0 grams, gained
on the average, 1.39 grams per day over intervals that averaged 44 days.
Six other young males first caught while in the weight range of 100 to
149 grams, were recaptured after intervals of 17 to 45 days and they had
gained, on the average, .92 grams per day. The corresponding figure for
four young females in the same size range was .71 grams per day. In
seven young males in the weight range 150 to 250 grams, that were caught
after intervals averaging 66 days, the gain in weight amounted to .83
grams per day. In seven females in the range 150 to 199 grams, gains
averaged only .68 grams per day. Fully grown females that are not
pregnant weigh, most typically, a little less than 250 grams while fully
grown adult males average a little more than 300 grams. Growth rate and
adult weight both are influenced to a large extent by season and even
more by individual differences. The underlying causes are obscure in
most instances, but individual rats that are still short of adult size
may stop growing for periods of months, and some individuals grow much
more rapidly than others. One male that weighed 108 grams when he was
first caught on July 3, 1951, was estimated to have been born in early
May. He was last captured 152 days later on December 2, 1951, and by
then his weight was 300 grams, representing an increase of 1.2 grams per
day. Another male that weighed only 75 grams when he was caught on
October 8, 1950, may have been less than two months old then. By
November 21, 1951, at a probable age of 15 months, he weighed 350 grams
having attained almost the maximum size. Other exceptionally large
individuals were known to be less than two years old, while those rats
that survived longest on the study areas did not much exceed average
adult size. These records seem to show that exceptionally large woodrats
are usually not those of advanced age, but are individuals which have
grown most rapidly through fortuitous circumstances, probably depending
upon both innate and environmental factors.

None of the woodrats handled was excessively fat, nor were any
emaciated. The habit of keeping on hand stores of food at all seasons
perhaps obviates the necessity for storing quantities of fat. Seasonal
trends in weight vary among individuals, and are not wholly consistent
from year to year. Rainey found that in late autumn and winter, rats
steadily gain weight reaching a peak in late February or March. However,
in the winters