The feet of a Common Emu look clumsy and dinosaur-like, but the Australian bird’s color vision is significantly more advanced than that of humans.
By Rex Graham
The order of flightless birds that includes the Emu diverged 110-120 million years ago from the families of birds that fly today. Even though Australia’s largest bird lacks a keel bone on its sternum to anchor wing muscles, it shares superior color vision and the ability to see ultraviolet light with modern birds.
Songbirds, parrots and other modern birds have 4 types of photoreceptors in their retinal cone cells that give them color vision. The cones are augmented by a sophisticated package of vision tuning and filtering accessories to enhance the entire package of color vision. Now, a study by Australian scientists in Proceedings B of The Royal Society reported that Emus share the same visual system.
Color score: birds 4 – humans 3
Humans have only 3 types of photoreceptors for color vision, and none of the spectral-tuning and filtering accessories that give birds superior color vision. While humans can distinguish shades of red, green and blue, birds can distinguish many more hues, plus ultraviolet or violet light.
“The Emu retina contains a similar photoreceptor complement to almost every other bird species studied to date,” researchers with Macquarie University and the University of Western Australia said in the paper in Proceedings B of The Royal Society.
The ultraviolet-sensitive (UVS) photoreceptor that is unique to birds (and reptiles and many fish) is the focus of intense research.
Scientists can’t realistically imagine the richer visual reality of birds and how it may compare to that of humans. Ornithologists had assumed since the time of Darwin that humans and birds share similar visual experiences.
That changed 20 years ago when scientists discovered that birds see UV light. Other researchers soon made the parallel discovery that many birds use UV-reflecting feathers and ornaments to signal hostility, romantic interest or other messages. The newly discovered “hidden communication channel” of birds and their surprising mental abilities is reshaping many ornithological theories of avian behavior and evolution.
Emus see the colors humans see plus UV light, but they don’t have UV-reflecting feathers. However, they and Ostriches, rheas, tinamous and cassowaries – members of the Struthioniformes order –use their sophisticated visual system to optimize their search for the most nutritious fruits, seeds, flowers, insects and foliage to eat. “Many of these food items have high UV reflectance or characteristic UV reflectance patterns that may be used by emus (as with other birds) to detect or discriminate them against other objects,” the Australian researchers said.
Most modern mammals, which evolved from a common, burrow-dwelling nocturnal ancestor rely on rods in their eyes to see in black and white in low-light situations. Many primates and other mammals later evolved color vision based on 3 types of photoreceptors: red, green and blue. However, primates will never see ultraviolet light because the ocular media inside their eyes blocks UV light from reaching the back of the eye, or retina.
Birds evolved from dinosaurs that slept at night. Their 4 color-sensitive photoreceptors are adaptations to see the world in full daylight. Each avian photoreceptors absorbs narrow wavelength-bands of light evenly spaced along the rainbow of visible colors. In addition, each cone cell in the avian eye has a tiny intracellular droplet of oil. Light passing through each droplet is filtered before it reaches the photoreceptor in such a way that a bird sees more shades of green, blue and red.
Birds’ UV exploitation
Similarly, birds have acquired adaptations to enhance their detection of either violet or ultraviolet light. For example, waterfowl, pheasants and falcons have violet-sensitive photoreceptors. Songbirds and parrots have UV-sensitive photoreceptors, and some gulls, auks, herons, egrets and other wading birds have both.
“Modeling suggests that these adaptations have served to enhance spectral sensitivity and color discrimination as birds have expanded their visual systems into the UV range,” a team of 15 researchers reported in the July 12, 2016 issue of the journal eLife.
Emus’ UV vision may be the ancestral avian form. “These flightless birds might not have been subjected to selection pressures of sufficient magnitude to drive a shift in the spectral range of their visual system, as appears to have occurred in birds that have subsequently evolved a violet-sensitive pigment,” the Australian scientists said.
Matthew Toomey, a research scientist at Washington University in St. Louis, Mo., and lead author of the eLife study, said scientists are eager to unravel more details of avian vision. Other exciting areas of scientific research include the neurological system of birds that processes visual signals and elicits behaviors.
Taste for Emu
Unfortunately for Emus, they are on the menu. They are regarded as larger, red-meat versions of chickens. Emus are raised on farms in Australia, Canada, Europe, U.S., and Japan, and slaughtered for human consumption.
The collagen extracted from Emu skin has increased in commercial value in recent years as a pathogen-free, non-allergenic ingredient in cosmetics, wound dressings, pharmaceuticals and other products.
The collagen extracted from the hide or skin of cows, pigs and domestic fowl is sometimes tainted with bovine spongiform encephalopathy (mad cow disease), food-and-mouth disease, transmissible spongiform encephalopathy, or avian influenza. Commercial suppliers of biodegradable collagen have turned to the Emu because it poses no risk of disease transmission and its collagen lacks the unhealthy antigenic components found in cow and pig collagen. Emu collagen also faces no religious barriers to human use.
“A large quantity of Emu skins as by-products have potential as a good alternative source of high-quality collagen for industrial purposes in the foods, cosmetics, and pharmaceutical and biomedical fields,” Japanese researchers said in a paper in the Journal of Food Science Technology.