Jameson (1947:146) found an average of 3.4 young per litter in 58
litters of _M. ochrogaster_ from northeastern Kansas, mostly from
Douglas County. Martin (1956:386) recorded a somewhat lower mean of
3.18 +- 0.24 in 65 litters on the Reservation in 1950, 1951, and 1952.
For a total of 82 litters recorded from 1950 through 1956, inclusive,
I obtained an average of 3.37 +- .075 young per litter. Several litters
that were recorded were excluded from this computation as in each
instance there was reason to suspect that they were incomplete. These
included instances of females found in traps with young several days
old, females that may not have completed parturition when they were
released with newborn young, and those litters that might have
sustained losses through cannibalism by the mother or her trap-mates.
Mean numbers of young per litter were found to vary from year to year
and from month to month, as shown by the following lists: 1950, 3.0
(13 litters); 1951, 3.5 (23 litters); 1952, 3.5 (11 litters); 1953,
3.4 (5 litters); 1954, 3.4 (15 litters); 1955, 4.1 (7 litters); 1956,
3.8 (5 litters); January 2.0 (1 litter); February 3.5 (4 litters);
March 4.5 (4 litters); April 3.9 (12 litters); May 3.3 (25 litters);
June 3.0 (9 litters); July 2.7 (4 litters); August 2.9 (7 litters);
September 2.8 (6 litters); October 3.4 (7 litters); November 5.0 (2
litters); December 4.0 (1 litter).
These differences can be logically explained on the basis of changes
in the average age of the breeding females in the population. On the
average, with greater length, weight and age, females produced
progressively larger litters, although individuals did not necessarily
conform to this general trend. For 24 females recorded in 1954-1956
and measured within a few days of birth of their litters, average
length was correlated with number of young as follows: 6 young, 163.5
mm.; 5 young, 158.0 mm.; 4 young, 157.7 mm.; 3 young, 154.6 mm.; 2
young, 160.5 mm.
For 48 other females, recorded in 1950-1953, that were not measured,
but that were mostly assignable to broad age groups on the basis of
their individual histories in the trapping records, the following well
defined trend was demonstrated.
Table 3. Number of Young per Litter Correlated with Age or Size
of Female.
=====================================+============+==================
| Number | Average
Age or Size Group of Female | of females | number of
| in sample | young per litter
-------------------------------------+------------+------------------
More than one year old | 4 | 4.25
6 to 12 months old | 16 | 3.50
Large (age indeterminate) | 9 | 3.44
2 to 5 months old | 9 | 2.90
Small and medium (age indeterminate) | 10 | 2.80
-------------------------------------+------------+------------------
It seems that the exceptionally high average numbers of young per
litter in March and April result from the breeding females in those
months being nearly all fully mature survivors of the previous year.
In summer, when many females that are only a few weeks old become
pregnant, the average litter declines to less than three young. The
small average litter of 3.0 young for 1950 probably resulted from the
fact that the population on the Reservation was then expanding rapidly
in the newly favorable habitat created by one year's crop of
vegetation after discontinuance of grazing, and had an unusually high
percentage of breeding females that were not fully adult.