BULLETIN FLORIDA STATE MUSEUM VOL 34(6)


the proportion of non-reproductive individuals nearly tripled in April 1984, in
comparison to the preceding March.
 The onset of the regular breeding season in April is indicated by the
sudden appearance of perforate females in that month in all three years (Fig.
10). The female cohort, viewed by themselves during the main breeding
season, shows that at any one time most females were either pregnant or
lactating (Fig. 11, Table 11). Structure during the main breeding season is
very similar for 1982 (Fig. 14) and 1984 (Fig. 16). The sample shown for
August 1983 (Fig. 15) reflects the origin of the sample: all P. natalensis
examined for that month came from the area in which the mark and release
study was done, which showed progressively disparate sex ratios in favor of
females in each of the three years (Tang Christensen pers. comm.). This was
also the only area on campus from which viral seropositives were obtained. It
is tempting to speculate that infection by this virus, which cross reacts with
Lassa and Mopeia viruses, caused differential mortality against males.
 Influence of habitat.-- From July through April, trap success for Praomys
natalensis in uncultivated grass areas was approximately half that obtained in
fallow fields (Table 12). In Ma and June, however, there was no difference
between habitats.
 Population structure was similar in both habitats when densities were at
peak, as shown by the female cohort in October (Fig. 22).
 In other seasons, though, there were differences which probably reflect
the influence of both rainfall and habitat upon reproduction. This is shown by
the composition of the female cohort from December 1982 through March
1983 (Fig. 23).
 In December, a few reproductive females were taken in fallow field.
Those taken in January from grass areas (Fig. 25) were either young, non-
reproductives or post-reproductive females of the preceding year. In February
and March, there were many newly post-reproductive females present in fallow
field as well as many newly weaned juveniles, demonstrating production of a
litter in January (Fig. 26). The proportion of post-reproductive females had
increased in samples from grass in February (Fig. 23), but no juveniles were
taken and these females could have been from the preceding year. Even more
post-reproductive females were found in fallow field in March (Fig. 23), while
there was evidence that reproduction was just beginning in grass habitats.
Another contrast appeared between habitats in August 1984 (Fig. 24), when
females collected in grass had already completed reproducing, yet in post-
harvest maize fields, 20-35 percent were still breeding. Among males from
grass only 10 percent showed visible epididymal tubules, in comparison to 29
percent from fallow field.
 Influence of rainfall upon growth.-- Growth data on P. natalensis were
obtained during the capture-mark-release-recapture study (Tang Christensen
unpubl.), and will be presented elsewhere. Data obtained from monthly