The thick, creamy, foul-smelling liquid which is light gray to brown in colour contained in the anal sacs is squeezed onto faeces when the cat defecates. This imparts a particular smell onto the faeces which acts as a calling card for the cat.
The scent is particular to that individual cat allowing others to know that they’ve been there. It is part of the well-known behaviour of the domestic cat: scent marking.
Anal sacs are also referred to as anal glands. The walls of the sacs are lined with a lot of sebaceous glands that produce this foul-smelling liquid.
The diagram on this page shows you where they are located and where the liquid is deposited onto faeces as the cat defecates. It’s just before the anus at the extreme end of the colon called the rectum.
At a more detailed scientific level, a study looked at the bacteria in the anal glands which through fermentation produce volatile compounds which provide this “signalling”. The word “signalling” in this instance means that the domestic cat is signalling to others of their presence.
Volatile compounds (VOCs) are organic chemicals that have a high vapour pressure at room temperature and emit gases.
Below is a summary of the study on a Bengal cat which I’ve referred to:
The study discusses the role of scent secretions and marking behaviors in communication among social animals, specifically mentioning the anal sacs found in species in the Order Carnivora as an important source of odors.
These secretions from the anal sac may be used for a variety of behaviors, such as defense, species recognition, and signaling reproductive status. The passage also introduces the fermentation hypothesis, which suggests that microorganisms living in association with animals contribute to the chemical signals used in communication and that variation in these signals reflects variation in the microbial community.
The study looked at the anal sac secretions from a male Bengal cat, a hybrid of the domestic cat and the leopard cat, to examine the fermentation hypothesis. The study involved collecting anal sac secretions, extracting DNA, and performing PCR amplification and sequencing to characterize the microbial community.
The study also involves isolating bacterial colonies and using Solid Phase Microextraction gas chromatography-mass spectrometry to identify volatile organic compounds (VOCs) in the anal sac secretions and in cultures of selected bacterial isolates.
The results of the study show that the anal sac contains a bacterial community consisting mainly of a few abundant taxa, and that these taxa produce numerous VOCs that are also found in the anal sac secretions. These findings support the fermentation hypothesis and the idea that the anal sac may be maintained in part to house bacteria that produce VOCs for the host.
Although, normally, the anal sacs are emptied naturally by rectal pressure when the cat poops, they might become impacted although this is uncommon. It occurs when the sacs fail to empty normally. The ducts which carry the liquid to the rectum might be plugged because the secretions are so pasty. The condition might not be recognised until an infection has developed. An abscess might also develop. Your cat might scoot to try and relieve the irritation. A veterinary visit will be needed. Or at least a telephone consultation if that is permitted by your veterinarian. It is possible for a cat caregiver to express anal sacs if they have the knowledge. There is no substitute for a vet visit when needed and an infection in the anal sacs needs a vet’s attention.
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