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Glands found near the anus in many mammals

The anal glands or anal sacs are small glands near the anus in many mammals. They are situated in between the external anal sphincter muscle and internal anal sphincter muscle. Sebaceous glands within the lining secrete a liquid that is used for identification of members within a species. These sacs are found in many species of carnivorans, including wolves, bears, sea otters and kinkajous.

The anal glands are situated within the wall of the anal canal and communicate with the lumen of the canal via ducts that open at the anal valves, just proximal to the pectinate line. Humans have 12 anal glands on average (which are evenly distributed around the circumference of the anal canal).

In humans, the glands secrete mucin (which differs in composition from that secreted by the rectal mucosa). Their function is unclear.

Dogs and cats primarily use their anal gland secretions to mark their territory, and generally will secrete small amounts of fluid every time they defecate. Many will often express these glands when anxious or frightened as well. Dogs who are healthy can usually have a wide variety in the appearance of their sac's content. Discomfort may also be evident with impaction or infection of the anal glands. Anal sac impaction results from blockage of the duct leading from the gland to the opening. The sac is usually non painful and swollen. Anal sac infection results in pain, swelling, and sometimes abscessation and fever. Initial treatment usually involves the manual expression of the anal sacs, most often by a veterinary professional. The frequency of this procedure depends on the patient's individual degree of discomfort but can range from weekly to every few months. Increasing dietary fiber is also commonly recommended.

Opossums use their anal glands when they "play possum". As the opossum mimics death, the glands secrete a foul smelling liquid, suggesting the opossum is rotting. Opossums are not members of the order Carnivora, and their anal sacs differ from those of dogs and their relatives. Castoreum is the chemical compound that is secreted from the castor sacs and is originally a thin, yellowish liquid. It is composed of a mixture of varied metabolic compounds, from the sacs and other bodily systems, and excreted urine. Variations in color of anal glands range from a light straw color to brown. Compared to the castor sacs, anal glands secrete a much more sharp odor. The frequency of scent mounding is typically highest during the spring and after winter when the ice has melted. This is prevalent as the water sources are more available from which beavers can gather mud. Anal gland secretions (AGS) of badgers are not commonly studied but contribute to key information for communication due to volatile organic compounds (VOCs). The subcaudal gland contributes to individual specific communication, much like the AGS. Badgers have the reputation of being aggressive towards one another and cubs are often victims to infanticide. The anal sac remains covered in sebum and remains closed when the tail of the mongoose is down but opens when the tail is raised. The organ that provides this paste, the anal glands, are occupied by microbes. Although both species of hyena contain fermentative bacteria, the microbes found in the anal gland of spotted hyenas (Crocuta crocuta) differ from the microbes found in striped hyenas. The spotted hyena paste holds many populations of coccus and rod shaped bacteria. A survey found that Firmicutes, Actinobacteria, Bacteroidetes, and Proteobacteria are the most common bacteria found in the hyena paste.

The bacteria found in the feces and anal glands of dogs and cats are also found in their mouths due to the consistent exposure to their backsides (licking and chewing). In a healthy dog or cat the bacteria normally found in their feces are Streptococci and Enterococci, more specifically, Enterococcus faecium, Streptococcus bovis, and Enterococcus faecalis. There are many glands in dogs that secrete into the sac's lumen, filling them with fluid. The anal sacs are usually made up of about 88% water, 11.5% organic and 0.5% inorganic matter. The secretion of anal gland content contains mucin, that is rich in sialic acid and other anti microbial proteins, like lysozyme, immunoglobulin A, and lactoferrin. There is also an abundance of gram positive cocci since they are part of the anal glands normal flora. The organic components mainly consist of short chain fatty acids and trimethylamine. The anal sac fluid can be secreted or expressed into the anal channel to serve as a scent marker for an individual's territory through their feces.