Scent Marking in Mammals

1. What they did

Scent marking is behaviour in which mammals place urine, faeces or the secretions of special skin glands on the ground or on objects (and sometimes on other animals). It is often called “territorial marking”. Johnson reviewed the laboratory and field studies available in the early 1970s to see how well the evidence supports this and other ideas about what scent marks are for.

He first listed the functions that had been proposed. Scent marks might act as:

• a deterrent or substitute for aggression, warning other animals of the same species away from occupied territory;
• a sex attractant or stimulant;
• a way of labelling the habitat for an animal’s own use, to find its way or to feel familiar with an area;
• a sign of individual identity, perhaps including sex, age and dominance;
• an alarm signal;
• a measure of population size (Wynne-Edwards’ suggestion).

He then reviewed the evidence under three headings: (1) where scent marks are placed; (2) how often animals mark, according to their state and their surroundings; and (3) how other animals respond to scent marks.

2. What they found

Where marks are placed

• If marks defended a territory, they should be most frequent at the borders. Some studies fit this: the lynx and the wild cat bury urine and faeces inside the territory but leave them conspicuously near the boundary, and rabbits mark more often towards a neighbouring colony.
• In many other cases marks are not confined to borders. Beaver marking points are found throughout the territory and some are near the lodge; black rhinoceros marks were found more or less at random across the home range.
• Marking also happens outside an animal’s own territory: male European hamsters marked inside a female’s territory, and rabbits marked in neighbouring territories they sometimes visited.
• Scent may help animals find their way. The rhinoceros was able to lay and follow faecal scent trails, and rats and mice use urine trails. A sense of familiarity with an area may give animals a “sense of confidence”.
• Some animals mark each other. Male rabbits mark females and young; lemurs, sugar gliders, tree shrews and mongooses also do this, and spraying others with urine seems common in rodents and rabbits and hares. This may help identify group members.

How often animals mark

• Dominant animals may mark several times an hour (duiker, rabbit, mongoose).
• Many species mark more in a new cage or when given a new object. Gerbils in an unfamiliar area marked more and more for the first 10 minutes; on later days marking peaked within a few minutes and then fell away. In another study, gerbil marking increased over 11 days, especially from day 1 to day 5.
• Marking usually begins with sexual maturity, and males usually mark more than females: male gerbils about twice as often, male hamsters about three times, and male rabbits ten times as often.
• In some species gland activity and marking rise in the breeding season (European hamster, water vole, Siberian hamster, rabbit). In others marking goes on all year (otter, shrew, the gerbil Rhombomys opimus), and grey squirrels marked most outside the breeding season.
• Evidence on marking during oestrus is mixed and sometimes contradictory; female dogs urinate more often when in heat. In golden hamsters and gerbils the scent glands are not essential for mating.
• Dominant animals mark more than subordinates in several species, and marking often happens during aggressive encounters. In some species gland secretions are used in defence or as alarm signals (skunks, civets, mongooses, black-tailed deer, and the odour of stressed mice and rats).

How animals respond to marks

• Little evidence was found that marks attract or excite the opposite sex in natural conditions, although some laboratory studies showed attraction (for example to the urine of female dogs in oestrus, and mice to traps used by the opposite sex).
• Johnson found no observations of animals withdrawing after meeting a stranger’s scent mark. Cats and dogs do not seem to be kept out by marks, and intrusions into marked territories occur. Groups of cheetahs did appear to avoid one another, using marks perhaps “like railway signals”.
• Animals in a stranger’s territory may change behaviour: a rabbit became alert, changed posture, stopped feeding and fled if met by the owner. Gerbils were more hesitant near strangers’ marks. These effects might come from learning, or simply from an unfamiliar place.
• Many animals approach and investigate strangers’ marks. Beaver scent was used as bait in traps as early as 1847, and otters made journeys to investigate the marking points of neighbours.
• A stranger’s mark often makes the finder mark too (dog, beaver, tree shrew, mongoose, sugar glider, rabbit). Rabbits marked intensely when strange droppings were put in their own territory, but did not mark at all in unfamiliar territory.
• Many species share communal marking sites: rabbits, zebras, mountain goats, otters, cheetahs and lemurs, rhinoceros dung hills used by several individuals, and spotted hyena latrines, one covering about a quarter of an acre and visited by packs of up to 100 hyaenas.
• Marks probably carry identity: mice and mongooses could tell individuals apart by scent, and even human subjects could tell apart the gland secretions of different rabbits.
• Marking may change with population density. Otters marked more at high density (one otter per 3.6 km of river, about 80% of marking points visited, against one per 5 km and about 40% the next year). Other studies on rabbits and gerbils gave the opposite or unclear results.
• A threat display sometimes follows smelling a stranger’s mark (European hamster, beaver, mongoose). An animal’s own marks may raise its confidence: a frightened badger calmed down when given an object carrying its own scent.

3. What they concluded

• There is a shortage of data on where and why mammals scent mark, and a lot of room for experiments.
• Marking is clearly linked with aggression and dominance, and so probably with threat or territorial motivation, but its exact role in aggression is unknown. Marks near boundaries do not seem to cause withdrawal or warn intruders away.
• Evidence that marking defends territory is lacking. Many of the observations are anecdotal or were read with the idea of territory already in mind.
• Marks may also help an animal feel at home, signal identity and status, act as alarm signals, serve as places for exchanging information, and perhaps help regulate population density.
• Scent marking almost certainly has more than one function in any one species, and different functions in different species. A common mistake is to treat all scent marking as the same behaviour.
• Johnson considered the term “territorial marking” not appropriate, because much marking is unrelated to territory, and its role in territorial organization has not been shown.

6. Other technologies, skills or innovations

• Johnson recommends that the term “pheromone” be used only where a mark has a specific effect, because scent marks probably act through several kinds of olfactory cues.