Introduction
The phenomenon of eyeshine, also called tapetum lucidum, is what causes some dogs’ eyes to glow or reflect light in low light conditions. The color of this eye reflection is primarily determined by two key factors – the breed of the dog, and the genetics related to coat color. While some breeds tend to exhibit certain eye colors more than others, and color genetics play a role, there are additional elements that can impact the tapetum lucidum as well.
In this article, we will explore the anatomy behind eyeshine, examine the genetics and breeds associated with different colors, look at health implications, and summarize what’s known about why some dogs have glowing green eyes while others glow red.
Anatomy of the Canine Eye
The canine eye has several key anatomical structures and layers that enable dogs to see [1]. The outer layer, called the cornea, is transparent and allows light to enter the eye. Behind the cornea is the anterior chamber, filled with a watery fluid called the aqueous humor. Next is the iris, the colored part of the eye, which controls the amount of light entering through the pupil. The lens sits behind the pupil and focuses light onto the retina at the back of the eye.
The retina contains rods and cones, specialized cells that convert light into signals to the brain, allowing dogs to see. One unique structure dogs have is the tapetum lucidum, a layer of reflective tissue behind the retina [2]. The tapetum lucidum bounces light back through the retina, giving rods and cones a second chance to detect light and enhancing dogs’ ability to see in low light conditions.
Tapetum Lucidum
The tapetum lucidum is a reflective layer of tissue located behind the retina in the eyes of many vertebrates including dogs, cats, and other mammals [1]. It consists of stacks of overlapping cell layers containing crystals of guanine or riboflavin that reflect visible light back through the retina, enhancing vision in low light conditions [2].
The tapetum lucidum functions like a mirror, causing light that passes through the retina to be reflected back onto the retinal photoreceptors for a second chance at absorption. This “double passage” of light maximizes the amount of light detected, allowing animals with a tapetum lucidum to see much better than humans in darkness [3].
The tapetum lucidum layer gives canine eyes their distinctive shine or “eye shine” when light shines into them at night. It allows dogs and other animals to effectively see objects and terrain at night that would otherwise be too dark to detect.
Tapetal Color Variation
The tapetum lucidum is a reflective layer in the eye that causes the eye shine effect in dogs and other animals. The tapetum is located behind the retina and reflects light back through the retina to enhance vision in low light conditions [1]. The color of the eye shine depends on the composition of the tapetum.
The tapetum contains crystals called purines that are responsible for the color of the reflected light. The specific purines present determine if the tapetum reflects red or green light. Tapetums with high levels of riboflavin reflect blue-green light, while tapetums with more zinc cysteine reflect red light [2].
Genetics play a major role in determining the tapetal color. For example, Siberian Huskies lack a tapetum entirely and therefore have a red eye shine. In contrast, Akitas and Malamutes have high riboflavin levels in their tapetums and exhibit a greenish eye shine [3].
Breed Differences
Certain dog breeds are more likely to exhibit green or red eye shine due to differences in their tapetum lucidum.[1] Some examples:
- Breeds prone to green eye shine:
Siberian Husky, Australian Shepherd, Border Collie, Doberman Pinscher - Breeds prone to red eye shine:
Labrador Retriever, Golden Retriever, Cocker Spaniel, Beagle
In general, breeds with blue, green, or hazel eyes tend to have green tapetums resulting in green eye shine.[2] Breeds with brown eyes usually have red tapetums and red eye shine.
However, there can be variations within breeds. Some Siberian Huskies may have a red tapetum while others have a green tapetum, causing differences in eye color even within the same litter.[1]
Genetics
The presence or absence of the tapetum lucidum in dogs is determined by genetics. According to a study published in Veterinary Ophthalmology, an autosomal recessive gene controls the development of the tapetum lucidum in dogs reference. Dogs that are homozygous recessive (ll) will lack a tapetum, while dogs that have at least one dominant allele (Ll or LL) will have a normal tapetum.
Certain breeds, such as Labrador Retrievers and Beagles, are known to have a high prevalence of hereditary tapetum defects. Breeding experiments in Beagles have confirmed that the absence of the tapetum is inherited in an autosomal recessive manner reference. The specific genetics behind tapetal color variation in dogs with normal tapetum development remains unclear.
Overall, genetics plays a major role in determining not only the presence of the tapetum lucidum, but also aspects of its structure and function. Selective breeding in purebred dogs has likely influenced tapetal properties over generations in certain breeds.
Health Implications
The color of a dog’s eye shine does not directly indicate any health issues. However, changes in eye color or eye shine could potentially signify an underlying condition.
For example, a dog’s eyes normally have a bright, vibrant shine. If the eyes appear dull or lackluster, it could point to corneal ulcers, cataracts, glaucoma, or other issues (source). Some eye diseases like progressive retinal atrophy can also cause a dog’s eye color to fade or change hue.
Additionally, certain eye conditions may be more prevalent in breeds with light eye colors like blue eyes. For instance, albino dogs, which lack pigment, are prone to visual deficits. However, the eye shine color itself does not directly correlate with health.
Overall, while eye shine color does not indicate disease, paying attention to the eyes and any changes can help identify potential health issues that require veterinary attention.
Environmental Factors
Certain environmental and dietary factors can impact the properties of a dog’s tapetum lucidum, which in turn affects eye shine color. For example, vitamin A deficiency has been associated with retinal degeneration and degradation of the tapetum lucidum in dogs (Tapetum lucidum – an overview). Providing adequate levels of vitamin A through diet is important for maintaining a healthy tapetum.
Exposure to toxins like lead and arsenic has also been linked to tapetal degeneration in dogs. Avoiding environments with heavy metal contamination can help preserve normal tapetum lucidum function (Spatial relationships among the cellular tapetum, visual pigments, and rod photoreceptors in the canine retina).
Some medications like chloroquine and hydroxychloroquine are associated with ocular toxicity and changes to the tapetum lucidum. Consulting with a veterinarian before administering medications that could impact eye health is advised.
Overall, providing dogs with a nutritious diet rich in vitamins, avoiding exposure to heavy metal toxins, and using caution with certain ocular medications can help maintain a healthy tapetum lucidum and normal eye shine colors.
Conclusion
In summary, the tapetum lucidum is a unique reflective layer behind the retina in canine eyes that causes eyeshine. The color of eyeshine is determined by several key factors, including:
– Breed genetics – Some breeds have a higher prevalence of green/yellow eyeshine while others are more likely to exhibit red/orange eyeshine. Retrieved from: https://www.eyecareforanimals.com/conditions/eyeshine-veterinary/
– Age – Puppy eyes tend to show blue eyeshine that shifts to another color like green or orange as the dog matures.
– Health and environment – Illness, eye issues, and nutrition can impact tapetum lucidum pigmentation and therefore the observed eye color.
While subtle variations exist, these key genetic, developmental, and health factors interact to produce the distinct eyeshine colors seen in dogs. Proper eye care and veterinary monitoring can identify any issues impacting eye health. With this understanding of the tapetum lucidum, dog owners can better appreciate their pet’s unique glowing eyes.
References
American Kennel Club. (2019). A guide to dog eye anatomy, physiology and health. https://www.akc.org/expert-advice/health/dog-eye-anatomy-function/
Fitzpatrick, L. (2015). Dog eyes: Anatomy and function of the canine eye. Veterinary Partner. https://veterinarypartner.vin.com/default.aspx?pid=19239&id=4951460
Scoot, H. (2021). Why do some dogs’ eyes glow green and some glow red? American Kennel Club. https://www.akc.org/expert-advice/health/why-do-dogs-eyes-glow-in-the-dark/
VCA Hospitals. (2020). The genetics of eye color in dogs. https://vcahospitals.com/know-your-pet/the-genetics-of-eye-color-in-dogs
Walls, G. L. (1942). The vertebrate eye and its adaptive radiation. New York: Hafner.
