There are a number of reasons why dogs may develop dark or discolored rings around their nipples.
• Pregnancy
If a female dog has developed dark rings around her nipples, then it could be an early indicator of pregnancy. Melanin can build up in tissue under a dog's skin during pregnancy, and while this is more common in the later stages of pregnancy, it can occur at any time. This build up of melanin is completely safe, and in fact can occur in pregnant women. The discoloration should disappear when the pregnancy is over and the dog has given birth.
• 'False pregnancy'
If a female dog did not get pregnant during her season or heat cycle, then she can experience a false pregnancy. This is because her body expected to become pregnant, and was well-prepared to do so. As a result, the dog may experience a number of changes in behavior and appearance as if she were pregnant, and discoloration of the nipples or dark rings can be a sign of this.
• Changes in skin pigment
As dogs get older, like humans, their skin undergoes a number of changes. And, like in humans, one of these changes is in the pigmentation or coloring of the skin. As the skin around the nipples is slightly thinner and more sensitive, changes in pigment may be more obvious in this area than in other areas on the dog's body. Changes in pigment are usually nothing to worry about and are often a natural part of the aging process.
• Bruises
If a dog has been in a fight, has fallen from high up or had aggressive or high-pressure contact with another object, then the dog may show bruising, especially in delicate areas such as around the nipples.
• Pregnancy
If a female dog has developed dark rings around her nipples, then it could be an early indicator of pregnancy. Melanin can build up in tissue under a dog's skin during pregnancy, and while this is more common in the later stages of pregnancy, it can occur at any time. This build up of melanin is completely safe, and in fact can occur in pregnant women. The discoloration should disappear when the pregnancy is over and the dog has given birth.
• 'False pregnancy'
If a female dog did not get pregnant during her season or heat cycle, then she can experience a false pregnancy. This is because her body expected to become pregnant, and was well-prepared to do so. As a result, the dog may experience a number of changes in behavior and appearance as if she were pregnant, and discoloration of the nipples or dark rings can be a sign of this.
• Changes in skin pigment
As dogs get older, like humans, their skin undergoes a number of changes. And, like in humans, one of these changes is in the pigmentation or coloring of the skin. As the skin around the nipples is slightly thinner and more sensitive, changes in pigment may be more obvious in this area than in other areas on the dog's body. Changes in pigment are usually nothing to worry about and are often a natural part of the aging process.
• Bruises
If a dog has been in a fight, has fallen from high up or had aggressive or high-pressure contact with another object, then the dog may show bruising, especially in delicate areas such as around the nipples.