A “knot” on a dog can refer to three very different things: the reproductive structure that swells during mating, a lump or bump under the skin, or a tangle of matted fur. Which one applies depends on context, so here’s a clear breakdown of each.
The Mating Knot (Bulbus Glandis)
If you’ve seen two dogs stuck together during mating, the knot is the reason. The bulbus glandis is a bulb-shaped area of erectile tissue on a male dog’s penis. During mating, this tissue swells significantly once inside the female, creating a locking mechanism that holds the two dogs together. This “tie” keeps them connected during and after ejaculation, increasing the chances of successful fertilization. The gland also produces fluids that lubricate the urethra and neutralize acidity from residual urine.
A copulatory tie typically lasts anywhere from five minutes to an hour. The dogs will separate naturally once the swelling goes down. If you see two dogs locked together, do not try to pull them apart, as this can cause serious injury to both animals.
Lumps and Bumps Under the Skin
Many dog owners discover a mysterious knot while petting their dog and immediately worry. The good news is that most lumps in dogs are benign. The not-so-good news is that you can’t tell benign from dangerous just by feel.
Fatty Tumors (Lipomas)
Lipomas are among the most common lumps in dogs. They feel soft, squishy, and typically move freely under the skin when you press on them. They show up most often on the trunk and near the tops of the legs, and they’re especially common in older, overweight female dogs. Lipomas are benign and usually don’t need treatment unless they grow large enough to interfere with movement. Some lipomas, called infiltrative lipomas, sit deeper in the fat layer and can blend into surrounding tissue, making their edges harder to define.
Skin Cysts
Cysts form when hair follicles malfunction and fill with keratin, a protein the skin naturally produces. They feel firmer than lipomas, sometimes with a hard or solid core. If a cyst ruptures, the material inside is typically gray, brown, or yellowish and has a grainy, cheese-like texture. Never squeeze a cyst to try to drain it. This pushes the contents into surrounding tissue, and your dog’s immune system will react to it as a foreign substance, triggering severe inflammation.
Insect Bites and Allergic Reactions
A knot that appears suddenly, within minutes or hours, is often a hive or localized allergic reaction. Insect bites, bee stings, medications, and even shampoos can trigger these raised, red, swollen patches. They tend to show up on the back, flanks, neck, eyelids, and legs. These bumps are usually itchy and often disappear as quickly as they arrived. If the swelling spreads rapidly or your dog has trouble breathing, that’s anaphylaxis, which is an emergency.
Lumps That Need Attention
Some lumps are cancerous, and the tricky part is that they don’t always look scary. Mast cell tumors, one of the most common skin cancers in dogs, are sometimes called “the great pretender” because they can look like anything: a small, harmless-seeming bump, a rapidly growing mass, or a red and ulcerated sore. There’s no way to distinguish a mast cell tumor from a lipoma just by touching it. A simple needle test at the vet’s office can identify the cell type in most cases.
As a general rule, any lump that is growing or hasn’t resolved on its own within a few weeks warrants a vet visit.
How to Monitor Your Dog’s Skin
A monthly nose-to-tail skin check is the best way to catch new lumps early. Find a well-lit area, and if your dog is small enough, put them on a counter or your lap so you can see clearly. Start at the nose and work your way back to the tail, paying special attention to spots that are easy to miss: between skin folds around the face and lips, inside the ears, between the toes, in the armpits, under the belly, behind the back legs, and under the tail. Part the fur and look at the skin itself, not just the coat surface.
Keep a simple notebook where you record any lumps you find. Note the location, size, and how the lump feels. Counting and measuring bumps is especially useful if your dog has several, so you can track whether any are changing over time. This log gives your vet concrete information to work with rather than a vague “I think it’s bigger.”
Matted Fur Knots
The other kind of knot is a tangle of matted hair. Mats form when loose fur wraps around itself and tightens, creating clumps that pull against the skin. This is more than a cosmetic problem. Mats tug on the skin with every movement, causing redness, itching, and painful sores. The warm, moist environment trapped beneath a mat becomes a breeding ground for bacterial and fungal infections. Parasites and hidden wounds can lurk under severe matting without the owner ever seeing them. In extreme cases, mats restrict a dog’s ability to walk or run, and the trapped heat can contribute to overheating, which is dangerous in warm weather.
Preventing and Removing Mats
Dogs with long coats should be brushed two to three times per week using a slicker brush and metal comb. Breeds with thick, shedding undercoats benefit from a grooming rake. The key is getting the brush all the way down to the skin, not just skimming the surface of the coat. Regular brushing removes loose hair before it has a chance to tangle.
If mats have already formed, clippers are the safest removal tool. Scissors are risky because mats sit so close to the skin that it’s easy to cut your dog, especially if they move unexpectedly. Work on dry fur, since wetting matted hair only tightens the tangles. Check the clipper blades frequently with your hand to make sure they haven’t gotten hot enough to burn the skin, and disinfect the blades after use. De-matting sprays (leave-in conditioners) can help loosen very small or shallow tangles, but they won’t work on dense mats. After clipping out the mats, a full shampoo, condition, and thorough comb-through helps prevent them from returning.