Scent Communication

Weasels are solitary animals for most of the year, but that does not mean that they are totally nonsociable. Weasels must be well aware of their neighbors, even if only to avoid them. They keep in touch with each other by a well-developed system of scent communication—an efficient mechanism for small animals living on large home ranges covered with thick vegetation. Even under snow, the conditions are good for scent communication—cool, dark, humid, still, and quiet—and odor signals last well, although they cannot be advertised over long distances by a breeze.

Hidden under the tail of a weasel is a pair of large, muscular sacs, in which is stored a substantial quantity (up to 100 mL in male stoats) of musk, a thick, rather oily, yellowish fluid with a powerful and unpleasant (to most humans)

smell. The musk is produced in modified skin glands, which are grouped together at one end of the sac and empty into it. Musk is a complex substance, in which the important components are lipophilic compounds of low molecular weight, several containing sulphur (Brinck et al. 1983). When the musk is exposed to the air, it is metabolized by bacteria into various carboxylic acids, perhaps in different combinations according to which bacteria are present.

Voluntary muscles control both the openings of the glands and the walls of the sac, so a weasel is able to expel musk at will. Normally, only a little is produced at a time, but a severely frightened weasel is able to evacuate the entire contents of the anal sacs at once—the famous "stink bomb" well known to careless trappers. The effective defense system of skunks may well have evolved, eventually accompanied by warning coloration and behavior, from such a beginning.

All mustelids have a system of anal glands, constructed slightly differently in each genus. The anatomy of the scent glands gets progressively more complex in a series from Meles through Lutra and Martes to Mustela, which seems to be the most advanced of all (Stubbe 1970, 1972), apart from the skunks.

Weasels also have smaller glands in the skin of the body, especially along the belly and flanks, and on the cheeks. Weasels regularly mark their home ranges by depositing scent from these glands in strategic places. This behavior is hard to see in the wild, but can be observed in very tame captive animals. For example, Drabble (1973) described how a hand-reared female stoat marked her territory: "she pressed herself flat and anointed the surface she claimed by rubbing it with her belly."

Like most carnivores, weasels also make use of two other strongly scented substances, scats and urine. For example, in the course of traveling around his home range, a typical male will seldom pass a place where he has previously found or deposited a scat without visiting it again, carefully sniffing, and then turning and depositing a new one—often with an expression of fierce concentration. Weasels seem to take delight in taunting researchers by leaving proof of a visit on the top of an undisturbed live trap (Figure 8.5). Scats may be found singly along trails, sometimes in different stages of weathering, suggesting regular marking of familiar routes. They are obviously powerfully attractive to any weasel using the same path, because they make good bait in live traps (Rust 1968; R.A. Powell, personal observation).

Scent-marking behavior is well known but not well understood. At the very least, scent marks convey information on social and reproductive status, and probably also individual identity. The various forms of marking behavior have been closely observed in captive Swedish stoats by Sam Erlinge and his coworkers (Erlinge et al. 1982).

A stoat performing anal drag presses his anal area to the ground, with his tail raised, and wriggles forward, pulling himself along with his forelegs. Females do it, too, and also young as soon as they begin to move about outside their

Figure 8.5 Scats are objects of great interest and usefulness to weasels, and are carefully placed in strategic positions around a resident's home range—sometimes including the top of a live trap that had failed to catch the weasel.

nest. The idea is for a weasel to permeate the whole of its home range with its own scent, to mark new objects encountered, and to cover over the marks made by others. Stoats will do it when they are alone, and at any time of day.

Body rubbing, by contrast, is used as a threat signal, especially by a dominant of either sex during an aggressive encounter with a subordinate. The dominant vigorously stretches himself out along logs or stones, scraping the scent from his cheeks and sides against them (Figure 8.6). He does the same thing when he deposits a kill in a cache or when he takes over a den formerly occupied by another stoat. Captive stoats finding cloths impregnated with human scent (unwashed t-shirts) in their cages reacted with energetic body rubbing (Winder 2003). The message is definitely a belligerent one, rather than the mere labeling of property.

It seems, from watching the reactions of one weasel to marks made by another and the way that weasels use these cues to space themselves out with minimum open conflict, that the scents of each are not only individually distinguishable, but also very informative. For example, a subordinate stoat faced with the scents left by a body-rubbing dominant reacts with obvious fear. It shows various signs of uneasiness and anxiety; it gives the little trilling call that imitates the cries made by young and that would mollify the aggressive reactions of the dominant if it were still within earshot, and it will search around for a way to escape.

Figure 8.6 Resident weasels deliberately rub their body scent on prominent objects around their home ranges as a sign of ownership, which is recognized and understood by intruders.

By contrast, a dominant stoat will show no obvious reaction to the marks, apart from marking over them. Dominant individuals also set marks more often than subordinates. They simply appear more self-confident.

The world of a weasel is full of meaningful scents, which are just as informative in total darkness as in full sunshine, and whose messages last longer than those of the fleeting visual images on which we rely. Our noses are useless to read them but, fortunately, the chemical composition of the secretions produced in the scent-marking glands of weasels can be displayed by gas chromatogra-phy. If scent marks really do convey important information between individual weasels in the wild, as field and behavioral observations imply, the chromato-grams should show subtle differences between individuals, consistent over time in the same individual—and indeed, they do (Brinck et al. 1983).

The component molecules separate out according to their weights, and the patterns produced show distinct differences from one animal to the next. Better still, the differences reappear in successive samples from the same animal. The origin of the differences is uncertain. Perhaps the molecules of one animal's musk are structurally different from those of any other, or perhaps each animal has a unique combination of bacteria producing a different set of metabolites of musk (Gorman 1976). Either way, it seems likely that, to a weasel, scents are as unique as faces are to us. Like faces, scent marks give one weasel a lot of useful information about another—not only its sex, identity, social status, and breeding condition, but also the probable outcome of a confrontation.

The advantage of scent marks is that they persist for some time, so that, for example, a resident male can give out information to potential intruders in many places at once. An intruding male, on the other hand, has the information on which to decide whether to risk an encounter with his unseen neighbor. He knows how long ago his neighbor was last here. Weasels can fight ferociously (Figure 8.7), and are well able to injure each other, so it is an advantage to a stranger to assess the likely outcome while escape is still possible. In fact, avoidance and retreat are much more common than all-out offense in deciding the local dominance

Figure 8.7 Stoats normally avoid each other but will fight vigorously when necessary. Here, two females fight, possibly over territory boundaries. (Redrawn from a series of photographs by Derry Argue.)

hierarchy and distribution of individuals. But running battles do happen, especially in spring. Vernon-Betts (1967) gave a particularly vivid account:

During the last week in April I was driving down a lane when I saw two [common] weasels fighting on the grass verge. I stopped the car within a few feet and watched them for several minutes fighting with the concentrated ferocity of a couple of bull terriers. I released the brake and free-wheeled after them as they conducted a running fight for more than 100 yards down the road. .. . Both animals went onto the verge opposite a ... gate in the hedge. Through the gate I saw a hunting cat with ears pricked advancing on tiptoe; clearly it could hear the squeaks of the weasels but had not yet seen them. Suddenly the pursuer saw the cat and made a run for the hedge. . . . The pursued ran for the gate and apparently straight into the jaws of the cat.. .. When I reached the gate, only the weasel was to be seen, white belly up, paws in the air, a bright bead of blood behind one ear and apparently dead. I picked it up. ... It was an undoubted male, and the same size as its antagonist. I began to carry it back to the car. As I was passing through the gate I noticed its belly was pumping up and down; after a further two steps it gave a violent wriggle so that I dropped it. It hit the ground running and darted into the hedge where it immediately burrowed into a pile of dead leaves.. .. Half an hour later I returned the same way, and, though I stirred the leaves with a stick, there was no sign of it.

This extraordinary incident illustrates not only a particularly vigorous dispute between males in the breeding season, but also the so-called "sham-dead" trick of weasels in response to immediate mortal danger. It may be, as some believe, a deliberate defensive ploy. Others interpret it as the effect of violent exertion on a brain already under pressure from parasitic worms (see Figure 11.8).

When weasels live at high density, such encounters between a resident and a neighbor or an intruder are more likely. For example, Lockie (1966) had already inferred, from the distribution of trapping records, that the resident common weasels he was observing worked to keep others off their "own" ground. Then he actually saw it happening:

I ... once [saw] a territory holder escort a transient from its territory. . . . Both animals suddenly appeared running towards me, the chaser shrieking now and then. They paid no attention to me and passed close by. At the known boundary of the territory the owner broke off and returned into his territory where he was shortly after trapped and examined. The other animal kept running and disappeared from view a quarter of a mile down the track. I was unable to catch and examine the chased weasel, but since none of the known residents was missing I presumed it to be a transient. On another occasion I trapped a transient stoat which squealed as it came out of the anesthetic.. . . Immediately the presumed owner of the territory appeared racing towards me apparently to see what was happening. . . .

Lockie recognized that such incidents were rare, and it is certainly in the weasels' interests to avoid them by use of more subtle means of communication. In 852 hours of radio tracking common weasels, Macdonald et al. (2004) heard three and witnessed only one fight between a collared and an unknown male; collared (resident) males simply avoided each other.

Likewise, Sandell (1988) witnessed only one encounter between stoats, even though he was tracking several males in the same area at the same time. He was waiting for a collared male to come out of a den, when an unmarked male approached from the other direction and entered the same den. Soon after, the unmarked male rushed out with the radio-collared male at his heels, and the chase continued for several hundred meters. Such encounters are rare, Sandell concluded, because resident males normally use scent marks to locate and avoid each other.

In a detailed study of scent marking by wolves, Peters and Mech (1975) documented patterns of behavior that are easier to observe in these larger animals and help to explain how weasels use scent marking to maintain their spacing patterns. Wolves scent mark everywhere within their territories, especially whenever and wherever they meet the marks of other wolves. A dominant wolf would even sneak 100 to 200 m into the neighboring wolves' territory to leave a scent mark, then retreat. Consequently, even though wolves spend relatively little time at their territory boundaries, the boundaries get a disproportionate share of scent marks. In addition, the boundaries tended to be fuzzy, from constant pushing from each side.

The density of scent marks allows wolves to know where they are with respect to territory boundaries, and it also lets nonresident wolves know the safest places to travel: along boundaries where scent marks are dense but wolves are not. Weasels may well do something similar. When a weasel notes that his neighbor is not renewing scent marks, he knows that something is amiss and the neighbor could well be out of the way. Transient weasels, moving along a territory boundary, would also be able to note that scents on one side are old and not renewed, and so identify a possible new home.

Game theory, a branch of mathematics originally developed to understand gambling by humans, was quickly applied to animal behavior and has provided some fascinating insights into why scent marking works (Maynard Smith 1979). Game theory helps to analyze the costs and benefits of developing rules for territorial behavior, such as (1) chase your neighbor out of your territory when he invades (but don't chase fast enough to catch him); (2) run from your neighbor when he finds you on his territory (do not fight on his ground if you can help it); and (3) scent mark on top of his marks. If weasels follow these rules, they minimize their chances of getting hurt in a fight while maintaining their territories. More important, (4) when conditions change and no territory has enough prey to support a single weasel, move out.

And, indeed, weasels do follow these rules. Neighbors do not fight when they meet at territory boundaries, but a resident will chase an intruder (who always runs). When vole populations crash, weasels no longer attempt to maintain territories, and the stable spacing system breaks down.

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