A Reassessment of the Function of Scent Marking in Territories

1. What they did

Many mammals leave scent marks around their territories, using dung, urine, saliva and gland secretions, but why they do it was still unclear in 1982. The oldest idea, that marks keep rivals out, had been disproved: observers had seen intruders walk through marked territories in many species without being put off.

Gosling first reviewed eight existing explanations for marking territories. He then proposed a new one, the “scent matching” or “assessment” hypothesis, set out the predictions it makes for the behaviour of territory owners, intruders and neighbours, and checked those predictions against published descriptions of behaviour. Most examples come from hoofed mammals with one male defending a territory, especially Coke’s hartebeest, which he had studied himself.

2. What they found

Existing explanations fall short

Marks were said to deter intruders, intimidate them, give owners confidence, help owners find their way, attract or stimulate mates, help form pair bonds, regulate population size, or help owners forage. Gosling found each either poorly supported, impossible to test, or unable to explain key behaviours, such as why owners rub the same scents onto their own bodies.

The new hypothesis

Scent marks let an intruder recognise the owner. An intruder remembers the smell of the marks around them. When they meet an animal whose smell matches those marks, that animal is very likely the owner. Owners are usually high quality animals with more to gain from keeping the territory, so they are ready to fight hard. It therefore pays most intruders to back down early, and the owner saves the cost and risk of fighting every intruder. The intruder does not need to know which individual the owner is, only that they are the owner.

Owner behaviour fits the predictions

• Marks are placed where intruders will find them: along boundaries and trails, at head height on projecting twigs, and on conspicuous sites. Some animals make the site more visible: hartebeest thrash small trees to a bare stump, oribi bite grass stems to a good height, roe deer fray saplings white, beavers build mounds, pumas mark on scrapes, and many hoofed mammals paw at dung piles.
• Owners mark themselves with the same scents: hartebeest rub gland secretion on their shoulders and kneel in dung, wildebeest roll in dung, deer spray urine on their legs and wallow in urine-soaked mud, and cats rub their faces in their own urine. Rolling in carrion or strong-smelling plants might also give an animal the smell of its area.
• Owners make themselves available to be smelled: a territorial hartebeest stands still while an intruder slides their nose down the owner’s neck to the shoulder. Owners also mark more often during encounters, linking their smell to a fresh mark in front of the intruder.
• Owners remove or cover marks that are not their own: oribi bite off and eat old marks before re-marking, hartebeest sniff, scatter by pawing and then defecate on another male’s dung, and beavers and rabbits over-mark strange scents placed in their territories.

Intruder and neighbour behaviour

There are fewer observations of intruders, but those available fit: black-tailed deer entering a new area sniff many twigs; intruding hartebeest, impala and black buck sniff or taste the owner’s scented areas; and most intruders then withdraw. High status hartebeest challengers instead go from dung pile to dung pile marking at a high rate, as if marking the territory before taking it over. When a hartebeest territory falls vacant, the new owner marks intensely and neighbours intrude deeply until, after a day or two, the territory is fully marked and the intrusions stop.

Wider uses

The same idea may explain marking in dominance hierarchies, males marking females, members of family groups marking each other, and females choosing mates by matching a male’s smell to the marks around him. Subordinate animals tend to mark less and present their scent for inspection, which could signal that they are not the owner.

3. What they concluded

• Territory owners probably mark so that intruders can identify them, which reduces the number of escalated fights. The cost of marking, mainly time and energy, may be outweighed by the saving in defence costs and risk of injury.
• The hypothesis offers one framework for behaviours that other explanations did not predict: self-anointing, marking during encounters, and removal of old marks.
• It has two parts that can be tested separately: that owners benefit from letting intruders assess them, and that intruders do this by scent matching.
• Scent matching is a cheat-proof signal. Marking a whole territory costs a lot of time and energy and can only be done by an animal that has actually held the area for some time, so scent marks give a record of an animal’s past behaviour in both time and space that a weaker animal cannot fake.
• Smell is not the only clue. Low status intruders may flee on sight alone, while high status ones have more reason to get close and check the smell before deciding.
• The author notes that the extensions to hierarchies, marking of other animals and mate choice are only briefly treated and need more evidence.