Dogs can catch cancer simply by sniffing each other, with male animals at most risk, scientists have found.
A rare cancer called "canine transmissible venereal tumour" - or CTVT - can jump between creatures when they smell each other's intimate areas.
The living cancer cells physically "transplant" themselves from one animal to the other.
Dogs often sniff each other because it tells them about the other animal's identity, gender, health, mood, diet, and confirms whether they've met before, but it can transfer disease.
CTVT is usually transmitted during mating, but sometimes the cancer can affect other areas like the nose, mouth and skin.
Most facial cancers in male dogs
To find out what was causing the oddly-located tumours, researchers reviewed a database and found that 84 per cent of facial cancers were in male dogs.
In contrast, genital cases of CTVT occur in roughly equal numbers of male and female dogs.
"We found that a very significant proportion of the nose or mouth tumours of canine transmissible cancer were in male dogs," said Dr Andrea Strakova in the University of Cambridge's department of veterinary medicine, the first author of the paper.
"We think this is because male dogs may have a preference for sniffing or licking the female genitalia, compared to vice versa.
"The female genital tumours may also be more accessible for sniffing and licking, compared to the male genital tumours."
Rise in cases linked to animal imports
CTVT is now found in dog populations worldwide, and is the oldest and most prolific cancer lineage known in nature. Although it is not common in Britain, cases have been rising in the past decade, linked to the importation of animals from abroad.
The disease occurs worldwide but is mostly linked to countries with free-roaming dog populations.
The most common symptoms of the oro-nasal form of the cancer are sneezing, snoring, difficulty breathing, nasal deformation or bloody and other discharge from the nose or mouth.
"Although canine transmissible cancer can be diagnosed and treated fairly easily, veterinarians in the UK may not be familiar with the signs of the disease because it is very rare here," added Dr Strakova.
Transmissible cancers are also found in Tasmanian devils, and in marine bivalves like mussels and clams.
The researchers say that studying this unusual long-lived cancer could also be helpful in understanding how human cancers work.
The findings were published in the journal Veterinary Record.