Wednesday, July 15, 2009

Mor Kwan's Elephant Clinic: "The Elephant's Eye"

Vision of Elephant

The natural elephant habitat composes abundant significant components as follows: the biodiversity as the variability among living organisms from all sources, the ecosystems and the complexity of physical structures in the environment such as plants and geological features. Therefore, the elephants need to have several living factors to support it for survival in their habitat variety. As an illustration, an elephant's legs are great straight pillars, as they must be to support its bulk. Social learning also is important for the acquisition of behavior in elephants, for instance, young elephants learn what to eat by reaching up and sampling what is in their mother’s mouths. Nevertheless, it is interesting why elephants can move and do the activities greatly in the night. For this reason, there are numerous doubts about it which elephant’s sight may be important to avail for life.

The ability of sight of an elephant is not clear to understand. Some researches mention that the elephant has a poor vision limited to 20-25 meters but some proposes that its vision is quite well.

The elephant eye is comparatively small in relation to body size. The general eye structure is similar other mammals except absence of a tear apparatus in elephants. The eyes of the elephant are located on the sides of the head between the trunk and the auricular orifice; therefore, the elephant has monocular side vision and binocular vision ahead. Lateral placement of the eyes is a physical attribute essential for elephants to the survival to give the widest possible field of view as it allows them to increase side or peripheral vision. Side vision is a sensitive detector for motion or movement. Peripheral vision allows animals to efficiently scan for hazard. The visual field for an elephant, the binocular field is 67 degree creating depth perceptions and the uniocular field is 123 degree. Thus, the monocular field for each eye including binocular overlap is 190 degree. A blind spot of 47 degree at backside involves the circumscribed head and neck movement by the large head, the shortness and slight flexibility of the neck and laterally eyes set. In short, the elephant can see a total field of view of 313 degree with a small blind spot just in front of the trunk. Moreover, the elephant has long eyelashes and lots of folds in their eyelids, which hinder their vision.

According to the animal’s living, many ungulates, large carnivores, and elephants can be classified into arrhythmic diurnal-nocturnal species. The elephant’s activities are quite well in dim moonlight suggests the high adaptation of their eyes at low illumination levels. The elephant typically contain abundant rod photoreceptor cells in its retina that are specifically created for high visual acuity during the night. Furthermore, Tapetum lucidum fibrosum, a prominence reflective structure, enhances the elephant’s vision in shades by reflecting light back onto the retina and increasing sensitivity to available light. To compensate for increasing brightness in daytime, the cone cell receptors are activated. Many ungulates and carnivores have similar rod/cone ratios of 85–99% but the proportion of rods and cones in elephants is unknown. Regulation the amount of light reaching the retina is necessary when the light level changes. The elephant has not only the regulation of rods and cone cells function, but also the regulation by changing the size of pupil.

The perception of color vision in elephants described to study the visual pigments possess is currently inconvincible. Also, there is absent a behavioral measurement on their vision. However, the capabilities of the visual pigments in elephants may infer by comparing with other arrhythmic species. Most of ungulates have two spectrally distinct cone types, a short wavelength sensitive cone (439-456 nm) and a middle to long wavelength sensitive cone (531-551 nm). Human with trichromatic color vision see not only four primary colors (blue, green, yellow, and red), but also various intermediate colors between them. Instead of sight four primary colors, however, color-blind person detect only two primary colors (blue and yellow) and do not see intermediate color. Hence, when the two primary colors are mixed, the color-blind individuals detect either achromatic, i.e., white or gray, or one of the two basic colors. Therefore, it is highly likely that elephants have the same sets of visual pigments as color deficient people, giving them the dichromatic color vision of deuteranopes during the daytime. An early research in an Asian elephant presents that a little mistake with green and blue blocks was found more than with red, white, or yellow blocks. Nevertheless, no identification and sensitivity document of color receptor cells in the retina has been recorded in elephants.

The visual streak in an elephant resembles in other mammals except the one unique of an elephant which is a second retinal ganglion cell-rich area identified a pattern of the superior temporal retina. Thus, it is assumed that there is an adaptation to monitor the movement and appearance of the trunk. However, no evidence of an area centralis, or cone-rich area was found in the same investigation.

Although the visual acuity of an elephant is unknown in the true optical ability, documentation of visual acuity has been based more on ophthalmic anatomy than ocular reaction and sensation. The elephant acuity is less than domestic cats, but greater than domestic rabbits. What’s more, the development and progressive growth of the eye as the elephant matures may influence on elevating the greater visual acuity of the adult elephant.
The vision is not only significant sense for elephant’s effective survival but its hearing, smell, and touch also are excellent. Nonetheless, researching an additional knowledge about elephant eyes is necessary since an absence of an absolute optical ability is reported.
Hanggi, E. B., Ingersoll, G. F. and Waggoner, T. L. 2007. Color Vision in Horses( Equusc aballus): Deficiencies Identified Using a Pseudoisochromatic Plate Test. Joumal of Comparative Psychology. 121(1):65-72.
Kern, T. J., Murphy, C.J. and Howland, H.C. 1993. Physiological Optics and Ocular Anatomy of the Asian Elephant (Elephas maximus). Proc. Am. Assoc. Zoo Vet, St. Louis, Missouri. 406.
Nissani, M., Hoefler-Nissani, D., Lay, U. T. and Htun, U. W. 2005. Simultaneous Visual Discrimination in Asian Elephants. Journal of the Experimental Analysis of Behavior. 83: 15-29.
Suedmeyer, W. K. 2006. Special Senses. In: Fowler, M.E. and Mikota, S.K. (1st ed.). Biology, Medicine, and Surgery of Elephants. 399-407.
Yokoyama, S., Takenaka, N., Agnew, D. W. and Shoshani, J. 2005. Elephants and Human Color-Blind Deuteranopes Have Identical Sets of Visual Pigments. The Genetics Society of America. 170:335-344.

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