Blind animals

expand|date=September 2008Visual perception plays a huge role in the animal kingdom, most notably for the identification of food sources, and avoidance of prey. For this reason, blindness in animals is a unique topic of study.

In general, nocturnal or subterranean animals have less interest in the visual world, and depend on other sensory modalities. Visual capacity is a continuum, with humans falling somewhere in the center.

Totally blind species

*Mole (animal)The Star-nosed Mole can detect, catch and eat food faster than the human eye can follow (under 300 milliseconds). [ [http://www.eurekalert.org/pub_releases/2005-02/vu-mmg013105.php Marsh-dwelling mole gives new meaning to the term 'fast food' ] ]

Blind animals include the blind cave fish and cave crickets, the Texas salamander, blind flatworms, eyeless shrimp, eyeless fish, cave beetles, cave crayfish, and some bristletails, isopods and copepods.

Some animals live only in caves - they are called troglobites (meaning 'cave dwellers'). These animals are adapted to life in the dark. "See- A List of Troglobites"

Partial blindness

Although the eyes of most bat species are small and poorly developed, leading to poor visual acuity, it is incorrect to assume that they are nearly blind. Vision is used as an aid in navigation especially at long distances, beyond the range of echolocation.

Colour blindness

Humans and primates are unique as they possess trichromatic color vision, and are able to discern between violet [short wave (SW)] , green [medium wave (MW)] , and yellow-green [long wave (LW)] [Dulai, K. S., and M. von Dornum. 1999. The evolution of trichromatic color vision by opsin gene duplication of new world and Old World primates. Genome Research. 9: 629-638.] .Mammals other than primates generally have less effective two-receptor color perception systems, allowing only dichromatic color vision; marine mammals have only a single cone type and are thus monochromats. Honey- and bumblebees have trichromatic color vision, which is insensitive to red but sensitive in ultraviolet to a color called "bee purple".

Other animals, such as tropical fish and birds, have more complex color vision systems than humans. [Kelber, A., Osorio, D., Vorobyev, M. (2003) [http://www.blackwell-synergy.com/doi/abs/10.1017/S1464793102005985 "Animal colour vision--behavioural tests and physiological concepts."] Biol Rev Camb Philos Soc. 2003 Feb; 78(1):81-118. ] There is evidence that ultraviolet light plays a part in color perception in many branches of the animal kingdom, (especially insects) though there has not been enough evidence to show this. [Timothy H. Goldsmith. "What Birds See", Scientific American, July 2006, Vol. 295, Issue 1.] It has been suggested that it is likely that pigeons are pentachromats. "Papilio" butterflies apparently have tetrachromatic color vision despite possessing six photoreceptor types. [Arikawa, K. (2003) [http://www.springerlink.com/content/whjepqnhpulyeevk/ "Spectral organization of the eye of a butterfly, Papilio"] . J. Comp. Phys. A 189, 791-800.] The most complex color vision system in animal kingdom has been found in stomatopods with up to 12 different spectral receptor types which are thought to work as multiple dichromatic units. [Cronin T.W., Marshall, N.J. (1989) [http://www.nature.com/nature/journal/v339/n6220/abs/339137a0.html "A retina with at least ten spectral types of photoreceptors in a mantis shrimp"] Nature 339, 137 - 140.]

Birth blindness

Blindness at birth serves to preserve the young who are dependent on their parents. (If they could see, they could wander off.) Rabbits are born with eyes and ears closed, totally helpless. Humans have very poor vision at birth as well. See: Infant vision

Natural selection

Darwin cites moles as an example of mammals that have organs that have become vestigial and are being phased out by natural selection:

The eyes of moles and of some burrowing rodents are rudimentary in size, and in some cases are quite covered by skin and fur. This state of the eyes is probably due to gradual reduction from disuse, but aided perhaps by natural selection. In South America, a burrowing rodent, the tuco-tuco, or Ctenomys, is even more subterranean in its habits than the mole; and I was assured by a Spaniard, who had often caught them, that they were frequently blind. One which I kept alive was certainly in this condition, the cause, as appeared on dissection, having been inflammation of the nictitating membrane. As frequent inflammation of the eyes must be injurious to any animal, and as eyes are certainly not necessary to animals having subterranean habits, a reduction in their size, with the adhesion of the eyelids and growth of fur over them, might in such case be an advantage; and if so, natural selection would aid the effects of disuse. (Charles Darwin, The Origin of Species [Charles Darwin, The Origin of Species, ] )

Research

The blind forms of the Mexican tetra have proven popular subjects for scientists studying evolution:A recent study suggests that there are at least two distinct genetic lineages among the blind populations, arguing that these represent a case of convergent evolution.cite journal
author=Dowling, T. E., D. P. Martasian, and W. R. Jeffrey
title=Evidence for Multiple Genetic Forms with Similar Eyeless Phenotypes in the Blind Cavefish, "Astyanax mexicanus"
journal=Molecular Biology and Evolution
volume=19
year=2002
pages=446–55
url=http://mbe.oxfordjournals.org/cgi/content/full/19/4/446
pmid=11919286]

One theory is that because of its dark habitat, the fish embryo saves energy it would normally use to develop eyes to develop other body parts, and this developmental choice would eventually dominate the population. This is called economical adaptation.However, studies have shown that blind cave fish embryos begin to grow eyes during development but then something actively stops this process and flesh grows over the partially grown eyes.Another theory is that some Mexican tetra randomly don't develop eyes (which is represented by broken genes in the fish's genome), and this lack of eyes spreads to the rest of the population despite having no advantage or disadvantage. This is called the Unified neutral theory of biodiversity.

In one experiment studying eye development, University of Maryland scientists transplanted lenses from the eyes of sighted surface-form embryos into blind cave-form embryos, and vice versa. In the cave form, lens development begins within the first 24 hours of embryonic development, but quickly aborts, the lens cells dying; most of the rest of the eye structures never develop. Researchers found that the lens seemed to control the development of the rest of the eye, as the surface-form tetras which received cave-form lenses failed to develop eyes, while cave-form tetras which received surface-form lenses grew eyes with pupils, corneas, and irises. (It is not clear whether they possessed sight, however.)cite journal
journal=Science
title=Central Role for the Lens in Cave Fish Eye Degeneration
author=Yamamoto, Yoshiyuki, and William R. Jeffrey
year=2000
volume=289
issue=28 July
pages=631–3

doi = 10.1126/science.289.5479.631 ] cite journal
journal=Science
title=Embryonic Lens Prompts Eye Development
author=Pennisi, Elizabeth
year=2000
volume=289
issue=28 July
pages=522–3
doi=10.1126/science.289.5479.522b
pmid=10939956]

The evolution of trichromatic color vision in primates occurred as the ancestors of modern monkeys, apes, and humans switched to diurnal (daytime) activity and began consuming fruits and leaves from flowering plants. [Steven Pinker. "How the Mind Works", 1997. p. 191. ISBN 0-393-04535-8.] ("see-Evolution of color vision, Evolution of color vision in primates")

Injury, disease and disability

Blindness often afflicts pets, especially glaucomain old dogs. [http://www.blinddogs.com/ http://www.blinddogs.com/] is a support community.

References