The number of mammae characteristic of a species of vole may be a rough
guide to the average size of a litter for that species. The prairie vole
has fewer mammae (three pairs) than some other voles in North America,
and might, therefore, be expected to have smaller litters. Fifty-eight
gravid females of _Microtus ochrogaster_ examined by me had an average
of 3.4 embryos each; the number of embryos ranged from one to seven.
Hamilton (1936a) gave 5.07 as the average number of young per litter in
_M. pennsylvanicus_. Hatfield (1935) stated that _M. californicus_ has
an average of 5.7 young in a litter. Both _pennsylvanicus_ and
_californicus_ normally have four pairs of mammae. The expectation as to
the size of the litter seems to be realized. In the prairie vole one
pair of mammae is pectoral and two pairs are abdominal. Usually a
lactating vole showed evidence of only the abdominal mammae having been
in use.
The size of litters was found to vary with the season of the year (see
table 9). Gravid females were collected in every breeding month except
September.