Eileen Koval, CDBC, CBCC-KA, CPDT-KA
Mounting behavior is one of the most misunderstood behaviors that nearly all owners witness their dogs perform at some point during their lives. The reasons for mounting can vary widely, regardless of the age, breed, sex, or spay/neuter/intact status of the dog. I find that owners tend to be less surprised when this behavior happens with young, growing puppies prior to spaying/neutering, but they may become concerned if their spayed/neutered dog is displaying this behavior. Most of the time, the behavior is not sexual, including when it involves intact dogs. Even so, the times when it can be sexual are often not what most owners might suspect. Neutered dogs may not have a wild libido due to decreased testosterone but they can (and do) have sex! Intact males and neutered males (who may smell more like females) may enjoy sex together (Warnes 2014). Conversely, neutered males may get annoyed at sexual attention from intact males. Neutered males may be attracted to and have sex with bitches in heat, including becoming "locked" with a swelled bulbis gland. Still, mounting behavior in dogs is rarely about sex, and even more rarely about dominance.
So, why do dogs mount each other? The causes are varied and may include hormonal, medical, behavioral, or even possibly environmental issues. It can be difficult to discern which one it is at times without careful consideration.
As far as hormonal goes...
The sex pheromone in dogs has not been identified and semiochemical reproductive communication is not yet understood (Dzięcioł 2014). We do know that intact males and some neutered males are highly aroused by intact females during estrus (or "in heat"). Males can always be interested in mating, whereas females are only interested seasonally when in estrus. Young puppies experience a surge of sex hormones, with male puppies experiencing the first peak around 5 months of age. By seven months of age, they may start displaying interest in intact females in heat (Dodman 2020). Males have significantly reduced testosterone after surgical neutering, but that does not always stop sexual behavior. A study in 2004 found that 27.3% of male and female dogs continued to display sexual behavior following surgical spaying or neutering (Spain 2004). Spayed females may attract males or even display sexual behavior themselves if part of their ovaries were left behind during the surgery. Females do not have to be nearby to attract the attention of males. Someone in your neighborhood could have a female in heat that could result in your male dog crying, whining, or displaying mounting behavior toward other dogs or objects in your home.
Males may show sexual interest and mounting of spayed females with vaginitis, urinary tract infections (UTIs), and urinary incontinence. All of these issues are common with spayed females. Vaginitis and UTIs are common with females spayed at a young age, as they are predisposed to having recessed vulvas. The skin folds trap moisture in a dark area between the warm skin folds, making it an ideal breeding ground for bacteria (Khuly). Dogs of either sex with anal gland infections may give off a scent that attracts intact and some neutered males and lead to mounting behavior (Warnes 2014). Additionally, skin allergies, bug bites or anything else causing itchiness may lead to mounting behavior (UC Davis). We have personally witnessed this when we had chiggers in the grass outside where the dogs often play or lie down to relax. The first indication that there were chiggers was that our two male dogs -- one intact, one neutered -- began taking turns mounting each other every day with penises in full display and ejaculation. After a few days, we noticed bumps on their skin under all of the fluffy fur. We treated the grass to get rid of the chiggers, and the behavior went away once the bumps and itchiness on their skin were gone. The only times we had ever noticed them having sex was when they were stressed about an environmental change.
Now let's discuss behavioral causes...
Mounting behavior is observed during play in puppies as young as five weeks old, long before puppies are able or interested in sex (Dodman 2020). Play behavior involves role playing and simulation of sex and fighting, even long before dogs reach physical maturity. Sexual role playing is part of normal, healthy play, although it would be wise to redirect a puppy if the recipient of the mounting does not appreciate it. Dogs also hump objects such as pillows, stuffed toys, and dog beds to masturbate. This is normal behavior, particularly during the hormonal surges that puppies experience.
Mounting can also be a displacement behavior -- a normal behavior that is displayed out of context -- which appear during times of anxiety or stress as a way of deflecting those emotions and soothing themselves. A change in the pet parent's work hours, a new addition to the family/household, separation issues, a move to a new home, less exercise or playtime, frustration or punishment during training, and other stressors can cause displacement behavior such as mounting. Overly excited dogs may also mount, as is seen sometimes with visitors to the home and at the dog park. The recipient of the mounting is frequently not appreciative of the attention! Repeated instances of mounting behavior in a particular place/time/context is a heads up that training or other changes may be needed.
On to environmental causes...
What has been known for decades is that intact males become highly aroused when methyl p-hydroxybenzoate (methyl paraben) is applied to the vaginas of spayed female dogs (Goodwin 1979). While not a pheromone, methyl paraben can be present in some female dogs' urine. It is also in some shampoos! As a result, your dog -- male or female -- may attract sexual attention if bathed in a shampoo with methyl parabens (Warnes 2014). Make sure to check the ingredients list on your dog's shampoo.
What about dominance?
Dominance is behavior to gain and maintain priority access to all resources. Given that pet parents control all the resources, it is highly unlikely that your dog is attempting to dominate you. Also, there have not been any documented instances of cross-species dominance behavior. Dominance behavior toward other dogs can occur, but is rare. It involves the other dog demonstrating submission in any and all circumstances. Most likely, the mounting behavior you are witnessing can be attributed to one of the other causes described above.