Blind animals
Encyclopedia
Visual perception
plays an important role in the animal kingdom, most notably for the identification of food sources, and avoidance of predators. 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.
The Star-nosed Mole
can detect, catch and eat food faster than the human eye can follow (under 300 milliseconds).
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
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.
. (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
Statements that certain species of mammal
s are "born blind" refers to them being born with their eyes closed and their eyelids fused together; the eyes open later. One example is the rabbit
. In humans the eyelids are fused for a while before birth, but open again before the normal birth time, but very premature babies are sometimes born with their eyes fused shut, and opening later. Other animals such as the blind mole rat
are truly blind and rely on other senses.
Mammals other than primates generally have less effective two-receptor color perception systems, allowing only dichromat
ic color vision; marine mammal
s have only a single cone type and are thus monochromat
s. 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.
There is evidence that ultraviolet
light plays a part in color perception in many branches of the animal kingdom
, (especially insect
s) though there has not been enough evidence to show this. It has been suggested that it is likely that pigeons are pentachromat
s. Papilio butterflies apparently have tetrachromatic color vision despite possessing six photoreceptor types.
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.
cites moles
as an example of mammals that have organs that have become vestigial and are being phased out by natural selection
:
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
.
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 lens
es from the eyes of sighted surface-form embryo
s 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 pupil
s, cornea
s, and iris
es. (It is not clear whether they possessed sight, however.)
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.
(see-Evolution of color vision, Evolution of color vision in primates
)
in old dogs.
, tells the story of a boy who blinds six horses. Theodore Taylor
's classic young adult novel, The Trouble With Tuck, is about a teenage girl, Helen, who trains her blind dog to follow and trust a seeing-eye dog. In non-fiction, a recent classic is Linda Kay Hardie's essay, "Lessons Learned from a Blind Cat," in Cat Women: Female Writers on their Feline Friends.
Visual perception
Visual perception is the ability to interpret information and surroundings from the effects of visible light reaching the eye. The resulting perception is also known as eyesight, sight, or vision...
plays an important role in the animal kingdom, most notably for the identification of food sources, and avoidance of predators. 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
Visual acuity
Visual acuity is acuteness or clearness of vision, which is dependent on the sharpness of the retinal focus within the eye and the sensitivity of the interpretative faculty of the brain....
is a continuum, with humans falling somewhere in the center.
Totally blind species
- Mole (animal)Mole (animal)Moles are small cylindrical mammals adapted to a subterranean lifestyle. They have velvety fur; tiny or invisible ears and eyes; and short, powerful limbs with large paws oriented for digging. The term is especially and most properly used for the true moles, those of the Talpidae family in the...
The Star-nosed Mole
Star-nosed mole
The star-nosed mole is a small mole found in wet low areas of eastern Canada and the north-eastern United States, with records extending along the Atlantic coast as far as extreme southeastern Georgia...
can detect, catch and eat food faster than the human eye can follow (under 300 milliseconds).
Eurycea neotenes
Eurycea neotenes is a species of entirely aquatic, lungless salamander native to the United States. It is endemic to central Texas, near Helotes, in Bexar County.- Description :...
, blind flatworms, eyeless shrimp, eyeless fish, cave beetles, cave crayfish
Crayfish
Crayfish, crawfish, or crawdads – members of the superfamilies Astacoidea and Parastacoidea – are freshwater crustaceans resembling small lobsters, to which they are related...
, 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 batBat
Bats are mammals of the order Chiroptera "hand" and pteron "wing") whose forelimbs form webbed wings, making them the only mammals naturally capable of true and sustained flight. By contrast, other mammals said to fly, such as flying squirrels, gliding possums, and colugos, glide rather than fly,...
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.
Infant blindness
Blindness at birth serves to preserve the young who are dependent on their parentsAltricial
Altricial, meaning "requiring nourishment", refers to a pattern of growth and development in organisms which are incapable of moving around on their own soon after hatching or being born...
. (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
Infant vision
The science of infant vision gives a verifiable basis for some practices of pediatric ophthalmology and gathers measurements intended to describe, monitor and predict:* development of retinal photoreceptor cells...
Statements that certain species of mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s are "born blind" refers to them being born with their eyes closed and their eyelids fused together; the eyes open later. One example is the rabbit
Rabbit
Rabbits are small mammals in the family Leporidae of the order Lagomorpha, found in several parts of the world...
. In humans the eyelids are fused for a while before birth, but open again before the normal birth time, but very premature babies are sometimes born with their eyes fused shut, and opening later. Other animals such as the blind mole rat
Blind mole rat
The genus Spalax contains the blind, fossorial, or subterranean mole rats, which are one of several types of rodents that are called mole rats. The hystricognath mole rats of the family Bathyergidae are completely unrelated, but some other forms are also in the family Spalacidae...
are truly blind and rely on other senses.
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)].Mammals other than primates generally have less effective two-receptor color perception systems, allowing only dichromat
Dichromat
Dichromacy is the state of having two types of functioning color receptors, called cone cells, in the eyes. Organisms with dichromacy are called dichromats. Dichromats can match any color they see with a mixture of no more than two pure spectral lights...
ic color vision; marine mammal
Marine mammal
Marine mammals, which include seals, whales, dolphins, and walruses, form a diverse group of 128 species that rely on the ocean for their existence. They do not represent a distinct biological grouping, but rather are unified by their reliance on the marine environment for feeding. The level of...
s have only a single cone type and are thus monochromat
Monochromat
Monochromacy, also known as "total color blindness", is a complete inability to distinguish colors. This is distinguished from more common forms of color blindness, in which the affected individual can perceive color differences, but cannot make the same distinctions between colors as can an...
s. 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
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
and birds, have more complex color vision systems than humans.
There is evidence that ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
light plays a part in color perception in many branches of the animal kingdom
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
, (especially insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
s) though there has not been enough evidence to show this. It has been suggested that it is likely that pigeons are pentachromat
Pentachromat
Pentachromacy describes the capability and capacity for capturing, transmitting, processing, and perceiving five independent channels of color information through the primary visual system. Organisms with pentachromacy are termed pentachromats...
s. Papilio butterflies apparently have tetrachromatic color vision despite possessing six photoreceptor types.
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.
Natural selection
DarwinCharles Darwin
Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...
cites moles
Mole (animal)
Moles are small cylindrical mammals adapted to a subterranean lifestyle. They have velvety fur; tiny or invisible ears and eyes; and short, powerful limbs with large paws oriented for digging. The term is especially and most properly used for the true moles, those of the Talpidae family in the...
as an example of mammals that have organs that have become vestigial and are being phased out by natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
:
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 SpeciesThe Origin of SpeciesCharles Darwin's On the Origin of Species, published on 24 November 1859, is a work of scientific literature which is considered to be the foundation of evolutionary biology. Its full title was On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the...
)
Research
The blind forms of the Mexican tetraMexican tetra
The Mexican tetra or Blind Cave Fish is a freshwater fishof the characin family oforder Characiformes....
have proven popular subjects for scientists studying evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
:
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
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
.
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
Unified neutral theory of biodiversity
The unified neutral theory of biodiversity and biogeography is a hypothesis and the title of a monograph by ecologist Stephen Hubbell...
.
In one experiment studying eye development, University of Maryland
University of Maryland, College Park
The University of Maryland, College Park is a top-ranked public research university located in the city of College Park in Prince George's County, Maryland, just outside Washington, D.C...
scientists transplanted lens
Lens (anatomy)
The crystalline lens is a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. The lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a...
es from the eyes of sighted surface-form embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
s 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 pupil
Pupil
The pupil is a hole located in the center of the iris of the eye that allows light to enter the retina. It appears black because most of the light entering the pupil is absorbed by the tissues inside the eye. In humans the pupil is round, but other species, such as some cats, have slit pupils. In...
s, cornea
Cornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...
s, and iris
Iris (anatomy)
The iris is a thin, circular structure in the eye, responsible for controlling the diameter and size of the pupils and thus the amount of light reaching the retina. "Eye color" is the color of the iris, which can be green, blue, or brown. In some cases it can be hazel , grey, violet, or even pink...
es. (It is not clear whether they possessed sight, however.)
The evolution of trichromatic color vision in primates
Evolution of color vision in primates
The evolution of color vision in primates is unique compared to most eutherian mammals. A remote vertebrate ancestor of primates possessed tetrachromacy, but nocturnal, warm-blooded, mammalian ancestors lost two of four cones in the retina at the time of dinosaurs...
occurred as the ancestors of modern monkeys, apes, and humans switched to diurnal
Diurnal animal
Diurnality is a plant or animal behavior characterized by activity during the day and sleeping at night.-In animals:Animals that are not diurnal might be nocturnal or crepuscular . Many animal species are diurnal, including many mammals, insects, reptiles and birds...
(daytime) activity and began consuming fruits and leaves from flowering plants.
(see-Evolution of color vision, Evolution of color vision in primates
Evolution of color vision in primates
The evolution of color vision in primates is unique compared to most eutherian mammals. A remote vertebrate ancestor of primates possessed tetrachromacy, but nocturnal, warm-blooded, mammalian ancestors lost two of four cones in the retina at the time of dinosaurs...
)
Injury, disease and disability
Blindness often afflicts pets, especially glaucomaGlaucoma
Glaucoma is an eye disorder in which the optic nerve suffers damage, permanently damaging vision in the affected eye and progressing to complete blindness if untreated. It is often, but not always, associated with increased pressure of the fluid in the eye...
in old dogs.
In fiction
The theme of blind animals has been a powerful one in literature. Peter Schaffer's Tony-Award winning play, EquusEquus (play)
Equus is a play by Peter Shaffer written in 1973, telling the story of a psychiatrist who attempts to treat a young man who has a pathological religious fascination with horses....
, tells the story of a boy who blinds six horses. Theodore Taylor
Theodore Taylor (author)
Theodore Taylor was an American author of more than 50 fiction and non-fiction books for young adult readers, including The Cay, The Weirdo , Ice Drift, Timothy of the Cay, The Bomb, Sniper, and Rogue...
's classic young adult novel, The Trouble With Tuck, is about a teenage girl, Helen, who trains her blind dog to follow and trust a seeing-eye dog. In non-fiction, a recent classic is Linda Kay Hardie's essay, "Lessons Learned from a Blind Cat," in Cat Women: Female Writers on their Feline Friends.