The seasonal cycles of molt and reproduction are closely related and are coordinated by the neuroendocrine system. The nerves receive stimuli from the outside world (Is it day or night? Is it warm or cold?); the brain then instructs the endocrine organs to secrete the appropriate hormones. Hormonal messages travel, via the bloodstream, more slowly than the electrical messages that flash along the nerves, but they reach every cell of every limb, organ, and tissue in the body. Cells for which the message is not intended will ignore it; cells sensitive to it will swing into coordinated action, even if they are spread from one end of the body to the other—as are the skin follicles.
The pivot of this system is the pituitary, a small gland under the base of the brain (see Figure 9.5), constructed so as to maximize the contact between brain and circulation. When the eyes perceive the lengthening of the days after the winter solstice, the pineal gland produces less melatonin, a substance that is released only in darkness and has an inhibitory effect on prolactin. Higher levels of prolactin in the circulation are a signal to the pituitary to release gona-dotropins, which stimulate the gonads to prepare for the approaching breeding season. The same message is also received and understood by the hair follicles. So, in early spring the testes and the hair follicles of males enlarge together, and the testes begin to manufacture testosterone and sperm while the hair follicles accumulate melanin and manufacture hair (Wright 1942b).
The autumn molt takes place in the opposite conditions, with shortening days, increasing levels of melatonin and falling levels of plasma testosterone. The connection between the two systems can be demonstrated experimentally by showing that it fails in stoats in which the pituitary has been removed (Rust 1965). The same effect can be achieved by inhibiting the pituitary, for example, with melatonin. The large testes of five intact captive stoats, given a melatonin implant in spring, regressed within 6 weeks to near-winter condition, while at the same time the animals molted into their normal winter coat (Feder 1990). Conversely, there is a significant correlation between the spring rises in testis weight and in readiness to molt, well documented in wild stoats in New Zealand (King & Moody 1982).
In females the pituitary gonadotropins also have a stimulating effect on both the hair follicles and the ovaries, at least to start with. But when the ovaries begin to manufacture large quantities of the female sex hormone estrogen, things suddenly change. Among the common weasels examined by King (1979) were some that had started their spring molt and had produced a dark dorsal stripe of new brown hair, but had no molt patches on the insides of their skins. These females were all in breeding condition, and it appears that, when they came in heat, molting had stopped. In the domestic ferret, molting is inhibited when the blood contains high quantities of estrogen; possibly the same thing happens in common weasels. However, when the heat period subsides, molting is resumed, and several female weasels were found to be still molting late into spring.
Molt and reproduction are both active processes for which the small female must find energy, in addition to the already considerable energy required just for daily living. Van Soest and van Bree (1969) suggested that female stoats complete their spring molt earlier than males, before their embryos implant, because they could not provide the energy for molt and gestation simultaneously. But female common weasels are able to do this, because they resume molting after they have been fertilized; and if they can do it, there is no obvious reason why stoats could not.
If there is any difference between the two species, it is probably less to do with energy conservation than with their different reproductive cycles. Female stoats are in heat in summer, so their spring molt is not interrupted by estrogen, and there is time for them to finish growing their summer coats before their embryos implant. The winter coat is a different matter, since it is thicker and no doubt more expensive in energy to grow, but by that time both stoats and common weasels are in their winter (anestrous) period, and no energy need be diverted from the serious business of preparing for the winter.
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