Why Do Dogs Sniff Each Other's Rear Ends?

Dogs sniff each other's' rear ends for a very good reason, there is a lot of information hiding under a dog's tail! In order to better understand a dog's "butt sniffing behavior," it helps to take a closer look into what hides under the hood, and what type of information dogs may gather.

Let's face it: dogs sniff each other's rear ends and dog owners may get upset about it. Dogs are often pulled away with a quick leash tug or they are blamed for being "rude." However, how would you feel if your dog's butt sniffing behavior was compared to exchanging business cards with a new acquaintance? Most likely the behavior would be more acceptable!

Well, sniffing each other's rear ends in dogs is normal canine etiquette and many compare it to giving a handshake and exchanging pleasantries. A little lesson in anatomy and a closer insight into what really happens when dogs sniff each others' butts can provide some elucidations on the behavior.

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The dog's vomeronasal organ is responsible for detecting pheromones

A Matter of Pheromones 

The term pheromone was coined by Karlson and Luscher. Pheromones are special airborne chemical substances that are secreted externally in the dog's urine, feces and several glands. These pheromones are excreted and meant to cause specific reactions in the receiving dog.

To detect pheromones, dogs are equipped with a special organ that is known as the vomeronasal organ (VNO). There are actually two VNOs which are located on each side of the nasal septum. Each VNO is estimated to measure around 4 centimeters in length (Wada et al., 1991) and they are innervated by three nerves. 

In dairy animals and horses, suction of pheromones by the VNO occurs when they raise their upper lip (flehem). In dogs, suction of pheromones often occurs by teeth chattering and moving the tongue (tonguing) against the incisive papilla, a special duct that is found right between the dog's front teeth.

When dogs chatter their teeth, and perhaps even foam at the mouth, they are therefore sending large scent molecules to their incisive papilla so they can reach the Jacobson organ and then the dog's brain center (the amygdala to be precise), where they can trigger emotional or physiological responses.

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Dogs release pheromones from several glands under their tails

What's Under a Dog's Tail?

Dogs release pheromones from several places. In particular, under the tail, dogs are equipped with several pheromone-releasing glands.

So exactly what glands are found under a dog's tail? They consist of the supracaudal glands, the circumanal glands, and the anal glands. Let's take a closer look into each of them. 

The supracaudal glands, also known as violet glands, may be undeveloped in some dogs, but they are significantly more developed in male cats. These glands are not really found under the tail, but are rather on the top surface of the dog's tail approximately 5 to 40 mm below the base. These glands are more developed in male dogs. 

The circumanal glands (surrounding the anus) consist of sebaceous and modified sweat glands found around the dog's anus. These glands are more developed in male dogs and their size increases with age. These glands release secretions that play an important role in the social life of dogs. In female dogs in heat, these glands produce trimethylamine-rich secretions which are known to attract male dogs. 

The anal glands are two sacs that consist of modified sweat glands and sebaceous glands. These sacs are surrounded by muscular fibers which allow dogs the expulsion of secretions during urine marking and alarm marking as it happens during fear-induced reactions. In female dogs, secretions of these sacs during heat are highly attractive to male dogs.

When dogs wag their tails, they spread around pheromones. With those wide, sweeping motions, dogs are therefore spreading their personal information with others, sort of like we do when we hand out our business cards.

 At a closer insight, here's what more precisely happens: every time the tail is wagged, the muscles around the dog's rectum contract and end up pressing on the anal glands triggering the release of volatile molecules associated with pheromones. 

According to research by Natynczuk, Bradshaw and McDonald, the anal gland compounds excreted by dogs are different between one dog and another and these secretions also differ on a day-to-day basis. They provide information about a dog's age, sex, health, and possibly, also diet.

Did you know? Anal sac infections in dogs may evoke aggressive behavior in dogs, even among dogs sharing the household, and therefore, it's important that veterinary behaviorists have a dog's anal sacs examined to rule out an anal sac infection, explains Patrick Pageat in a paper.

Extra Glands in Female Dogs

Female dogs have several sebaceous glands under their tail and they are part of the genital area. These glands are categorized as "the genital complex."

Not surprisingly, in the female dog, this complex is very explored during each social contact as secretions from this area participate in both social and sexual behaviors. 

During a female dog's heat cycle, secretion of methyl dihydroxybenzoate is highly attractive to male dogs. Interestingly, these compounds are nowadays used as a preservative in many cosmetic products for humans.

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<em>"Head-on encounters on leash may lead to "incomprehension" among dogs</em>

Now That You Know...

As seen, dogs find the area under the tail quite interesting to investigate and therefore sniffing each other's rear ends is normal. So if your dog is interested in sniffing other dogs' butts, don't yank the leash to correct your dog and expect him to meet new doggy friends "head on." 

Head-on encounters among leashed dogs puts dogs face-to-face promoting direct eye contact and unnatural postures that may be perceived as threatening. and may evoke aggressive responses. 

"Direct head-on approaches can be threatening to dogs, especially shy ones meeting a person or dog whom they don't know."~Patricia McConnell, Ph.D. The Other End of the Leash: Why We Do What We Do Around Dogs

References:

Anim Reprod Sci. 2001 Mar 30;65(3-4):157-70. The role of pheromones and biostimulation in animal reproduction. Rekwot PI(1), Ogwu D, Oyedipe EO, Sekoni  

Pageat P. La communication dans l’univers des carnivores domestiques. Point Ve´t 1997;28:1055–63  

Current research in canine and feline pheromones Patrick Pageat, DVM, PhD*, Emmanuel Gaultier, DVM, MA  Vet Clin Small Anim 33 (2003) 187–211 

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