Cone Photoreceptors Allow for Specialized Vision

Biological Strategy

European starling

AskNature Team

Image: Alan D. Wilson /

Transduce/Convert Signals

Sometimes a signal, such as sound or vibration, reaches a living system and needs to be converted into a different, usable form. This requires converting from one type of energy form to another. For example, organisms with eyes have photoreceptors at the back of the eyes that convert light into electrical pulses. These pulses travel to the brain and allows it to register color, shape, and detail.

Search this Function

Sense Shape and Pattern in a Living System

Living systems must identify other living systems and objects to navigate, feed, escape predators, find resources, and more. The ability of living systems to “see” varies widely, with “seeing” including not only sight, but other means of sensing shapes or patterns, such as smell or echolocation. For example, a hawk can see much more detail than humans while other organisms can see far less detail. Nevertheless, each living system is capable of detecting shapes and patterns to the extent it needs to for survival. In addition to perceiving physical shapes or patterns, living systems can also sense landscape patterns at various scales. The purple sea urchin, for example, does not have eyes, yet can pick out fine details in its environment by using its entire surface as a compound eye. The urchin’s spines shield light coming from wide angles, further refining its focus.

Search this Function

Sense Light (Non-visible Spectrum) From the Environment

Living systems interact with each other and with their environments to gain information. Sometimes that information is in the electromagnetic spectrum. Wavelengths in the electromagnetic spectrum are also called the non-visible spectrum, because humans can’t detect them with the naked eye. These include ultraviolet (UV) light, infrared (IR) light, radio waves, and other wavelengths. Detecting within these spectra requires strategies beyond those used for visible light, so many living systems that depend on these signals have specialized organs to do so. For example, beetles that feed on burned trees have sensory organs that detect infrared radiation emitted by fires, enabling them to quickly locate a burned area.

Search this Function

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.

Search this Function

Birds

Class Aves (“bird”): Eagles, hawks, sparrows, parrots

Birds are evolutionary engineering marvels. They are descended from dinosaurs, but are far from our idea of heavy, scaly reptiles. Of the specific adaptions that set them apart, most notable is flight—although some mammals can fly, birds take the prize for abundance in the skies. Many birds have hollow, lightweight skeletons and specially-designed wings to help them stay aloft. They also have feathers made of keratin that help them stay warm, attract mates, and improve navigation and aerodynamics in flight. In contrast to their dinosaur ancestors, they lack true teeth and have replaced them with specialized beaks and bills.

Search this Living System

The starling is a bird, meaning it is active during the day. Like many animals, the starling contains photoreceptors in the back of its eyes that allow it to see color, shape, and detail. Yet unlike most animals, the starling has a distinct composition of photoreceptor types that make it capable of seeing heightened levels of detail, and UV light.

Photoreceptors are cells in the back of the eye that convert ambient light into electrical impulses. The electrical impulses then travel to the brain where color, shape, and detail are registered. There are two types of photoreceptors: rods and cones. Rods register general forms, and are optimized in darkness. They are more common in nocturnal species. Cones register color vision and fine detail, and are utilized in bright environments. They are more common in diurnal species.

In the starling, there are significantly more cones than rods making the starling better at seeing fine details. More significant, however, is that the types (or classes) of cones vary between the left and the right eye. This makes one eye more specialized for seeing the general features, and the other eye better seeing fine details.

There are two types of cones: single and double. Double cones are larger in size, and and absorb a wider range of wavelengths (light, in this case) than single cones. Single cones are divided into four classes that correspond to the maximum wavelength that can be absorbed. These are long wave sensitive (LWS), medium wave sensitive (MWS), short wave sensitive (SWS), and ultraviolet sensitive (UVS) cones.

Double cones are good at distinguishing between general shapes, but not good at distinguishing between individual colors and details. Single cones, on the other hand, absorb a smaller range of wavelengths but can distinguish more detail within the wavelengths they do absorb. For example, a double cone is like a box of markers with four colors (red, yellow, green, blue). You can draw an image, but there will not be a lot of detail. Now, if each color were in its own box of four varying hues (i.e., the red box contains maroon, burgundy, red, red-orange) the detail of the image would be far greater.

What this creates is a specialized system of visualization. The right eye of the starling contains a greater amount of rods and double cones. These will be utilized for distinguishing movement, shape, and general color. The left eye contains more single cones that will be used to distinguish between complex hues, details, and UV frequencies.

This summary was contributed by Allie Miller.

Image: Alan D. Wilson /

European Starling, Cabin Lake Viewing Blinds, Deschutes National Forest, Near Fort Rock, Oregon

Image: Alan D. Wilson /

European Starling , Desert Botanical Gardens, Phoenix, Arizona

Last Updated August 23, 2016

References

Book

The Hidden Powers of Animals: Uncovering the Secrets of Nature

26/02/2021 | Dr. Karl P. N. Shuker

embedly preview

Reference

Journal article

AskNature