Scent-marking in wolves

1. What they did

Wolves have long been thought to use scent marks to hold their territories, but in 1975 there were few hard field data. Radio-tracking made it possible: each radio-tagged pack could be identified, found at any time so their tracks could be followed in the snow, and given a history (number of members, sex and age of some members, and the size and location of their territory).

In the Superior National Forest of northeastern Minnesota, 96 wolves in 13 neighbouring packs had been radio-tagged from 1968 to 1973. Over three winters (1971–72 to 1973–74), the authors found packs by aerial or ground radio-tracking and then followed their tracks in the snow. In the last two winters they concentrated on two neighbouring packs near their field headquarters: after watching the wolves from the air, they followed their tracks on the ground to the same place, which let them check both the aerial observations and their reading of the sign.

They recorded four kinds of possible scent mark on a detailed sketch keyed to a topographic map:

• Raised-leg urination (RLU)
• Squat urination (SQU)
• Scats
• Ground scratching

In all, they analysed 110 samples (stretches of track followed along their whole length) covering 240 km of ground tracking, plus 40 km of aerial tracking. They compared marking rates by time of winter, type of travel route, territory edge versus centre, pack size, and time between a pack’s visits. They also watched captive wolf packs at the Brookfield Zoo and the St. Paul (Como) Zoo to see which wolves marked, and in what mood.

2. What they found

Who marks, and how

• In the captive packs, only mature, dominant wolves, mainly the alpha male or female, urinated with a raised leg. In the Brookfield pack, 22 of 27 RLUs were made by the alpha pair, 20 of them by the alpha male. Over 60% of RLUs went with self-assertion, snarling, growling or biting; squat urinations did not (Table 1).
• Twice in the field, radio-locations showed the tracks being followed were those of pups. Over more than 6 km of those tracks there were several squat urinations but no RLUs.
• Only high-ranking wolves scratched. A scratching wolf usually steps away from the urine or scat first, and the thrown material is almost never aimed at it.

What the marks look like in the snow

• In 240 km of ground tracking the authors recorded 1006 possible marks: 584 RLUs, 193 scats, 170 squat urinations and 59 scratches.
• RLUs averaged about one every 450 m, ranging from none in 7 km to 20 in 1 km. All but 4 were aimed at conspicuous objects: blocks of snow, trees, shrubs, rocks, snowbanks and once a plastic bag. The authors could not measure the urine in an RLU, so they copied the mark by squirting 5 cc of coloured water on snow. In winter an RLU stands out against the snow and can be seen several metres away.
• In more than 70 places, wolves on roads had left a nose-shaped dent in snowbanks at RLU height. Under about 30% of these, blowing away the snow revealed urine.
• Scats count as marks when they are on prominent objects (snowbanks, stumps, shrubs, even empty beer cans), when many accumulate in one place over months, or when the same wolf urinates or scratches on them. Wolf scats could be smelled by people up to 10 m away, even in still air below −20°C. Large scats, sometimes up to six together, were left at strategic points around rendezvous sites (where pups wait while adults hunt in summer).
• A squat urination and an RLU together were often found in the breeding season. One observation from the air and six in captivity suggest a female squats and a male then sniffs and makes an RLU on the spot.

When and where they mark

• RLUs rose from about 2.5 per km in December and January to a peak of 3.5 per km in late February (the breeding season), then dropped to about 1 per km in March. Squat urinations also peaked in late February. Scat and scratch rates stayed level all winter (Figure 3).
• About 40% of RLUs and scratches, and 50% of squat urinations and scats, on roads and trails were at junctions, even though wolves rarely pause there for more than a few seconds.
• Wolves marked more on roads and trails (3.4 RLUs per km) than in the bush (1.7 per km), even though the bush has more objects to mark. Rates were lowest on frozen lakes and waterways (0.4 per km), where there is little to mark (Table 3).
• Territory edges (a strip about 1 km wide) had about twice the RLU rate of territory centres: 2.67 versus 1.27 per km (Table 4).
• Larger groups left more scats, squat urinations and scratches per kilometre, but not more RLUs, because RLUs are made mainly by the alpha male whatever the pack size (Figure 4).

What prompts marking

• One pack re-marked the same 2.4 km stretch of road on at least ten occasions with no other wolves in between, sometimes while their old marks could still be smelled by a person. The fresher the old marks, the more new RLUs the pack added (Figure 5). Marking fell back to the base rate when marks were about a week old, though the authors recorded marking in response to the pack’s own sign up to 23 days old.
• Four times a pack was tracked meeting fresh sign of a neighbouring pack. Twice the marking rate jumped from none in over 2 km to about 10 marks (at least 6 RLUs) in the first kilometre after the encounter. In the other two the rates were the highest recorded.

Turning back at the border

• A pack of seven to nine wolves walked about 2 km along a lake shore at their territory’s edge, approaching the shore 13 times and turning back each time before reaching land.
• A pack of eight crossed the two-week-old tracks of a neighbouring pack of five, marked heavily, went on about 1 km and turned back.
• Soon after a boundary river froze, seven wolves of the pack north of the river crossed to the south shore and went back. The next day the south pack came within 50 m of the river; one wolf made five approaches toward it, making RLUs and scratches at the northern end of each. Where the south pack met the north pack’s tracks, a 300 × 600 m network of trails held 30 RLUs, 10 scratches, 2 squat urinations and 1 scat.
• The north pack chased and badly wounded a deer across the river, then stopped following it, marked heavily and went home. A day later the south pack found and ate the deer.

3. What they concluded

• Each pack’s territory is marked throughout with their own scent, and regular use of marked routes renews it: fresh marks prompt more marking (positive feedback), not mainly the marks of strangers.
• Because neighbours both mark heavily where they meet, each territory becomes an olfactory “bowl” with a dense rim of marks from the resident pack and their neighbours.
• In winter a travelling wolf meets or leaves a sign every 240 m on average, including an RLU every 450 m; at the usual speed of 8 km per hour that is a sign about every 2 minutes and an RLU every 3 minutes. Wherever a wolf is, they can tell whether they are in their own territory, and lone wolves can tell where they are in relation to each pack.
• Raised-leg urination is probably the main way packs hold their territories. Squat urination, scats and scratching may carry information mainly within the pack, such as who travelled with whom or whether an area was hunted recently.
• Aversion to strange marks is probably learned, not inborn: captive wolves did not avoid the scent of unfamiliar wolves. It may work together with wolves’ “cognitive maps” of their routes and territory.
• The authors propose that an empty area, with no marks or only marks too old to matter, prompts wolves to mark it. Lone wolves could find vacant land and each other by scent marks, and pairing could start a new pack. Such a system would keep all available habitat occupied and limit the population to the space available.

4. How tracking was or was not used

Tracking was the main method. The authors found packs by radio and then followed their tracks in the snow, on the ground and from the air, recording every raised-leg urination, squat urination, scat and scratch along each stretch. They classed scats as marks or droppings by where they lay and whether they had been urinated or scratched on. They read gait from the tracks (almost all samples were at a walk), related sign rates to the number of wolves making the tracks, found nose dents in snowbanks where wolves had sniffed, and blew away fresh snow to check for urine underneath. Following tracks also showed how packs behaved at borders: approaching a shore and turning back, crossing a neighbour’s trail, or abandoning a wounded deer. Aerial observations of behaviour were checked by following the same tracks on the ground.

5. Tracker or observer certification

Not reported. The authors did the snow tracking themselves and do not describe anyone’s tracking experience. Every finding rests on reading tracks and signs in the snow, such as telling raised-leg from squat urination and judging which scats were marks.

6. Other technologies, skills or innovations

• Radio-tagging and aerial and ground radio-tracking to find and identify packs, so that tracks could be tied to a known pack.
• Pairing aerial observation with ground tracking of the same animals, so each could check the other.
• Sketch maps keyed to topographic maps for recording sign along each tracked stretch.
• Copying an RLU with 5 cc of coloured water on snow, since the amount of urine could not be measured.
• Observations of captive wolf packs to learn which animals made which marks.