![]() This is unusual because most other organisms have heterochromatin lining the inner membrane of the nucleus. Each rod acts as a lens by packing its heterochromatin in the center of the nucleus as opposed to on the periphery. In addition to nocturnal mammals having more rods than cones in comparison to non-nocturnal mammals the rods themselves are different. In these conditions rods become saturated with light bleaching rhodopsin and cones are relied on for image transduction. Essentially, having enough cones to produce an image during the day. This ability is conferred by having enough cones for a secondary image forming system. Some animals that can function during the night are also capable of 24 hour activity. ![]() Thus, in nocturnal animals visual acuity is sacrificed for being able to see well at night. However, cones allow for greater visual acuity. Cones need higher intensity photons to be excited and thus are rendered essentially useless in low light conditions. However, in nocturnal animals rods predominate and in low light conditions rhodopsin is regenerated fast enough for the animal to effectively use these rods. In these animals the rhodopsin in the rods breaks down or is “bleached” faster than it can be regenerated making the rods useless and vision is primarily sensed by the less sensitive cones. In animals that are functional during the day many have both rods and cones. The rhodopsin in rods is very sensitive to light, which enables it to sense low levels of light. There are very few cones compared to the amount of rods present in the nocturnal mammals retina. The retina of nocturnal mammals have many more rods than cones. Rods are sensitive to low light conditions. These muscles are much more efficient at closing in either direction as opposed to radially. The efficiency is due to the way that the cilary muscles bunch as they contract. In animals with slit pupils the pupil opening and closing is controlled by two cilary muscles. However, this limits the diameter in which the pupil can dilate to and is not an efficient system for opening and closing the pupil aperture quickly. In species with round pupils the pupil opening and closing is controlled by circular cilary muscles. Īdditionally, slit pupils are much more efficient at opening and closing. This allows nocturnal mammals to function during the day if necessary. Thus, slit pupils allow for use of the entire diameter of the eye and each zone of differential focus even when high intensity light is encountered. If the animal had circular pupils then if the animal experienced intense light during the day when the pupil constricts it would cut out many of the zones of differential focus creating a very blurry image. Due to multifocal lenses and the nocturnal mammals eye being highly sensitive to bright light circular pupils would not work. ![]() These concentric zones focus a specific spectral range onto the retina allowing for better resolution of the image. However, many animals have adapted by having multifocal lenses with concentric zones of differential focal lengths. ![]() This chromatic defocus can cause blurry images. This chromatic defocus is due to light hitting a medium and differing wavelengths travelling at different speeds causing chromatic defocus or chromatic abberation. This is due to the pupil being so large in comparison to the focal length. ĭue to these very large pupils nocturnal mammals have a small depth of field. Light can further be reduced by having a vertical slit and partially closing the eyelids. The way that nocturnal or diurnal mammals accommodate such a wide range of light intensities is through having a slit pupil. ĭue to this increased sensitivity the animal needs to protect its eyes from the high intensity light that it could possibly experience in the day. The way that nocturnal mammals adjust for this is by increase “quanta” or light capture by having large pupils and short focal lengths. Additionally, at lower light levels it is harder to distinguish edges and borders and thus shapes because there is a decrease in contrast in low light conditions. Thus, the large pupil helps to receive as much light as possible in low light conditions. Having a larger eye and larger pupil allows a nocturnal mammal to receive more incoming light thus gather more photons due to the larger surface area exposed. Nocturnal mammals tend to have larger eyes as well as larger pupils that can open widely in low light. ![]()
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