August. For example, a young female first
caught on June 15, 1951, weighed only 150 grams, but by August 10 she
had gained to 220 grams (probably in pregnancy) and had a vaginal
orifice. Of 35 adult and subadult females examined by Fitch in October,
eleven had a vaginal orifice, the latest on October 18. Of these eleven
showing signs of breeding, four at least had not yet produced litters,
judging from the undeveloped condition of their mammae, and others that
showed evidence of recent lactation probably included young of the year
that had bred in August or September. One female gave birth to a litter
in a trap on the night of October 6, 1950. Of 32 adult and subadult
females recorded by Fitch in November, all were sexually quiescent, with
the possible exception of one having a partially open vagina on November
10. All females taken in December, and most of those taken in January,
also were sexually quiescent. January 20 was the earliest recorded date
for a female with a vaginal orifice. Females examined in February mostly
were perforate and many of them appeared to be in oestrus. One female
trapped on February 19, 1950, weighed only 140 grams and was still
imperforate. Another, weighing 200 grams on February 3, 1952, still was
imperforate, but by February 27 she was perforate and appeared to be in
oestrus. An adult female that appeared to be in oestrus on February 3,
1952, was imperforate on February 10.

_Growth_

At birth woodrats weigh approximately 10 grams or a little more. In a
litter born in captivity and kept by Rainey, the average gain amounted
to a little more than 1.5 grams per day during the first two months, but
in the third month it was somewhat less. As this was an unusually large
litter, of five young, one more than the female's teats could
accommodate, their growth may have been a little less rapid than in most
of those under natural conditions. At an age of three months they
averaged approximately 120 grams. The three males consistently exceeded
the two females.

[Illustration: FIG. 3. Typical growth curves for male and female
woodrats; early stages are based on the litter of a captive female,
later stages on average gains of recaptured juveniles and subadults,
excluding those that seemed to be stunted. Solid line represents males
and broken line represents females.]

Young weighing less than 100 grams are rarely caught in live-traps. Four
young, all males, first caught at an average weight of 8