From Birth to the Opening of the Eyes

Infant weasels look rather alike in all species, both in their appearance at birth and in their early physical development. They are all born completely helpless, and all grow in the same way, but least and common weasels develop more rapidly than do stoats and longtails, and they reach the milestones of development at younger ages (Table 9.3). For example, although young stoats and longtails are born larger than young common weasels, they grow more slowly, and are 6 to 8 weeks old before they are again larger than common weasels of the same age.

Newborn young weigh 1 to 4 g, and at birth, or soon after, their pink skin is covered with a fine, pale natal down, which gives them a silvery appearance. They do not grow the characteristic brown and white fur of adults for several weeks. The eyes can be seen only as indistinct bluish-black dots crossed by the sharp horizontal line of the tightly closed eyelids. The ears are very small, pressed against the side of the head, and often have a waxy deposit in the ear channels. They can easily be told from infant rodents, because they already have the long necks typical of the weasel family, and the front limbs seem to be placed almost halfway down the total length of the head and body.

All four limbs are short, weak, and hardly jointed, but already furnished with broad paws, and the very short toes have fine well-developed little claws. The prominent ribs make the tiny bodies (the width of a pencil) appear to be closely segmented; Ternovsky (1983) commented that "in appearance they are reminiscent of large ants." The tail, little longer than the limbs, is cone-shaped. The urethral opening of the males lies halfway between the navel and the prominent anus; the vulva of the females lies directly in front of the anus.

Newborn young have no teeth, but they do have powerful equipment and instincts for sucking. The jaws are relatively shorter than those of adults, and a massive shovel-shaped tongue takes up most of the space inside the mouth. Left alone, newborn young lie in a heap together, crawling under each other to avoid disturbance; if one is separated it immediately struggles back into touch with the rest. Almost from birth they can produce a fine chirping sound, usually a sign of protest or distress. Most observers report that, at this stage, males and females are roughly the same size, and both are, on average, equally represented.

Table 9.3 Development of Young Weasels

Common Long-tailed

Common Long-tailed

Table 9.3 Development of Young Weasels

Least weasel

weasel

Stoat

weasel

Total gestation

34-36 days

35-37 days

220-380 days

205-337 days

Birth weight

1-2 g

1-3 g

1-3 g small races

3-4g

3-4 g Britain,

New Zealand

Birth coat

Naked

Naked

Fine white hair on

Fine white hair

back, rest naked

all over

Growth of mane

None

None

14-22 days

none

dorsal fur

18 days

21 days

21 days

28-35 days

black tail tip

None

None

42-49 days1

21 days (?)

Teeth, milk

11-18 days

14-21 days

18-28 days

21-28 days

permanent

4-6 weeks

8-10 weeks

10 weeks+

10 weeks+

Opening of eyes

26-30 days

28-32 days

30-42 days

35-37 days

Eat meat

3-4 weeks

3-4 weeks

4-5 weeks

4-5 weeks

Lactation lasts

4-7 weeks

4-12 weeks

4-12 weeks

5-12 weeks

Play outside nest

4 weeks

4 weeks

5-6 weeks

5-6 weeks

Kill prey

6-7 weeks

8 weeks

10-12 weeks

10-12 weeks

Adult size, M

3-6 months

3-6 months

12 months

12 months

Adult size, F

3-4 months

3-4 months

6 months

6 months

Sexually mature, M

3-4 months

3-4 months

11-12 months

11-12 months

Sexually mature, F

3-4 months

3-4 months

4-6 weeks

3-4 months

1. Tail tip begins to darken at about 18-20 days.

(Hamilton Jr. 1933; Sanderson 1949; East & Lockie 1964, 1965; Hartman 1964; Heidt et al. 1968; Heidt 1970; Müller 1970; Blomquist et al. 1981; Ternovsky 1983; Polkanov 2000; Svendsen 2003).

1. Tail tip begins to darken at about 18-20 days.

(Hamilton Jr. 1933; Sanderson 1949; East & Lockie 1964, 1965; Hartman 1964; Heidt et al. 1968; Heidt 1970; Müller 1970; Blomquist et al. 1981; Ternovsky 1983; Polkanov 2000; Svendsen 2003).

The litters of common and least weasels tend to be somewhat smaller (usually four to eight) than those of stoats and longtails (usually six to 12; Table 9.2), for reasons explored in Chapter 14.

At first, nestling weasels have no proper fur and cannot maintain their own body temperatures. When the mother is not in the den they huddle tightly together for mutual warmth. Even so, if their body temperatures drop below 10°C to 12°C (Segal 1975), the young go into a temporary cold rigor. Their pulses and breathing slow, their metabolism and growth slip into a lower gear, and they become stiff and cold to the touch. The effect is somewhat like hibernation, except that it is rapidly reversible as soon as the mother returns to the nest.

The reason for this curious habit appears to be connected with energy conservation. When the mother is with them, she provides both warmth and food, and they cuddle up close to her and channel as much as possible of the energy she provides into growth (Figure 9.7). When the mother is away, if they tried to maintain normal temperature they would have to draw, from their own meager resources, energy that they need for growth. The advantage to the young of

Figure 9.7 A mother weasel must spend as much time as possible with her young while they are very small. When the young get chilled, they stop growing. Here a female stoat sleeps with her young, who have recently nursed.

not attempting to keep themselves warm when left alone is that they maximize their growth rate when their mother is with them, and when she is away they reduce the chances of running out of energy altogether, even for staying alive, before she comes back. The disadvantage is that, if hunting is bad and the mother has to be away a lot, the young have few chances to grow at all. When that happens, they end up permanently smaller than usual, or dead. In a very large sample of stoats collected from beech forests in New Zealand, where food supply varies substantially between years, we showed that the young of both sexes born in hungry seasons were fewer and grew into smaller adults than those born in good seasons (Powell & King 1997).

Unweaned nestlings spend most of their time asleep. They squeak and chirp in response to disturbances near them, but otherwise they wake up only to suckle and to defecate. The den is kept quite clean because their mother licks up the feces of the young—and, indeed, they apparently perform this service for each other. They make no attempt to leave the den, although the mother may move them if the family is threatened by any interference or by bad hunting in the immediate vicinity. Then she carries each one in turn (Figure 9.8), darting through the undergrowth to the new den. She leaves each of them there, one by one, and returns for the others.

Michael Hitchcock, an English gamekeeper, was once standing by a pile of rotten logs when a common weasel came out, and studied him for a few seconds at a range of hardly more than an arm's length. She then ducked back into the pile of logs and emerged carrying a tiny, blind young one. She carried the young high off the ground as she ran along the hedgerow out of sight. Hitchcock waited, and she returned to repeat the procedure with a total of six young. After the first one, Hitchcock kneeled down by the burrow and watched the operation at close range. He was especially struck by the total fearlessness of the mother weasel in her domestic crisis. The young remained limp and passive in the tragschlaffe ("carry-sleep") position while being carried, just as adult females do when being carried about by a male during mating, and made no sound unless the mother grabbed them in a sensitive place.

When the young are 2 to 3 weeks old, their milk teeth begin to erupt, as razor-sharp miniature editions of their adult meat-eater's teeth. As with most mammals, weaning is a gradual process in young weasels. While still blind and deaf (still <4 weeks old; Table 9.3), they chew vigorously on mice that their mother has opened for them, or on small pieces of meat, skin, or bones. Nonetheless, they continue to suckle for several more weeks. They begin to be aware of their surroundings, and they respond to squeaking noises or to human speech by raising their heads, opening their mouths, and hissing faintly. Some make feeble attempts to strike if provoked.

Eventually they try to get to their feet, usually collapsing again immediately. Crawling begins as an unsteady, circular exercise. Before long, the young are

Figure 9.8 The mother weasel carries her kits in the typical carnivore way, grasped gently around the body.

able to crawl short distances, in a more or less straight line, before their mother can retrieve them. From about the time that meat enters the diet at 3 to 5 weeks, depending on the species, youngsters learn to defecate outside the entrance of the den, and from this time onward, the size difference between males and females becomes more and more noticeable.

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