June 7, 1924 
Variation in Microtus Montanus Yosemite 
987 
of obtaining as nearly as possible a correct average of measurement for thoroughly 
normal, sexually mature individuals of the two sexes. The tables give informa¬ 
tion that may be desired concerning such specimens as might possibly be consid¬ 
ered unusually large adults and juveniles. 
For males the average head-and-body length is 122 mm., with extremes of 134 
and 110—a difference of 24 mm., or 19.7 per cent of the average. The females 
average 118.2 mm., with extremes of 130 and 106, again a difference of 24 mm., 
or 20.3 per cent. The above figures indicate that males are consistently about 4 
mm. larger in this measurement than are females, and the writer believes that this 
sexual difference in size is real, as it is corroborated by other data. 
LENGTH OF TAIL 
This measurement was secured by placing a millimeter rule at the upper side of 
the base of the tail of the unskinned animal and then bending this member at a 
right angle to the axis of the body, measuring to the tip but not including the 
terminal hairs. 
In the case of Microtus californicus sanctidiegae it was found that thereunay be 
as much as 5 per cent variation in this measurement, depending on the length of 
time after death that it was taken. Add to this another 5 per cent to account 
for the different results obtained by various collectors, as well as the fact that 
throughout life the tail tip is especially subject to injury, and it is evident that this 
dimension is none too reliable. Extreme variation occurs even in the case of 
females. In males this measurement should be considered with decided caution, 
because the tails are so often injured and materially shortened while the animals 
are fighting among themselves. The average length of tail in males (with oldest 
and youngest specimens eliminated) is 48.2 mm., with extremes of 55 and 42—a 
variation of 13 mm., or 27 per cent, of the average. For females these figures are 
46.5 mm., with extremes of 50 and 40, showing a variation of 10 mm., or 21 per 
cent. This difference of 6 per cent in the amount of variation in the two sexes 
may be partially ascribed to the greater propensity of the males to injury. In 
both sexes the tail averages 39 per cent of the head and body. Age causes very 
little variation from this figure. In the case of the six youngest females the per¬ 
centage rises to 41, but this difference is almost too slight to be taken into con¬ 
sideration. 
LENGTH OF HIND FOOT 
There is probabl} 7 less error in measuring the length of the hind foot from the 
heel to the tip of the longest claw than in either of the other two body measure¬ 
ments. There is little effect of rigor mortis upon this member; hence for all 
practical purposes the same figure will result regardless of how long after death 
the measurement may be taken, and there need be no difference in the results 
obtained by various collectors. The normal error in making this measurement, 
if half millimeters be recorded, will be about 0.4 mm., or slightly less than 2 per 
cent. In figure 1, of course, this possible error for individuals is magnified al¬ 
most five times, although the average for the series is not affected. 
There is considerable individual variation in the size of the foot, but practically 
none with age in the specimens at hand. In fact, the 12 largest males have feet 
that average a shade shorter than those of the 12 smallest; but this difference is 
so slight (0.1 mm.) that it is of little significance. It will thus be seen that full 
growth is attained by the feet at an early age, and for this reason foot length 
often is of much value in identifying juveniles of different species. 
The hind foot of males averages 22 mm. in length, with extremes of 23 and 21, 
a variation of 9.1 per cent of the average; that of females, 21.6 mm., extremes 
