Size Variation In Weasels On The Northern Hemisphere Islands

There are thousands of islands off the mainland coasts of North America and Eurasia, ranging in size from Newfoundland and Britain down to scattered rocks.

All those now separated from the mainland by water less than 100 m deep must have been joined to it during the last glaciation, so all those that were not glaciated, or were freed from the ice before the rising sea level cut them off, could presumably have been colonized across dry land by animals from the nearest continental fauna living at that time.

Some islands were no doubt colonized by weasels then and have been continuously inhabited by weasels ever since. Other islands have been colonized since their isolation, once or many times, by weasels swimming from the nearest point on the mainland or, occasionally, deliberately or accidentally carried in boats by farmers or traders, not necessarily from the nearest mainland port. Only the largest islands provide enough space and enough prey for a permanent population of weasels: Among the offshore islands of Britain the lower limit is around 60 km2 (King & Moors 1979a), and, except on mainland Britain (230,000 km2), Skye (1,600 km2), and the Isle of Wight (380 km2), only one species of weasel lives on each island.

Weasels on islands are often at least slightly different in size from those on the nearest mainland, especially if they have lived there in isolation for a long time. The published data on island weasels are sparse and inadequate, and the origin, route, and date of colonization of the immigrant weasels are seldom known. Yet, in recent years we have begun to compile a reasonable general explanation of the patterns we see. Included with Eger's (1990) analysis of the distribution of size and shape of the skulls of stoats across North America were several representative populations living on islands. For example, the stoats on Newfoundland, Kodiak, and Baffin Islands are significantly different in size, but not necessarily in shape, from those on the adjacent mainlands. The skulls of the stoats living on the Alexander Archipelago of southeast Alaska are not only much smaller and distinctly different in shape compared with skulls from the adjacent mainland, but also different from skulls collected in the far north and in the Pacific Northwest (Eger 1990). In the British Isles, the stoats of northern Ireland are very different in size and coloration from those in Scotland just across the northern Irish Sea.

Where did these differences come from? One obvious possibility is that islands such as Ireland and mainland Britain were occupied by different colonizing stocks—one larger in body size than the other (Kratochvil 1977). It is true that some physical characters remain stable over many generations, but size is not one of them. Size is too important to be determined merely by ancestry; it is acutely sensitive to contemporary conditions. The mean body size that we observe in a population of weasels is the one that best suits the present environment. To understand how that best size is determined, it is interesting and important to know the history of the colonizing stock, and the size the colonists were when they arrived, but other things also help to determine the outcome.

On the other hand, shape is less critical to survival than size, and local variation in skull shape of weasels may still show discontinuities that can be explained only in terms of different historical origins. Eger's (1990) analyses of skull variation in North American stoats show that pattern well (Chapter 1). In other words, contemporary adaptation and ancestry both matter. What we see now is a result of the interaction of contemporary ecological imperatives and historical dispersion.

Stoats in the Alexander Archipelago provide a locally detailed and elegant example of this interaction. Cook et al. (2001) describe three distinct genetic lineages of stoats inhabiting the islands (Chapter 1). Molecular data confirm that these observable differences are not only real and taxonomically significant but also much more locally complex than morphological data can reveal. So the present stoats have not only adapted to local conditions (in this part of the world, stoats are larger in cold, dryer areas on the continent and smaller in moist, warm ones, like the islands), but they also retain traces of their different historical origins (Eger 1990).

The stoats of Ireland have provided endless puzzles. For many years, the only measurements of Irish stoats available were those of Fairley (1971), whose material came from the northern part of the island. The stoats there are intermediate in size between the stoats and the common weasels of mainland Britain, and they were assumed to represent all Irish stoats. One early theory, put forward in a famous paper by Hutchinson (1959), linked the small size of Irish stoats with the absence from Ireland of common weasels.

Hutchinson based his paper on the theoretical idea of character displacement, which proposes that two similar, sympatric species must be different in size to coexist, using resources of different sizes (Chapter 14). This hypothesis predicts that, where one of such a pair of species lives alone, it could shift to an intermediate size suitable for exploiting all resources available. Hutchinson noted that Irish stoats seemed to fulfill these predictions very well. In fact, so also do the very large common weasels in the Mediterranean, especially on islands such as Sardinia that have no stoats (Figure 4.2 and 4.3). The same argument was applied to the very large common weasels of Egypt by Dayan and Tchernov (1988).

Unfortunately, all these examples are invalid. The common weasels in the Mediterranean are large because common weasels consistently become larger toward the south of Europe (Figure 4.3). The large common weasels in Sardinia and Sicily could be merely continuing the trend, and in Egypt they could actually be a different species (Chapter 1).

The stoats of Ireland get more and more interesting as new data appear. Although the stoats in the north are indeed very small, those only 250 km away in the south are about as large as British stoats (Table 4.1; Fairley 1981). Since there is also a southward increase in the proportion of Irish stoats that have the straight-line belly pattern typical of British stoats, Sleeman (1987) suggested that both trends might have been influenced by an unrecorded introduction of British stock into southern Ireland at some time in the past. Lynch (1996) confirmed that small-sized northern Irish stoats are very different in their skull characteristics from the stoats of England or Scotland, and are unlikely to be descended from ancestors that crossed to Ireland from the east on a hypothetical postglacial land bridge. Lynch suggested that the ancestors of the present Irish stoats could have survived the last glaciation on the exposed continental shelf south of Ireland, feeding on cool-climate rodents such as lemmings in the periglacial tundra, but he added that more recent human-assisted colonizations from England cannot be excluded.

Plausible as this idea is, problems remain. For example, we now have to explain why stoats in northern Ireland have not yet (after about 8,000 years) become larger in body size in response to the mild, damper climate of postglacial Ireland. It looks as if some other important factor is involved besides climate and ancestry.

Could the development of small, island stoats be stunted in some way? Those on Terschelling Island, off the coast of the Netherlands, are small on average, and they suffer badly from skrjabingylosis, an unpleasant condition caused by a parasitic nematode in the nasal sinuses (Chapter 11). Van Soest et al. (1972) suggested that this infection could be the cause of the distinctly small size of the Terschelling males. Much less evidence supports this suggestion than contradicts it, even as an explanation specific to Terschelling Island, for reasons discussed further in Chapter 11.

Another possible cause of stunting is that young stoats cannot reach their full potential size if they are not well fed in the first few months of their lives. Sleeman (1987) suggested that the limited choice of prey and short growing season in northern Ireland might restrict the growth of young stoats there. Such an effect can be directly demonstrated in feast-or-famine habitats such as New Zealand beech forests (Powell & King 1997), so there could be something in the idea. Nonetheless, there is no obvious reason why it should be confined to northern Ireland.

In Britain, the northward increase in size of common weasels is matched by a similar trend in body size of field voles (Corbet & Harris 1991). Voles are among the favorite prey of weasels and, in general, the body sizes of hunter and hunted are closely linked (Chapter 6). Unfortunately, we have no information on what aspect of body size of either predator or prey to measure, or any precise data on the size ranges and relative abundance of the prey eaten by weasels on any of the northern-hemisphere islands. We do not know how long we might expect any local adaptation to take or how far it might go. As Levins (1966) pointed out, no theory can be general, accurate, and precisely relevant to local conditions. If the size of the weasels on each island is a unique, local compromise, no general theory will explain the whole pattern unless it includes detailed information on the weasels living in all the island habitats, how long they have been there, and where they came from.

On the other hand, we do have some of this information for one group of islands in the South Pacific, and any attempt to construct a general theory about what determines body size in weasels should take account of what is happening there.

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