The Reproductive Anatomy Of Weasels

The testes of males are simple oval sacs within the furry scrotum. The coiled tube of the vas deferens leaves the epididymis, at the distal end of the testis, and ascends back into the body cavity again. The penis is stiffened by the baculum, a small rod-shaped bone attached to the pelvis by muscles at one end, which acts as a rigid support during copulation. The urethra fits within a groove on the underside of the baculum, shown in Figure 9.1. Normally the whole apparatus is hidden inside the body, and in living animals the baculum can only be felt through the skin, like a matchstick lying under the midline between the small tuft of hair at the orifice and a point just forward of the testes. But when the baculum can be dissected out, as from carcasses, it becomes one of the most informative items of a weasel's anatomy.

The baculum is useful to biologists for two reasons. One is that the shape and size of the baculum are reliable characteristics of each species (Figure 9.1), so that an incomplete skeleton of a male can often be identified from the baculum alone (Baryshnikov et al. 2003). The bacula of all species are roughly the same in general design: Each has a more or less straight shaft, with a curve at the distal end and a knob at the proximal end. But the baculum in nivalis is relatively short and thick, with the distal curve formed into a distinct hook; the bacula of erminea and frenata are longer and more slender, and their distal curves are more gentle.

young adult

Figure 9.1 The bacula of male weasels are diagnostic of both age and species. (Redrawn from Burt 1960.)

frenata

Figure 9.1 The bacula of male weasels are diagnostic of both age and species. (Redrawn from Burt 1960.)

The second reason is that, within each species, the baculum indicates age. Wright (1950) showed, by a series of laboratory experiments, that the development of the baculum is controlled by androgens. The proximal knob is characteristic only of adults; juveniles have a thin shaft hardly broader at the end than along its length. In males castrated as juveniles, the knob did not develop at all. The essential role of hormones in this development was proven by the classic reverse experiment: Castrated juvenile males treated with implants of testosterone propionate developed nearly normal knobs. In intact males, the knob first develops at puberty and continues to grow in size, and therefore in weight, for several years, and probably throughout life (Figure 9.2). Presumably, the strengthening of the bone stimulated each breeding season by testosterone has a cumulative effect.

In male stoats and longtails, it is possible to define the minimum baculum weight marking sexual maturity, although the actual threshold figure depends on general body weight. For example, in the small male stoats of northern Ireland (average body weight 233 g), the minimum baculum weight of adults was 30 mg (Fairley 1971); in the larger males from the Netherlands (284 g), it was 32 mg (van Soest & van Bree 1970); and in the even larger males from New Zealand (324 g), it was 38 mg (King & Moody 1982). Similarly, in the small subspecies of longtails the bacula are smaller than in the larger subspecies (Wright 1947). In erminea and frenata the weight of the baculum clearly distinguishes the young males, but in nivalis the transition from the juvenile form

Figure 9.2 The weight of the baculum has long been a useful method of distinguishing subadult males (independent but not yet fully grown or sexually mature) from adults. These bacula of New Zealand stoats, however, all of known or part-known age, show that baculum weight increases for several years, perhaps throughout life. (Redrawn from Grue and King 1984.)

Figure 9.2 The weight of the baculum has long been a useful method of distinguishing subadult males (independent but not yet fully grown or sexually mature) from adults. These bacula of New Zealand stoats, however, all of known or part-known age, show that baculum weight increases for several years, perhaps throughout life. (Redrawn from Grue and King 1984.)

and weight to those of the adult is smooth, allowing no separate age categories based on baculum weight (Table 9.1).

In females, the uterus is a simple tube with two branches (usually called horns) joined at the base and pressed against the dorsal side of the body cavity. The ovaries are quite conspicuous, round and flattened and rather yellowish, lodged in the free ends of the two uterine horns. The ova develop in follicles just under the surface of each ovary, and when they are ripe, the follicles burst and the ova are released to pass down the fallopian tubes to the uterus. The uterus enlarges somewhat before estrus, but this phase is shortlived and seldom observed except in unmated females in captivity. Estrus is best detected externally, from the swollen, moist, doughnut-shaped vulva. A preserved uterus betrays no sign of whether it is carrying young, or has done so before, until the embryos become visible as evenly spaced swellings about 3 weeks before full term.

Table 9.1

Baculum Weight

in Relation to Age and Species

nivalis

erminea

frenata

Young

5-21 mg

10-30 mg

14-29 mg

Adults

15-59 mg

50-89 mg

53-101 mg

Locality

Britain

Britain

Montana

Reference

(Hill 1939)

(Deanesly 1935)

(Wright 1947)

The mammary glands are set toward the rear of the long abdomen, and are invisible except during lactation. The nipples are tiny pimples hidden under the fur, both in juveniles and in adults until shortly before a litter is born. The nipples of adult females that have recently suckled young remain elongated for some months, but those of adults that have lost their young or failed to rear them remain practically invisible. Female stoats and longtails usually have four or five pairs of nipples, and common and least weasels have three or four pairs. Only those nipples that are being suckled remain active, so fewer than the total possible number will be visible on a female with a small litter.

The most useful feature of the reproductive anatomy of females, at least for researchers interested in weasel reproduction, is the corpus luteum. Each ovum released at ovulation leaves a space behind, which is quickly filled with a dense mass of hormone-producing cells. These cells are slightly yellowish in color and are clearly visible to the naked eye, hence the name (corpus luteum is Latin for "yellow body"). One corpus luteum forms for each ovum released. The function of the corpora lutea is to produce progesterone, the hormone needed to maintain the pregnancy. The beauty and usefulness of the corpora lutea (plural) lie in the two facts that ovulation in weasels has to be induced by copulation and that each corpus luteum marks the site of an ovum that has been released. Hence, counting corpora lutea is a convenient way to estimate which animals have mated, plus their total potential fecundity (King 1981a), and these are very important data for population studies (Chapter 10).

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