The link between the sizes of a predator and its prey is well demonstrated in "sets" of species with similar hunting habits and tastes but different sizes, such as the three species of Mustela in the northern hemisphere. Least and common weasels do usually kill more small rodents, and fewer medium-sized rodents and lagomorphs, than do stoats or longtails living in comparable habitats. Likewise, the stoats of smaller than average body size living in northern lands do concentrate on small rodents much more than do the large British and European stoats, which eat many more rabbits (Chapter 5). But these comparisons can be tricky, since merely finding rabbit hair in a weasel's gut does not prove the weasel killed the rabbit. Lagomorph hair is very distinctive under the microscope and unlikely to be misidentified, but it looks the same in lagomorphs of any age, and the same in those that have been killed by a weasel and in those that have been scavenged on the road. Another problem of interpretation arises because predators living in different places do not always have the same opportunities to choose between large and small prey. One of the reasons that Arctic stoats eat fewer rabbits than British ones is that they meet fewer.
We can avoid that geographic problem by examining the prey choices made by male and female weasels of the same species from the same population. Because males are so much larger than females, but hunt the same prey fauna, we can study the size relationships between predators and prey independently of location.
The latest and most comprehensive analysis found no evidence of any difference in diet between the two sexes of common weasels in Britain, when the analysis of a substantial sample was corrected for age and season (McDonald et al. 2000). Remains of large prey, such as lagomorphs, were more often found in spring, and small rodents in summer, and these seasonal biases can sway simple annual averages. Earlier studies that claimed that male common weasels ate more lagomorphs, and females more small rodents, were not always well enough checked for sampling error. By contrast, McDonald et al. found firm evidence that the diets of stoats were different in the two sexes, and this difference was itself significantly affected by season. Lagomorphs were always more commonly eaten by male than by female stoats, especially in winter, and small rodents more often by females than by males throughout the year. Both sexes were equally likely to take birds' eggs.
In the far north, female stoats are matched to the sizes of vole tunnels better than are males (Simms 1979a); so, females are better at searching for and catching voles (Raymond et al. 1990), and they increase their foraging efficiency for voles more effectively when hungry than do males (Vaudry et al. 1990). It may be that the small northern female stoats are less able to kill a full-grown rabbit than a male, but, more important, they may need to eat fewer lagomorphs because they can make a good enough living on small rodents without the extra effort of tackling large prey. In most places in the northern hemisphere, female stoats can rank lagomorphs so low that they fall off the bottom of the profitability list.
By contrast, female stoats in New Zealand do not have that option. They do eat some small prey (mice and insects) more often than do males, but in many places they also take large prey, lagomorphs and rats, as often as do males (King & Moody 1982; Murphy et al. 1998). How do they do it? Some unknown proportion of rabbits and rats eaten by female stoats everywhere would be young ones taken from nests, which are not too much of a challenge. But for a stoat stalking an adult rat or rabbit, it is a matter of adjusting the equations, balancing the risk of injury if it attacks versus the risk of starvation if it does not. When the second risk looks larger and more certain than the first, an attack is probably worth while.
New Zealand stoats are the prime example supporting the conclusion of Carbone et al. (1999), that carnivores with high energy requirements cannot live on invertebrates, even though they do eat lots of them, especially the females (King 1991b). The traditional small mustelid trump card, their specialization on small rodents, is of no advantage in New Zealand, and the abundant large insects are not an adequate substitute. Stoats there often have few options other than to rely on their relatively large size and their extraordinary boldness and tenacity, and perhaps also to exploit the paralyzing shock to the victim of being attacked by a stoat of any size. For breeding females it is a life-or-death calculation. The fact that stoats have prospered so well in New Zealand shows that they often do beat the odds.
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