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JOURNAL OF MAMMALOGY 
NOTES ON THE BREEDING HABITS OF CAPTIVE DEERMICE 
By Ernest Thompson Seton 
At Cos Cob, Connecticut, on August 22, 1905, I caught a female 
deermouse {Peromyscus leucopus novehoracensis) , and put her in a large 
cage on my study table, so as to observe her continually. On Sep- 
tember 15, she was found to have a nest of young — three in number 
and apparently newly born; therefore gestation was at least 23 days. 
The house mouse is said to carry for 21 days. My notes are as follows: 
October 7. The young mice are three weeks old, but already half 
grown and very active. They are of a lead color, blended into dull 
whitish below. One has a weak hind leg; this one often dances; that 
is, whirls around and around. I once counted 12 turns right to left, 
then changing he made 11 turns left to right, then a dozen the other 
way, then a lot so fast I could not count them. This one is evidently 
troubled with a nervous disease, but one of the stronger ones also 
whirls at times. A sudden fright sometimes makes them whirl. The 
sickly one died when five weeks old. 
October 27. The young deermice in the cage are now 42 days old 
:and two-thirds grown. One shows a general tinge of brown on fianks 
;and hams. The other is clearly moulting. Along each flank is a line 
of long unmoulted hair, on the back it is short, sleek and blue. 
April 18, 1906. On my return home, after an all winter’s absence, 
I find the old deermouse and one young one remaining. The young 
one, a female, is now of the same fawn color and white as the mother, 
but though seven months old is not quite so large, which is doubtless an 
individual peculiarity. They are active chiefly at night. (But two 
which I had later commonly came out of the nest at noon to sit for a 
time in the sun, which fell on their cage. This habit I have also seen 
in the wild ones.) 
April 27. To-day for the first time I heard the deermice squeak 
Rnd twitter; they seem to be quarrelling in the nest. On May 4 I put 
a male deermouse into the cage, and, a few days later, a second one. 
May 14. The old deermouse female has ceaselessly persecuted the 
smaller male. To-day he got out and escaped up the chimney, leaving 
the older male with the two females. The little female showed signs 
of heat to-day and yesterday, following the old male, sniffing at his 
rear and nibbling his legs. To-day I gave them a convenient nesting 
box in the cage. The young female was wildly excited, rushing in and 
out, preparing a nest. The old male is heard drumming frequently. 
