Section II. LIFE HISTORY AND BEHAVIOR
1-7.
GENERAL
Information on the life history and behavior of rodents suggests a great similarity
in their birth, development, and general activity. Facts on any one species are sporadic
and incomplete. Because of this, these phases of their biology are treated as a unit
rather than by individual species. In this way a more coherent idea of the rodent's life
may be gained.
1-8.
BIRTH AND DEVELOPMENT
a. The life of the average rat or mouse is fairly short--approximately one year--
and the young mature rapidly. The Norway and roof rats are born, on the average, 22
days after successful mating has been accomplished. The house mouse requires even
less time, producing young at the average of 19 days after mating. Female rats and
mice can mate within 48 hours after giving birth, which makes it possible to produce
young almost continuously. Under ideal conditions, a single pair of captive rats has
produced 1,500 descendants in one year. Fortunately, several things act to slow this
reproduction. Mating is not always successful, or is not even attempted immediately
after the young are born. Then, too, if the female is nursing young and is also pregnant,
birth of the new litter may be delayed as much as a week, with the delay depending on
the number of the nursing young and size of the unborn litter. No other rat matches the
lesser bandicoots reproductive rate. The females begin bearing young at 2 months and
produce 7 pups every month thereafter.
b. Young rats and mice enter this world none too gently, and birth is precarious
at best. Large litters are the rule, and in the confusion at birth some of the newborn
may be killed and eaten. The female is very nervous, and disturbances to the nest, or
intrusions by others, may result in destruction of the litter by the mother. Often following
such disturbances, the mother will move the young to another place and many litters
thus moved do not survive. It has been observed that slightly more than 50 percent of
all rats and mice die before being weaned, and that only 5 percent live for a year.
c. Newborn rats and mice are practically helpless. They are hairless, their eyes
and ears are not open, and they have a very limited means of mobility. During this early
period, mice and rats respond largely to heat and touch, although they probably have
some ability to smell. Their ears open in about 3 days, but rats show no signs of
hearing until 12 days of age, whereas the mouse may be sensitive to sound at only 4
days of age. In all species, fine hair appears on the body in about a week and their
eyes open at about 12-14 days of age. At this age, the young are already active and
enter a period of intense investigative behavior. They begin to take excursions out of
the nest, often as result of following the mother when she leaves. For about 3 weeks,
the young depend on the mother for food but begin to take solid food in the middle of
the third week. If forced, the young can survive without their mother after 3 weeks of
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