Biology Reference
In-Depth Information
own size and weight. Incidentally, although weasels have excellent eyesight and hearing, they often use
their keen sense of smell to track their prey.
Delayed implantation, already mentioned prominently, has some major advantages for a predator
with the weasel's habits. In order to understand these advantages, let's start with the birth of a litter of
kits, normally in April. There are usually six or seven kits, but litter size can vary widely.
The kits are blind, nearly hairless, and extremely tiny—premature by the standards of most mam-
mals. This arrangement is vital to the mother, who must retain her slender outline and light weight in
order to hunt successfully and survive throughout her entire pregnancy.
Breeding occurs three months later, in July or early August. The male comes to the den and woos the
female with presents of dead mice and other prey; this allows the female to spend time breeding, rather
than hunting.
Now comes the most unusual part—one that clearly demonstrates what a great advantage delayed
implantation is to weasels. Not only does the male breed the mother, but her daughters as well! Because
the fertilized eggs simply float around for about eight months before attaching to the uterus walls and
starting to grow, the little juvenile females have time to grow to adult size before the fetuses develop
within them. Since the average life span of weasels is so brief—about eighteen months—this breeding
strategy ensures ample numbers of young to perpetuate the species.
By virtue of one of evolution's quirks, the least weasel is one of only two North American mustelids
that don't utilize delayed implantation. Perhaps this is because the tiny predator breeds at various times
of the year and may have more than one litter annually. Instead, the least weasel has a gestation period
of thirty-five days; however, the kits develop with enormous rapidity during the final few days before
birth, thus sparing the female from carrying a heavy load while hunting during most of her pregnancy.
Weasels seem to have little innate fear of humans. In fact, in their pursuit of mice they'll occasionally
enter people's houses and remain there for some time, and they commonly search and inhabit barns,
sheds, and other outbuildings.
I've been fortunate enough to observe this behavior on a number of occasions. In the most recent
episode, I had skinned a deer in our shed a day or two before, and draped the hide over a rack. When I
entered the shed, a weasel slithered off, oozing through a crack with lithe, serpentine movements. Mo-
ments later it reappeared, inspected me for a moment with curious, beady eyes, then ducked out of sight
again.
Gradually becoming bolder, it soon began to gnaw at pieces of meat still attached to the deer hide,
and became almost oblivious to my presence. I went back to the house, grabbed my camera, and pro-
ceeded to take a number of pictures of the weasel from a distance of no more than five feet, while it
largely ignored me. This was typical of the weasels I've observed in our outbuildings.
We generate our own electric power by photovoltaic cells, with a backup generator. One winter even-
ing I stepped into the generator house to start the generator, turned on the light, and was startled when
