds, Reptiles)

Subphylum Vertebrata (“jointed”): Mammals, fish, birds, reptiles

Vertebrates are a subgroup of chordates, which all have a flexible, rod-shaped structure that supports the body called a notochord. Non-vertebrate chordates include tunicates, hagfish, and lancelets. In vertebrates the notochord ultimately becomes part of the spine, usually encased in bony joints. All chordates also have a dorsal hollow nerve cord that forms the nervous system and pharyngeal slits that open outside the body during development (and persist to form gills in aquatic animals). Lastly, chordates have a tail at the back of the body––it’s just that sometimes you need an x-ray to see it.

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The tapetum lucidum of many vertebrates enhances night vision by reflecting light back to photoreceptors in the eye.

“The tapetum lucidum is a biologic reflector system that is a common
feature in the eyes of vertebrates. It normally functions to provide
the light-sensitive retinal cells with a second opportunity for
photon-photoreceptor stimulation, thereby enhancing visual sensitivity
at low light levels…

“Some species (primates,
squirrels, birds, red kangaroo and pig) do not have this structure and
they usually are animals. In vertebrates, the tapetum lucidum
exhibits diverse structure, organization and composition. Therefore,
the retinal tapetum (teleosts, crocodilians, marsupials, fruit bat),
the choroidal guanine tapetum (elasmobranchs), the choroidal tapetum
cellulosum (carnivores, rodents, cetacea), and the choroidal tapetum
fibrosum (cow, sheep, goat, horse) are described…

“The tapetum lucidum
represents a remarkable example of neural cell and tissue
specialization as an to a dim light environment and, despite
these differences, all tapetal variants act to increase retinal
sensitivity by reflecting light back through the photoreceptor layer.
These variations regarding both its location and structure, as well as
the choice of reflective material, may represent selective visual
adaptations associated with their feeding behavior, in response to the
use of specific wavelengths and amount of reflectance required.” (Ollivier et al. 2004:11)

Last Updated October 26, 2016

References

“The tapetum lucidum is a biologic reflector system that is a common feature in the eyes of vertebrates. It normally functions to provide the light-sensitive retinal cells with a second opportunity for photon-photoreceptor stimulation, thereby enhancing visual sensitivity at low light levels…

“Some species (primates, squirrels, birds, red kangaroo and pig) do not have this structure and they usually are diurnal animals. In vertebrates, the tapetum lucidum exhibits diverse structure, organization and composition. Therefore, the retinal tapetum (teleosts, crocodilians, marsupials, fruit bat), the choroidal guanine tapetum (elasmobranchs), the choroidal tapetum cellulosum (carnivores, rodents, cetacea), and the choroidal tapetum fibrosum (cow, sheep, goat, horse) are described…

“The tapetum lucidum represents a remarkable example of neural cell and tissue specialization as an adaptation to a dim light environment and, despite these differences, all tapetal variants act to increase retinal sensitivity by reflecting light back through the photoreceptor layer. These variations regarding both its location and structure, as well as the choice of reflective material, may represent selective visual adaptations associated with their feeding behavior, in response to the use of specific wavelengths and amount of reflectance required.” (Ollivier et al. 2004:11)

Journal article