Sexual Dimorphism

Male weasels of all species are substantially larger than females (see Table 4.1; Figure 14.1). The reason for this difference was at first considered to have something to do with food (Brown & Lasiewski 1972). Because the two sexes are so different in size, they tend to eat different things; so, the argument ran, the difference must have arisen so that each could avoid trespassing on the other's food supplies. In times of food shortage, this trick might be valuable to both. Indeed, the overlap in the diets of males and females is substantial at all times, especially when food is short (Chapter 5).

Reasonable as this argument sounds, there is no evidence to support it. Fortunately, there is a much better hypothesis, which proposes that males and females are different in size for reasons to do with that old driving force, reproduction (Erlinge 1979a; Moors 1980). Large and small predators tend to choose prey appropriate to their size, for good reasons (Chapter 7); therefore, the observed differences between male and female weasels in their diets are the consequence, not the cause, of the difference in their body size.

Figure 14.1 Male weasels are significantly larger than females. (See also Figure 9.9.)

The fluctuations of populations of voles and lemmings over a 3 or 4-year period create alternate feasts and famines for weasels. Across these fluctuations, the chances of a female weasel producing young that survive to breed in their turn range from practically nil to very high. The population densities of the weasels themselves vary over the same period, during which the chances of a male weasel gaining several mates, or any at all, also range from good to bad.

The probability of a given individual weasel of either sex meeting a disastrous season, in which successful breeding is impossible, is high; and because weasels have relatively short life spans, the probability of surviving to a better season is low. The common problem for weasels of both sexes, living in such a variable environment, is the risk of failing to leave any young at all. The two sexes have responded to this problem in different ways.

A female weasel is an energetic little hunter, and she always raises her offspring with no help from a male. In good years the food resources available are sufficient in quantity, distribution, and rate of renewal for the females to manage very well alone. A female breeding in a good year can harvest enough prey to feed her entire litter, and those breeding in the best of the intervening years can usually manage to produce at least some young. Since the effort of raising a litter is so great, a female with her first litter has a rather small chance of having another one later in life (except when food is very abundant), so her best chance of leaving surviving offspring is to invest the maximum possible effort into the one she has. Occasional lucky observations of weasel mothers with their young show that they certainly do take this advice seriously (Chapter 9).

Of course, the task would be easier for her if the father stayed with her and helped protect and provide for the litter. But from his point of view, that would not be a good policy. In good years, the male who spreads his genes liberally stands to gain many more descendants than the male who has stayed with one mate. The best bet for a male is, therefore, to invest the minimum possible time in each of as many litters as possible, and to rely on the females to work, for their own reasons, to raise the young he fathered.

Mammals that normally produce relatively large litters of blind and helpless young tend to have stable pair-bonds among the adults, whereas those that produce a few, well-developed young, born fully furred and with eyes open, are often polygamous. Weasels, however, have relatively large litters of helpless young, but no pair-bonds at all. Why are weasels so different?

The explanation has yet again to do with the consequences of the weasels' specialization as rodent hunters. A female weasel cannot afford to accommodate a growing litter internally for too long after they begin to ruin her slim figure (Figure 14.2) (Gliwicz 1988), but neither can she afford to reduce the number of young she could produce when conditions for breeding are favorable. Her solution is to divide her resources between a large number of small kits, and to avoid carrying them about with her by dropping them in a safe place as soon as they are viable. She is then free to concentrate on bringing food to them.

Figure 14.2 A pregnant female weasel builds up caches of extra food for as long as she can still squeeze into small spaces to search for rodents. When hunting becomes difficult, she retreats to her chosen birthing den and waits. By producing extremely altricial young, born as soon as they can live outside the uterus, a mother weasel minimizes the period when the alteration of her svelte outline reduces her hunting success. Once the kits are born, their mother can leave them safe in their natal den while she hunts.

Figure 14.2 A pregnant female weasel builds up caches of extra food for as long as she can still squeeze into small spaces to search for rodents. When hunting becomes difficult, she retreats to her chosen birthing den and waits. By producing extremely altricial young, born as soon as they can live outside the uterus, a mother weasel minimizes the period when the alteration of her svelte outline reduces her hunting success. Once the kits are born, their mother can leave them safe in their natal den while she hunts.

People do sometimes report seeing male stoats, longtails, or common weasels taking food to a den, or families of weasels accompanied by both female and male adults. These males are more likely to be motivated by sex than by paternal responsibility. In fact, two breeding dens observed by Erlinge (1979a) had such narrow entrances that no male could enter. In addition, a female weasel with small young protects her family so fiercely that she becomes temporarily dominant over even the larger males. Adult female weasels of all species may have postpartum estrus, so a male bringing food to the den is most likely to be attempting to placate the female's hostility and to curry her favor to gain a mating.

Male stoats and longtails are especially likely to bring presents, but only for strictly sexual reasons. Because of delayed implantation, the young born this year were fathered last year, very likely by another male, and it is not in any male's interests to invest time and effort into feeding another male's young. On the other hand, he might be willing to invest in the female young as sexual partners for himself, and the potential rewards of gaining access to them are great enough to be worth a considerable investment. Perhaps this explains why male longtails sometimes do, apparently, help provide for a litter (Hamilton Jr. 1933; Gamble 1980). Young female longtails have to be well grown before they are ready for mating, and a male might increase the number of young females becoming independent by bringing food to them in their den. Gehring and Swihart (2004) reported that, in Indiana in May, one radio-collared male was twice relocated within a natal den containing a litter of 6- to 8-week-old young.

Because the roles of male and female weasels in reproduction are totally different, it is not surprising to find that the adaptations that each has evolved are also quite different. Females need to be highly efficient providers of prey for their young, and males need to dominate other males in competition for access to the most females. Both aims are critically affected by body size.

Small size increases hunting efficiency for a female feeding her young on voles and mice, and small size allows her to minimize the food she must eat to maintain herself, leaving more of what she catches for her young (Moors 1977, 1980). On the other hand, large size increases a male's chances of success in confrontations with other males (Erlinge 1977a) and, perhaps, somewhat lessens his chances of being attacked by a swooping raptor (Korpimaki & Norrdahl 1989a). Hence, the balance of size-related advantages favors different average body sizes for male and female weasels.

Efficiency in hunting may mean either minimizing the time taken to collect a certain amount of food or maximizing the amount of food that can be collected in a given time. Female weasels could do either, according to the need and the conditions. For example, when the young are very small, they need the mother with them for as much of the day as possible, to keep them warm and to feed them milk. She would do best to avoid letting them chill while she is out of the den by minimizing the time she spends hunting at that stage. Later, as the young grow and their demands escalate, their growth and future prospects may depend on the extent to which she can fulfill their needs. They can keep each other warm by then, so the mother would do best to maximize the amount of food she can collect during the time she has the energy to go out to hunt. It would be interesting to know whether female weasels do switch from the one hunting method to the other as their young grow.

While the explanation of sexual dimorphism that we have presented appears sound and has broad support, the evolution of sexual dimorphism in mustelids in general, and in weasels in particular, is controversial. Tamar

Dayan and Dan Simberloff (1994, 1996) argued from patterns in canine sizes that sexual dimorphism in mustelids evolved to reduce competition between the sexes for food.

Because males of each mustelid species in a community are smaller than the females of the next larger species, the animals form a series of "morpho-species" that get bigger in size as one goes from common or least weasel female to male, to stoat or longtail female to male, to polecat or marten (or longtail where stoats and longtails both live) female to male, and so forth. The body sizes of these animals do, indeed, form such a series, and Dayan and Simberloff have shown that the largest diameters of their canines also form a stepwise pattern of in-

Figure 14.3 (A) The mean largest diameter of canine teeth and the mean condylo-basal lengths (CBLs) of skulls for female and male common weasels, stoats, and polecats in Britain. These six morpho-species form a controversial example of character displacement. Although the diameters of canine teeth of weasels, stoats, and polecats form a stepwise pattern from female to male common weasel to female to male stoat to female to male polecat in Britain, we note (B) that the diameters of the canines for males and females within each species are actually more similar (the sexual dimorphism is smaller) than are skull lengths. Holmes (1987) and Holmes & Powell (1994) showed that CBLs are less dimorphic than head and body lengths. Consequently, canines appear to be responding to a form of selection different from that which has generated the large sexual dimorphism in body sizes of weasels. (Data from Dayan & Simberloff 1994.)

Figure 14.3 (A) The mean largest diameter of canine teeth and the mean condylo-basal lengths (CBLs) of skulls for female and male common weasels, stoats, and polecats in Britain. These six morpho-species form a controversial example of character displacement. Although the diameters of canine teeth of weasels, stoats, and polecats form a stepwise pattern from female to male common weasel to female to male stoat to female to male polecat in Britain, we note (B) that the diameters of the canines for males and females within each species are actually more similar (the sexual dimorphism is smaller) than are skull lengths. Holmes (1987) and Holmes & Powell (1994) showed that CBLs are less dimorphic than head and body lengths. Consequently, canines appear to be responding to a form of selection different from that which has generated the large sexual dimorphism in body sizes of weasels. (Data from Dayan & Simberloff 1994.)

creasing size (Figure 14.3). They argued that this pattern could arise only if each morpho-species eats prey that differ in average size.

Unfortunately, sexual dimorphism in weasels gets even more complicated. While the perfect pattern appears almost to exist for canine diameters, it clearly does not exist for other measurements that should parallel such a pattern in canine diameter. Thor Holmes (Holmes 1987; Holmes & Powell 1994) analyzed over a dozen skeletal, skull, and dental measures from many mustelid species and found that the bodies of male and female weasels (and males and females of other mustelid species) were nearly perfect copies of each other at different sizes except that the heads were disproportionately more similar in size than expected. Those features most critically involved in catching and eating food, especially the jaws and teeth, showed less sexual dimorphism than did body size.

In addition, Holmes found that sexual dimorphism in North American weasels is smaller where their ranges do not overlap with competitors, such as other weasels or martens. Even the data presented by Dayan and Simberloff (1994) are consistent with those of Holmes: The canines of males and females in the same weasel species are more similar to each other than are their skulls, which Holmes showed to be more similar to each other than the bodies (Figure 14.3). We think that sexual selection drives the evolution of large size in males of each species, that efficiency of reproduction drives the evolution of small size in females, and that diets of males and females are more similar than expected from their differences in body size. It is true that diet studies have documented significant differences in the diets of males and females of sympatric weasel species, especially when corrected for seasonal effects (McDonald et al. 2000), but at some times and places diet overlap is substantial.

We consider that the best explanation of all these patterns is still that the differences in the diets of the two sexes arise as a consequence of their differences in body size, which itself is a consequence of sexual selection and reproductive constraints. In other words, sexual dimorphism came first, and different diets came second. Within each species, the members of the two sexes have similar diets because every weasel must catch a variety of prey to survive.

Males and females of each species are closer in size to each other than to members of other species and, therefore, the varieties of prey they catch must overlap extensively. In Dayan and Simberloff s data, the steps in canine diameter between females and males of the same species are smaller than those between species and, therefore, the stepwise increase in canine diameters is actually less even than expected from the animals' body sizes.

So, the stepwise increase in size from the smallest to the largest mustelids in any location and the relationship between body size and prey are still topics for discussion, awaiting a new set of data that might be able to support a conclusive explanation.

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