SUMNER — LONGEVITY IN PEROMYSCUS 
79 
LONGEVITY IN PEROMYSCUS 
By F. B. Sumner 
Various writers have doubtless recorded observations bearing upon 
the longevity of rats and mice, but very few of these have come to 
my notice. No careful search for such records seems profitable, how- 
ever, in connection with the publication of the following brief notes of 
my own. I shall therefore proceed without making any extensive 
display of references to ‘Hhe literature,^’ and shall ask the indulgence 
of those whose publications I have overlooked. It is worth mention- 
ing that I find rather wide differences among the estimates which lie 
at hand. Weismann, in his well-known essay on ^^The Duration of 
Life” states that a mouse” (presumably he means a house-mouse) 
lives six years, while the anonymous author of the booklet Fancy 
Mice” assures us that, if properly cared for, mice ‘‘will live for two or 
three years, and then die of old age.” The white rat, according to the 
extensive observations of Donaldson and his co-workers, is to be 
regarded as very old at the age of three years (i.e., comparable with 
a man of 90), while the female commonly becomes sterile at the age 
of 15 to 18 months. 
With deer-mice, as in the case of most wild animals, there would 
seem to be no way of testing the natural span of life, except by rearing 
them in captivity, from birth until the time when they finally succumb 
to “old age.” The period thus measured doubtless does not fairly 
represent the average life cycle in nature. On the one hand, the condi- 
tions of captivity, as I have more than once pointed out, do not make 
for normal development on the part of all individuals. Sterility, 
stunting and even deformity, may be the result of these artificial condi- 
tions. In the mouse’s favor, on the other hand, is the fact that it is 
protected and fed throughout life, a circumstance which is of particular 
importance during the period of old age. Thus, several of the mice 
referred to below became extremely feeble many months before death 
occurred. Such animals would doubtless have died or been killed 
much earlier in a state of nature. 
In September, 1916, ten mice (five of each sex) were selected from 
the first cage-bred generation of Peromyscus maniculatus gamheli, 
belonging to the local (La Jolla) strain. These mice were, at this 
time, about a year old. They were normal, healthy specimens, but 
not otherwise exceptional. Two of these animals met with an acci- 
dental death, when more than two years old; another met with a 
