My article last week about dogs in the rain landed on shop shelves at the same time as a burst of warm dry sunny weather: Ireland enjoyed a week of classic pre-Leaving Cert sunshine. For the first time this year, people had to start using sun tan lotion, and my local sea front was populated by sunbathers. As I walked my dog along the promenade, somebody shouted out a question at me: "Do dogs get sun tans?"
The short answer is yes, dogs can get sun tans, but they're usually difficult to see.
Now for the long answer. Pets' skin has a very similar structure to human skin. There are cells in the skin that are sensitive to sunlight: they're called melanocytes. These cells produce melanin, which is a type of dark pigment: the process is called melanogenesis. Pigment production happens all the time, even without sunlight: it's this basal level of melanogenesis which gives rise to the different colours of human skin. Lighter skinned individuals have low basal levels of melanogenesis, whereas darker skinned people have higher basal levels, even without any exposure to sunshine.
If the skin is exposed to UV-B radiation (such as sunlight), this activates the melanocytes to produce increased levels of melanin. The accumulation of many tiny pinpoints of melanin is the reason for the brown appearance of the skin which we all know as a "sun tan". The purpose of the increased melanin production is to stop the UV-B radiation from penetrating through the skin layer. The colour of the melanin is dark, allowing it to absorb most of the UV-B light and blocking it from passing through this skin layer into the deeper tissues of the body. UV-B light can damage sensitive cells beneath the melanocyte layer of skin, causing sun burn and potentially leading to cancer. A sun tan is nature's way of protecting the body from damage by too much sunshine.
Sun tans are often seen in humans because we lack fur, so our bare skin is easily exposed to sunlight (causing melanin production). And if we do get a sun tan, the results are clearly visible. If part of our skin is covered up (e.g. by watch straps), then so-called tan lines can be seen: the area of skin that was protected from sunshine remains paler, and there's a marked contrast with the surrounding brown, tanned skin.
What about dogs and cats? Their skin is very similar to our own, and they also have melanocytes which produce base levels of pigment all the time, as well as increased levels of pigment when exposed to sunlight. The colour of a pet's coat is related to the amount of melanin produced by the melanocytes in the area: dark coloured animals have higher levels of pigment than light coloured animals. Pets with patchy coats (like Dalmatians) have areas of skin with high levels of melanin production (the black spots) and areas with low levels ( the white areas).
The melanocytes of pets are activated to produce increased levels of pigment when exposed to sunlight, just like in humans. However there is one reason why sun tans aren't often seen in pets: their fur.
An animal's fur provides a physical barrier which protects the skin from sunlight. It's like a human wearing a long sleeved shirt to protect their skin from sunburn. For this reason, most of a pet's body is rarely exposed to full sunshine, so their melanocytes are not activated, and their skin does not get tanned. There are three parts of the body which are often not covered in fur, and the effect of sunshine is commonly seen in these areas.
First and second, the tips of the ears and the tip of the nose are often hairless, or covered in fine fur. In white animals, there is no natural pigment in these areas to protect against sunlight, and the deeper layers of the skin are exposed to dangerous UV-B radiation. As a result, it's common to see white dogs and cats with sun burn of the tips of the ears and the tip of the nose. In cats especially, this can progress to skin cancer which requires radical surgery to save the animal's life: it's common for white cats to need amputation of the ear tips and less commonly, removal of the tip of the nose due to skin cancer. Regular application of sun block to the ears and nose of white pets is the best way to protect the skin in these areas.
The third area of the body that's less hairy is the underside of the animal: the fur here is often thin and short. Many animals enjoy lying on their back in the sun, and so it's common for their undersides to be exposed to high levels of sunshine. I've seen some white dogs develop sunburn on their bellies and again, regular sunblock in sunny weather can be helpful to prevent this.
Interestingly, this is the also only area that I have seen pets developing a sun tan. An owner was worried because their dog had developed a strange-looking white line across the underside of the chest. Could this be an unusual skin condition that needed treatment? When the vet checked the dog, the owner had to remove the harness to show them the white mark, and it was suddenly obvious that the white line was in exactly the same place as the harness. It turned out that the dog liked lying on his back on the sun: the white band was just a "harness-line": the doggy equivalent of a watch-strap or bikini-strap line.