The Use of Urine Marking in the Scavenging Behavior of the Red Fox (Vulpes vulpes)

1. What they did

Red foxes urine mark often, and Henry noticed that most of it happened while they scavenged. He asked why. In Prince Albert National Park, Canada, some foxes scavenged around the park’s settlements and became used to people, so he could watch them closely for long periods in the forest. Over four years he saw 15 foxes, recognized individually by natural markings, urine mark more than a thousand times.

From these observations he proposed a “book keeping” hypothesis: a fox urine marks spots where food has already been eaten but where food smell or inedible scraps remain. When the same fox, or another fox, comes back, the urine says “no food here” and the fox moves on quickly, saving search time.

He tested this with three field experiments on free-ranging foxes:

• Experiment I (October 1974, three foxes). He set out seven substances, one at a time and in random order: trimethylamine, gasoline, oil, synthetic urea, water (a control), dry dogfood and soft dogfood. Soft dogfood was smeared on a rock or root so an inedible food remnant stayed behind after the fox ate. Each was set out 30 times, and he recorded whether the fox urine marked.
• Experiment II (October 1972, an adult female and an adult male). Each day on a new 1,000 m² plot he made 30 spots about 5 m apart, imitating fox caches: buried 30 g dogfood baits (Type I) and open holes with a 2 g smear of dogfood (Type II). Once a fox had visited, spots it left unmarked became Type III and spots it marked became Type IV. Spots the fox sniffed that he had not touched were Type V. He timed every investigation with a stopwatch.
• Experiment III (October 1973, a young male). At fresh urine marks the fox made while scavenging, he buried either a large (30 g) or small (3 g) dogfood bait about 10 cm from the mark, then watched whether the fox dug it out when it came back within two days.

He also compared his own earlier natural observations of how long foxes investigated unmarked and freshly marked spots.

2. What they found

How foxes urine mark

• A fox always investigated a spot before marking it, and marked after 42% of investigations (255 of 613). The urine usually landed within 15 cm of the investigated spot, often right on it.
• He described nine urine marking postures. Marking lasted about 2.25 seconds.
• In snow, each of 200 marks checked held less than 20 ml of urine, and he never saw a fox pass a large amount, so foxes appear to empty their bladders only in small marking amounts. All 200 times he checked snow after a marking posture, there was urine, so “pretend” marking seems rare or absent.
• While scavenging, foxes marked up to 70 times an hour and investigated up to 220 spots an hour.
• Common sites marked again and again by several foxes (a “social record”) made up only about 12% of marks. A few marks were made by the dominant fox at the end of aggressive encounters.

Experiment I

• Only soft dogfood, the one substance that left a visible, smelly food remnant, regularly set off marking: 23 of 30 times. Dry dogfood, which left almost nothing behind, was marked 4 times; oil 3; trimethylamine and water once each; gasoline and synthetic urea never.

Experiment II

• Foxes marked most food spots after investigating them (53% to 80% for Types I to III), but rarely marked unmarked natural spots (Type V, 0% to 3%).
• Foxes spent about 6 to 9 seconds on spots with food smell (Types I, II and III). Neither the open hole nor the smell of another fox shortened this.
• Spots with food smell plus a urine mark (Type IV) got only about 1.2 to 1.6 seconds, the same as spots with no food at all (Type V). The difference was highly significant (P < 0.001).
• The male, on his first visit each day, spent only about 1.55 seconds at spots the female had marked, so he was reacting to her urine, not to his own memory.
• In natural scavenging before the experiments, spots with a known fresh urine mark got about 1.48 seconds, against about 3.68 seconds for spots a fox then went on to mark.

Experiment III

• Near urine marks, the fox dug out 24 of 30 large baits but only 2 of 30 small ones. A strong food smell overrode the urine mark; a weak one did not.
• Henry ranked the cues: strong food smell, then urine mark, then weak food smell.

Other observations

• Of 225 caches he watched foxes make, they urine marked on or near the cache only twice. They often marked the empty hole after digging up and eating a cache.
• Smelling a fresh urine mark did not make foxes more likely to start scavenging (scavenging before: 25 of 57 times; after: 22 of 57).
• Foxes sometimes left scats on durable remnants such as bone, but also elsewhere, so scat marking during scavenging was only occasional.

3. What they concluded

• The results support the “book keeping” hypothesis: a urine mark on a food remnant tells a fox “no edible food here”, which may let foxes find more food per hour of scavenging, important in times of food shortage.
• Foxes responded to their own marks and to other foxes’ marks, so there may be a social side to scavenging in an animal usually seen as solitary. The social life of solitary animals may be more complex than we think.
• One urine mark can carry several messages at once (for example who passed, her breeding state, and that there is no food), which questions the idea of one chemical, one response.
• Korytin and Solomin’s idea that foxes use urine marks to find food again was not supported by their own data when reanalysed, and needs more testing; both ideas could be true.
• Manipulative field experiments can be done on free-ranging large mammals and are one of the best ways to test ideas about behaviour and wildlife management.
• Limitations named by the author: only five foxes, all from one region; he was in plain view of the foxes; most work was in autumn, so he cannot say whether foxes mark food remnants in the breeding season.

4. How tracking was or was not used

The study is mainly direct observation of habituated foxes, not tracking. Signs did play a part: Henry checked urine marks in snow (200 times) to estimate how much urine was passed and to confirm that every marking posture left urine, relocated urine marks from natural features or painted stones, and checked emptied cache holes and buried baits. The paper is about a sign trackers often meet, urine marks, and what they mean to the animals that read them.

5. Tracker or observer certification

Not reported. All observations were by the author, who says that with practice foxes can be sexed and individually recognized from natural markings. No test of this skill is described.

6. Other technologies, skills or innovations

• Field experiments on wild, free-ranging foxes, using artificial “caches” that imitate how foxes bury food.
• Stopwatch timing of each investigation, from when the fox stopped or turned towards a spot to when it lifted its head.
• Painted stones to mark and relocate spots; movie film and field notes to describe marking postures.

7. Implications for conservation (from the paper)

• Manipulative experiments on free-ranging animals are one of the best ways to test ideas about the management of wildlife species, and they are used too seldom.