The Economic Consequences of Advertising Scent Mark Location on Territories

1. What they did

Many mammals hold territories and mark them with scent. Scent marks tell other animals that the area is owned, and may also say something about the owner’s quality. An intruder who has smelled the owner’s marks can later recognise the owner by comparing the owner’s smell with the marks, which may settle a contest without a costly fight. But scent marks can only be detected from close by, so whether an intruder finds one is partly a matter of chance.

The authors argued that intruders gain from finding marks, so they should search for them, and owners should make their marks easy to find. They:

• Reviewed published examples of how animals seem to advertise where their scent marks are.
• Built a simple spatial model of one owner and one intruder in a circular territory, with a ring of scent marks inside the boundary. The model worked out the chances of five outcomes when an intruder crosses the territory: meeting the owner before reaching the marks; finding a mark and then meeting the owner (so the intruder can recognise them); finding a mark and leaving without meeting; missing the marks and then meeting the owner; or missing the marks and leaving without meeting.
• Varied the size of the territory, the number of marks and how far away each mark can be detected, to see how easier-to-find marks change the economics of scent marking.

2. What they found

How animals advertise their marks (review)

• Conspicuous, predictable sites. Klipspringer prefer to mark dead trees or branches, often of food plants, on slightly raised ground just above a break in slope. Many species place marks within a narrow band of height even when they could mark higher or lower. Some animals build their own marking sites, and some mark at sites used by other species.
• Visual signs beside the scent. Some ungulates thrash vegetation with their antlers; some cats and ungulates paw or scrape the ground; some animals damage or strip bark from trees where they mark. These signs are separate from the scent itself. There is no direct evidence that receivers are drawn to them, but the authors find their widespread existence hard to explain otherwise.
• Receivers do search. Black-tailed deer search for marks after entering a new area, and ring-tailed lemurs investigated 62% of scent marks within 10 minutes (median delay 30 seconds).

The model

• On small territories, the most likely outcome of an intrusion is meeting the owner after already having found their marks, so the intruder can recognise the owner.
• As territories get larger, this chance falls, until it becomes less likely than either meeting the owner without having found a mark or leaving without a meeting.
• The exception is very small territories, where the owner usually meets the intruder before the intruder reaches the marks.
• When marks can be detected from further away (10 m rather than 2 m), the range of territory sizes over which intruders can recognise the owner grows in a straight line.
• For any territory size, the number of marks needed to give the same chance of detection falls steeply (exponentially) as marks become easier to detect.

3. What they concluded

• Intruders gain from seeking out scent marks because it helps them avoid costly fights, and owners gain from advertising where their marks are, because their marks are more likely to be found.
• This raises the share of encounters in which the intruder knows it faces the owner, which should reduce escalated fights.
• Easier-to-find marks let owners mark at lower density, so receivers who search take on some of the cost of the signal.
• If marks attract attention by sight, the scent itself need not carry far. Information may then be gained only at close range, often by licking the mark, and less volatile scent lasts longer.
• Advertising marks and searching for them may play an important part in the economics of territoriality. Poorer competitors may gain most from finding marks (and leave), while strong competitors may need information before trying to take over.
• Scent marks are honest signals because the owner must hold the territory long enough to mark it; advertising them should reinforce this.
• Advertising could also help other receivers, such as potential mates, and help receivers find several marks, for example to predict the owner’s movements or navigate across territories.

6. Other technologies, skills or innovations

• A simple spatial model built on Roberts and Lowen’s (1997) model of klipspringer scent-mark patterns and a modified version of Waser’s (1977) gas model of random encounters, adding whether or not the intruder has found a mark before meeting the owner.