Adaptation In Body Size Of Stoats Transported To New Zealand

The stoats and common weasels that were introduced to New Zealand over the 20 or so years after 1884 were relatively large, because they probably all came from Britain (Chapter 13). They found an environment quite different from the one they left, with a generally milder climate (ranging from warmer than Britain in the north, where there are subtropical rainforests, to colder than Britain in the high mountains, where there are large, permanent glaciers). They also found a prey fauna completely different in size distribution. The newly established pastures where stoats were first released teemed with rabbits, which were familiar prey from England. In the neighboring forests, ship rats and birds were abundant (Chapter 5). But the staple prey of common weasels and stoats almost everywhere else in the world, the various kinds of voles, were completely missing. The only small rodents available were house mice, widely distributed but not nearly abundant enough to compensate for the absence of voles, and not particularly nutritious (Vaudry et al. 1990). Lizards and insects, especially large ground-dwelling forms, were then still common but, generally, the new environment offered more large and fewer small prey than did Britain.

Of course, the comparison is not simple, for two reasons. First, the factors determining the body sizes of male and female stoats are not exactly the same, so the two sexes might not respond in the same way to the same change in conditions. Second, the new environment offered by New Zealand to the immigrant stoats was not the same everywhere. Two main sorts of New Zealand native forest, one dominated by the five species of southern beech (Nothofagus sp.) and the other by native conifers (the podocarps) and broad-leaved trees, were quite different from each other and also from anything in the northern hemisphere. The alpine tussock grasslands, open gravel riverbeds, and second-growth scrub elsewhere in New Zealand were also quite unfamiliar. So, the way in which the immigrant stoats responded to the shift in prey size distribution need not be the same in both sexes nor in all habitats. And indeed, it was not.

We can ask two questions about the size of stoats in New Zealand: Have they developed any local differences in size related to habitat? and Have they changed relative to their British ancestors?

The first question is easy to answer in the affirmative. Within New Zealand, the adult males among the 1599 stoats collected in the 1970s from podocarp or mixed forest habitats, mostly at low elevations, were smaller than those from alpine southern beech forests and grasslands, by about 3% in skull length and about 4% in head and body length (King & Moody 1982). The same difference appeared, consistently but less clearly, in adult females and in young of both sexes. It even appeared in samples from the two kinds of forests taken within a short distance of each other; adult male stoats from podocarp forests on the west side of the Main Divide of the Southern Alps were smaller than their neighbors less than 20 km away in the beech forests on the east side, in two quite separate pairs of samples.

As in the Alexander Archipelago, these local variations in size of the New Zealand animals must have arisen since their ancestors arrived on the islands. In both cases, it stretches all credibility to suppose that the pattern could be due to sampling error or to systematic variation in body size of the original colonizing stock. On present data it appears that, within a hundred years (1884 to 1984), stoats in New Zealand developed a range of variation in body size between local populations no less than exists on the whole of continental Europe. Male stoats living in the foothills of the Southern Alps are probably among the largest in the world; in fact, they are near the top of the range of sizes of male long-tailed weasels in North America. Female stoats in New Zealand are larger than any Eurasian females, and near the middle of the range of sizes of female longtails (Table 4.1).

This response to a range of new environments has been extremely rapid, though not unique. Such remarkably rapid shifts of mean body size in colonizing populations have been observed before—for example, in the rats introduced to New Zealand and to other Pacific islands (Yom-Tov et al. 1999). The New Zealand example, however, is one of rather few in the world where substantial shifts in mean body size of stoats are correlated clearly with diet (Figure 4.8), sex, and habitat, and can be precisely dated (the first arrival of stoats in 1884 is well documented; Chapter 13). The key factors controlling body size are not fully understood; meanwhile, even simple phenotypic changes, when consistent, tell us that to a stoat, there is something different about New Zealand. The most obvious difference between New Zealand and Europe is in the frequency distributions of sizes of prey available (King 1991b; Chapter 5).

The second question, on how much stoats in New Zealand have changed size in the last century, is harder to answer. King and Moody (1982) did an extensive survey of the stoats in New Zealand, including the related question of how the contemporary animals have changed compared with their British ancestors. They concluded that male stoats have become larger in the beech forest/grassland habitats of New Zealand, and are unchanged, or possibly smaller, in the podocarp and mixed forests, over the 100 years since colonization. Unfortunately, few specimens of British stoats were available in the late 1970s, most collected in the first half of the twentieth century (before myxomatosis) and preserved in the Natural History Museum in London. King and Moody had to assume that the skulls in this rather modest sample of British stoats were still

Figure 4.8 Mean body weights of local populations of stoats in New Zealand (filled symbols, males; open symbols, females) plotted against an index of the distribution of prey sizes taken by each population. For comparison, we also plot the means and ranges for the index of prey size for North American stoats (plotted with a small, open triangle), northern European stoats (small, open diamond), North American longtails (small, open square), and British stoats (small, open circle). For diets of stoats and longtails, see Figures 5.1 and 5.2. (Data for New Zealand stoats redrawn from King 1991a: fig 6, and see Figure 5.4.)

Figure 4.8 Mean body weights of local populations of stoats in New Zealand (filled symbols, males; open symbols, females) plotted against an index of the distribution of prey sizes taken by each population. For comparison, we also plot the means and ranges for the index of prey size for North American stoats (plotted with a small, open triangle), northern European stoats (small, open diamond), North American longtails (small, open square), and British stoats (small, open circle). For diets of stoats and longtails, see Figures 5.1 and 5.2. (Data for New Zealand stoats redrawn from King 1991a: fig 6, and see Figure 5.4.)

much the same size as the skulls of the stoats that were transported to New Zealand in the late nineteenth century.

In the late 1990s, a large, new collection of British stoats and weasels was made by McDonald and Harris (2002). Preliminary (unpublished) measurements of this material have cast some doubt on whether the museum material quoted by King and Moody did in fact represent the stoats of the late 1800s. Tools for statistical analyses are also much more sophisticated now than in the 1970s, so we might get to understand this fascinating story better when extensive samples from both countries can be analyzed with new methods. For example, it would be helpful to combine new information on how early nutrition and intraguild competition can affect body size of stoats (Chapter 14) with DNA analyses and new statistical analyses of traditional morphological measurements. The first DNA data have been provided by Holland et al. (in prep.), who se-quenced mitochondrial DNA from 80 stoats from across New Zealand. They showed that five haplotypes (maternal lineages) survive at present, from an unknown number originally imported. The distribution of these haplotypes does not correlate with the variation in stoat body size. Unfortunately, that result tells us nothing about whether variation in body size of stoats is mainly a pheno-typic or a genetic response to local conditions, or some combination of both.

Meanwhile, it is still safe to assert that stoats in New Zealand are generally large compared with stoats in other countries, and that stoat body size consistently correlates with prey size (Figure 4.8). Since the climate of New Zealand is generally milder compared with Britain, and the prey available are on average larger rather than smaller, New Zealand stoats find more advantages than disadvantages in remaining, or becoming, large. The effect is most substantial in females and this, too, is just as expected. The cardinal advantage of small size for a female stoat, the ability to hunt rodents into their last refuges, is irrelevant in New Zealand, whereas larger size would help female stoats deal with rats, rabbits, and possums.

The real explanation for the body size of the weasels of Ireland, Terschelling, New Zealand and most other islands is still unknown (Dayan & Simberloff 1998). Our guess is that the average size of the weasels on any island will drift toward whatever gives them, in the local conditions, the best year-round compromise between the advantages of smallness (see Figure 2.8), especially the size distribution of the small mammals available to hunt, versus the upward pull exerted by sexual selection. The point of balance may be determined, in ways we do not yet understand, both by the climate and by the size distribution of available prey. For example, the range of potential prey available on islands is usually smaller than on continents. Weasels colonizing an island may find that familiar, staple prey items are missing and the alternative supplies are different in various ways. On cold islands such as Newfoundland, supporting abundant small prey and no large ones, small weasels would have some advantages and suffer few penalties. On warm islands with fewer small prey than large ones, large weasels would be favored. We hope that somewhere, someone will find a way to put this hypothesis to the test.

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