270 TELFORD: MULTIMAMMATE RAT POPULATION BIOLOGY IN TANZANIA

 Apart from those fluctuations possibly correlated with the reproductive
cycle, there was no marked influence of season upon abundance. Praomys
natalensis was more common in fallow fields from July through April than in
grass areas, but in May and June, when growing maize provided less cover,
there was no difference between habitats (Table 8).
 Within grass habitats, P. natalensis seemed to maintain the same
relationship to other species throughout the year. The contrast between
abundance in uncultivated vs. cultivated habitats was most marked from July
to December in 1981 and 1982, when P. natalensis comprised over 90 percent
of the catch in fallow fields, but less than 60 percent in grass (Table 9).
 The great increase in density observed in 1983 and 1984 was accompanied
by movement ofP. natalensis into grass, where it was found at about the same
relative abundance as seen in fallow fields in those years. From January to
April and in May-June (Table 9) in each year far less variation was found in an
annual comparison than from July to December.
 Population structure.-- Three cohorts can be identified in the population
from September to December (Figs. 14-17): adult males, post-reproductive or
parous females, and non-reproductive young of the year. A few females were
found still lactating or with perforated vaginae in September and October, but
these were clearly post-reproductive (Table 10).
 At this time, 60-80 percent of the population are non-reproductive young,
with the remainder representing more or less equal numbers of older males
and females. In January, with maturation of young males beginning in
December, the proportion of mature males increases, while post-reproductive
females diminish in representation (Figs. 14-17). In 1981-82, old females
disappeared completely by April (Figs. 14, 18). The same pattern probably
occurred in the other years of the study but was masked by the off-season
breeding in those years (Figs. 19-21), when young females born in the previous
season bred and entered the post-reproductive cohort. An apparent decrease
in mature males shown by the figures for 1983-84 (Fig. 16) and 1984-85 (Fig.
17) probably resulted from the classification of males by epididymal tubule
condition into reproductive category. Earlier, this had been based upon having
scrotal or non-scrotal testes in specimens with HBL of 105 mm or more (Figs.
14, 15).
 The occurrence of off-season breeding is shown clearly by the sudden
increase in parous females in February 1983 and 1984 (Figs. 19, 20), and in
January 1985 (Fig. 21), in contrast to the population structure seen in January-
February 1982 (Fig. 18) when the only parous females seen were survivors of
the previous year's breeding season (Table 10). A corollary to the sudden
increase in the proportion of females showing placental scars is, of course, the
capture of very small juveniles, 65-85 mm HBL, in the weeks following
appearance of females with scars, as shown most clearly by Figure 16, where