Sweden

Sam Erlinge and Mikael Sandell (Erlinge 1977b, 1977c, 1979b, Erlinge & Sandell 1986; Sandell 1986, 1988, 1989) observed the home ranges of stoats in southern Sweden, first by live trapping only and later also using radiotelemetry. Their main study area was a 40-km2 expanse of pasture, wet meadows, and marshes, crossed by streams and stone walls. The stoats' home ranges were concentrated in the most favorable spots, in the marshes and along the stone walls where small mammals were most abundant. The four main groups of residents were separated from each other by unoccupied open fields, and stoats from one group seldom visited another except in spring. Each group included several adult males and females plus a larger number of young of both sexes. Track surveys during snowy periods confirmed that the trap captures reflected the real distribution of the animals.

Erlinge's stoats established home ranges in late summer, as the year's crop of young entered the population and sought to establish home ranges for themselves for the winter. Males and females lived separately, with little contact. An adult female sometimes used part of an adult male's home range, but avoided him whenever possible, while young of both sexes kept well clear of his area altogether. Females had little to do with each other, and their home ranges were generally well spread out. They did forage throughout their home ranges, and on one occasion two happened to be at a common boundary at the same time. They came no closer than 60 m before moving away in opposite directions. Females generally spent a lot of time in deep rodent tunnels.

Males were in at least indirect contact with their neighbors of the same sex more often than were females. Two males whose ranges overlapped spent a lot of time in the boundary zone. There were never any signs of fighting or chasing or other direct confrontations, but the boundary was obviously set by social contacts between the two males. Adult males whose home ranges included or overlapped those of one or more females confidently moved about wherever they pleased, since they were always dominant in any encounter. When a male visited a female's den when she was not receptive to visitors, she hissed defensively or screeched, and he retreated.

All the resident stoats hunted over some parts of their ranges more than others, usually in short bursts of 10 to 45 minutes separated by longer periods (3 to 5 hours) of rest. The stoats obviously concentrated on the places where rodents were most abundant. Each visited the core of its home range almost every day, but other parts every few days. Some areas clearly known to the stoats were ignored altogether. The stoats covered more ground, and spent more time hunting, when rodents were scarce. They tended to be nocturnal in winter and diurnal in summer, and this change was connected with a pronounced seasonal reorganization of stoat society.

In spring, the settled system of winter ranges gradually broke down, just as in Lockie's common weasels. Some males set off on long excursions (up to 5 or 6 km), others stayed at home but moved about far more actively than before, and still others disappeared (Sandell 1986). The difference in behavior was correlated with body size and, presumably, social dominance. Large, assertive males ("roam-ers") wandered widely in spring, whereas small, low-ranked males ("stayers") stayed put. Erlinge and Sandell (1986) suggested that the reason for this seasonal change in behavior is that the decisive resource for males in early summer, receptive females, is more widely dispersed and less predictable and defensible than the decisive resource during the winter, usually food.

Dominant males can probably be sure of gaining access to any females they can find (Erlinge 1977a), so they can score the most matings by roaming in search of females across a large, but not too large, area. Seaman (1993) and Powell et al. (1997) found that wandering, dominant male mammals can keep good track of only a limited number of females. If a male tries to locate too many females, he cannot stay in close touch with each of them, and misses potential matings. The receptive period of the female stoats is short, and if a roamer mistimes his visits to a female, a low-ranked stayer already on the spot might take advantage. A low-ranking stayer cannot dispute the possession of a female with a dominant roamer if it comes to a fight, but by staying close to one female he could be in the right place at the right moment.

Mating tactics in general are too complex to be explained by single-factor models (Sandell & Liberg 1992), but this one does seem to account for changes in activity of stoats across the seasons, the pronounced differences in activity patterns of males and females in spring, and the strong seasonal swings in the sex ratio of stoats caught in traps.

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