June 7, 1924 
Variation in Microtus Montanus Yosemite 
989 
than for males, and if allowance is made for the fact that the three heaviest females 
were pregnant, the true variation due to age is yet smaller. As the average 
weight of females is considerably augmented by the presence of large embryos 
the true difference between the average weights of the two sexes must be con¬ 
siderably greater than the 4.5 gm. shown by the tables. 
It is readily apparent that weight is to a large extent correlated with both 
length of head and body and condylo-basilar length of skull, especially in the 
case of males; but there is less connection in the female series, complicated as are 
the weights of many of them by the presence of embryos. There are many 
exceptions to this rule, however, constituting the normal individual variation. 
EMBRYOS 
The period of gestation for Microtus, as determined by Bailey ( 8 ) in the case 
of the species pennsylvanicus, is 21 days. For the first few days after impregna-, 
tion the embryos are so small as to escape detection by the average collector, 
who gives but a hasty glance into the abdominal cavity. The writer has no 
means of knowing the exact time at which appreciable swellings within the uterus 
indicate clearly that the animal is pregnant, but it is certainly more than three 
days. If we say three days, to be conservative, then only six out of seven preg¬ 
nant females are recorded as in this condition by the careful collector. In the 
present series of 39 sexually mature females, 19, or roughly one-half, were defi¬ 
nitely with young. It is entirely possible for meadow mice to bring forth a litter 
regularly about every 20 days, as determined by Bailey, and although such 
fecundity is doubtless exceptional in the wild state it probably is closely ap¬ 
proached when a period of optimum population among the meadow mice is draw¬ 
ing near. The writer presented data elsewhere (5) which have caused him to 
believe that during the latter stages of and immediately following a peak popu¬ 
lation of small rodents which fluctuate markedly in numbers during certain periods 
there is a definite reduction in reproductive power. Indications in the present 
case, therefore, are that the females were bearing young at neither the maximum 
nor minimum rates and that the figures here presented are very likely average 
for meadow mice during midsummer. 
The measurements of embr3 r os given are merely for the length of the mem¬ 
branes in which they were contained, not for the total length of the fetus. These 
measurements vary from 2 to 16 mm., the latter figure being for examples at 
practically full term and which would probably have been born within 24 hours. 
Female No. 3844 contained 7 embryos, some of which were 3 and others 7 mm. 
in length, this variation probably being the result of some pathological condition. 
The average number of embryos for the pregnant females was 5.6. The 9 largest 
pregnant specimens contained an aggregate of 56 embryos, or an average of 6.2 
each, while the nine smallest held only 44, an average of 4.9. This difference in 
fecundity according to age may be fortuitous but is probably not, for it is in 
line with what is known of the subject from experience with domestic animals. 
The weight of the eight embryos, practically full term, of female No. 3868 (an 
alcoholic specimen) was 17.3 gm. Only 8 of the 19 largest females contained 
embryos, while 11 of the remaining 20 subadults were in this condition. These 
figures may indicate a slight waning of the fertility of some of the oldest indi¬ 
viduals. 
MYOLOGY OF THE HEAD 
The writer thoroughly believes that the development of the skull after birth is 
chiefly influenced directly by the muscles and their mechanical effects while exert¬ 
ing stresses upon their insertions upon the bones; hence indirectly by the character 
of the food and feeding habits (including manner of mastication) of the animal. 
