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of the
FLORIDA STATE MUSEUM
Biological Sciences
Volume 34 1989 Number 6
POPULATION BIOLOGY OF THE
MULTIMAMMATE RAT,
PRAOMYS (MASTOMYS) NATALENSIS
AT MOROGORO, TANZANIA, 1981-1985
SAM ROUNTREE TELFORD, Jr.
UNIVERSITY OF FLORIDA
GAINESVILLE
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POPULATION BIOLOGY OF THE MULTIMAMMATE RAT,
PRAOMYS (MASTOMYS) NATALENSIS
AT MOROGORO, TANZANIA, 1981- 1985
Sam R. Telford, Jr.*
ABSTRACT
The population of Praomys (Mastomys) natalensis on the campus of Sokoine University of
Agriculture in Morogoro, Tanzania, was studied by monthly samples from October 1981 to
February 1985. All 35 individuals that were karyotyped had a diploid number of 32
chromosomes. An annual species, the breeding season of P. natalensis normally begins in April,
with three or four litters of 3-23 young each produced by the end of August when reproduction
ceases. The average litter size was 11.7; females smaller than 105 mm HBL produced 1-2
embryos fewer than those larger ; maximum litter sizes averaging 12.0-12.6 were produced by
females 120-129 mm HBL. The actual number of young born was probably around 10.6 per litter,
due to resorption of 0.4-3.2 percent of embryos produced. Testes of adult males regressed from
September through November; spermatogenesis began in December and was completed by
February for young of the year and those adult males which survived into the next breeding
season. Off season breeding can occur in fallow maize fields in January or February, apparently
governed by the precipitation level during the short rains of November, and is limited to
production of a single litter before the regular season begins in April. Young produced during
this period reached maturity by April. Praomys natalensis comprised nearly 87 percent of the
9306 rodents and insectivores trapped on the university campus. Trap success was maximum
following reproduction, from September to December, declining thereafter until May when
recruitment of young to the population began again. The maximum recorded density was 1125
per ha, in October 1984. The population structure and density are influenced by both habitat and
rainfall. The critical factors which determine population levels following the main breeding
season are apparently heavy short rains followed by off-season breeding which alter the density
threshold at which the population begins reproduction in April.
* Denmark-Tanzania Rodent Control Project, University Campus, P. O. Box 3047, Morogoro, Tanzania.
Current Address: Adjunct Curator, The Florida Museum of Natural History, University of Florida, Gainesville, Florida, USA
32611.
TELFORD, S.R., JR. 1989. Population Biology of the Multimammate Rat, Praomys (Mastomys)
natalensis, at Morogoro, Tanzania, 1981-1985. Bull. Florida State Mus., Biol. Sci. 34(6):249-287.
250 TELFORD: MULTIMAMMATE RAT POPULATION BIOLOGY IN TANZANIA
RESUME
Se estudi6 la poblaci6n de Praomys (Mastomys) natalensis en el campus de la Universidad
Sokoine de Agricultura en Morogoro, Tanzania, por muestras mensuales de Octubre de 1981 a
Febrero de 1985. Se obtuvo el cariotipo de 35 individuos, que en todos los casos mostr6 un
nmmero diploide de 32 cromosomas. Praomys natalensis es una especie annual cuya 6poca
reproductive se inicia normalmente en Abril, con tres o cuatro camadas de 3-23 crias que nacen a
fines de Agosto, cuando terminal el period reproductive. El promedio de camada fue de 11.7;
las hembras menores de 105 mm de longitud de cabeza y cuerpo produjeron 1-2 embriones
menos que las mayores; los tamaios miximos de camada, con promedios entire 12.0-12.6, fueron
producidos por hembras con 120-129 mm de longitud de cabeza y cuerpo. El n6mero real de
crias nacidas fu6 probablemente de cerca de 10.6 por camada, debido a la reabsorci6n de entire
0.4 y 3.2 por ciento de los embriones producidos. Los testiculos de los machos adults se
mantuvieron abdominales de Septiembre a Noviembre; la espermatogdnesis se inici6 en
Diciembre y se complete en Febrero en los j6venes de ese afio y en los pocos machos que
sobrevivieron para una segunda 6poca reproductive. Puede ocurrir reproducci6n fuera del
period mencionado en campos abandonados de maiz en Enero o Febrero, aparentemente
controlada por el nivel de precipitaci6n durante el corto period de lluvias en Noviembre, y se
limita a la producci6n de una camada previa a la 6poca normal que se inicia en Abril. Los
j6venes producidos durante este period alcanzan la madurez en Abril. Praomys natalensis
represent casi el 87 por ciento del total de 9306 roedores e insectivoros capturados en el campus
de la universidad. El mayor 6xito de capture ocurri6 despuds de la reproducci6n, de Septiembre
a Diciembre, declinando entonces hasta Mayo, cuando el reclutamiento de los j6venes a la
poblaci6n vuelve a empezar. La densidad maxima registrada fu6 de 1125 por ha en Octubre de
1984. La estructura y la densidad de la poblaci6n son influenciadas por el habitat y por la lluvia.
Los factors critics que determinan los niveles de poblaci6n alcanzados despuds de la epoca
principal de reproducci6n consistent aparentemente de fuertes y breves lluvias seguidas de
reproducci6n fuera del period principal, que altera el umbral de densidad al que la poblaci6n
inicia la 6poca reproductive en Abril.
TABLE OF CONTENTS
Introduction............................................... ........................................................................................ 251
A know ledgem ents..................................... ...................................................................................... 252
M materials and M ethods........................................................................................................................... 252
R results and D iscussion........................................................................................................................... 254
Population Biology of Praomys natalensis in Morogoro............................................................ 260
C conclusions ........................................................ ........................................................................ ....... 280
L literature C ited ....................................................................................... ......................................... 286
BULLETIN FLORIDA STATE MUSEUM VOL. 34(6)
INTRODUCTION
The multimammate or shamba rat, Praomys (Mastomys) natalensis
(Smith 1834) is distributed as a species complex throughout virtually the entire
African continent below the Sahara. Although other species may have greater
significance in given localities, P. natalensis is the most important indigenous
species to public health and agriculture in Africa. Field studies on its biology
in East Africa are surprisingly few, in view of the economic importance of the
species. It has been reported to undergo periodic explosive increases in
numbers (Harris 1937; Taylor 1968), yet no study based upon adequate
numbers of individuals, taken at regular intervals over a substantial sampling
period is available. Most studies were conducted over less than two years, and
virtually none examined a thousand animals or more. Valuable information on
P. natalensis as part of small mammal communities in Kenya, Uganda, Malawi,
and Zambia was presented by Southern and Hook (1963), Hanney (1965),
Delany and Kansiimervhanga (1970), Sheppe (1972), Taylor and Green (1976),
and Delany and Roberts (1978). Only three papers present data derived from
Tanzanian populations: Harris (1937), Chapman et al. (1959), and Hubbard
(1972). In none of these studies was the specific identity of the P. natalensis
population precisely ascertained, although this may become obvious in the
future when adequate data are available on karyotype distributions in East
Africa.
The most valuable studies on the population biology of P. natalensis,
using an autecological approach, were those conducted by South African
workers (Coetzee 1967, 1975; Isaacson 1975). These have been supplemented
by observations on behavior (Veenestra 1958), postnatal development
(Meester and Hallett 1970), and community relationships (De Witt 1972).
Although there are the expected similarities in the biology of South African P.
natalensis to populations of the species in Tanzania, as discussed below, the
climatic factors which govern population dynamics between the two regions are
quite different, especially with respect to reproduction. Understanding of the
variability in reproductive response to local, seasonal, and annual variations in
climatic factors such as precipitation is critical to the development of effective
control programs.
In 1980 the Danish International Development Agency (DANIDA)
established the Denmark-Tanzania Rodent Control Project at Morogoro,
Tanzania. One of the objectives of the Project was to obtain, through suitable
long-term field studies, an understanding of the population biology of P.
natalensis adequate to assist in the design of a national control program
appropriate to the Tanzanian environment and economy. With my arrival as
252 TELFORD: MULTIMAMMATE RAT POPULATION BIOLOGY IN TANZANIA
Project Leader, studies commenced in June 1981 and were concluded in
February 1985. The results and conclusions derived from them are presented
here.
ACKNOWLEDGEMENTS
This study was supported by the people of Denmark, under their generous assistance to
the Republic of Tanzania through the Danish International Development Agency (DANIDA).
The splendid cooperation and support by DANIDA's Counselor for Development, Axel
Pedersen, enabled me, as Project Leader of the Denmark-Tanzania Rodent Control Project, to
carry out the research program with a minimum of the administrative and logistical frustrations
normally attendant upon bilateral aid projects in developing countries. Without his continuous
help the work would have been, at best, mediocre. Jens Tang Christensen, Project Ecologist, and
William R. Smythe, Rodent Control Specialist, were congenial colleagues and often assisted in
the routine sampling and laboratory procedures. The Government of Tanzania, through the
Ministry of Agriculture, provided field and laboratory staff for the project. In particular, I wish
to acknowledge the assistance of P. Mwanjabe, E. A. Nkya, M. Stambuli, C. Sabuni, and E.
Nzobakenga among the senior staff, and field assistants John, Yusuf, Peter, Clement, and Kasim.
Kim Howell, Department of Zoology, University of Dar-es-Salaam, provided information on the
distribution of small mammals in Tanzania, localities and habitats, and useful contacts within the
country.
Professional assistance was furnished by the Division of Mammals, British Museum of
Natural History, London (Jean Engels); Mogens Lund, Statens Skadedyrlaboratorium, Lyngby,
Denmark; the World Health Organization Collaborating Centre for Rickettsial Reference and
Research, Department of Microbiology, University of Maryland School of Medicine, Baltimore
(C. L. Wisseman, Jr., and R. Traub); and the Special Pathogens Reference Laboratory, Porton
Down, England. J. H. Greaves generously helped me with literature searches during visits to the
Pest Infestation Control Laboratory of the British Ministry of Agriculture,Fisheries and Food,
Tolworth, Surbiton, Surrey, in 1981 and 1985. Special thanks are due to Norman G. Gratz, then
Director, Division of Vector Biology and Control, World Health Organization, Geneva, for his
considerable (and characteristic) assistance in obtaining literature references, in facilitating
shipments of biomedical materials, in arranging technical briefings, and other helpful activities.
Upon my return from Tanzania to the University of Florida, Jerry F. Butler provided facilities
and equipment for preparation of the study.
My family contributed also to the success of the research program: Robert M. Telford and
Randolph S. Telford took part in the routine sampling during their several visits to Tanzania,
while Sam R. Telford, III, assisted both in the field and laboratory studies. Finally, Michiko M.
Telford continued to provide her typically efficient yet tranquil domestic environment for her
husband, despite often extreme difficulties in obtaining those material necessities that maintain
an acceptable standard of living.
MATERIALS AND METHODS
Sampling schedule.-- The Praomys natalensis population on the campus of Sokoine
University of Agriculture at Morogoro (0651'S, 3738'E) was studied by monthly removal
sampling from 1 October 1981 through 25 February 1985, a period of 40 months, taken
consecutively except for November 1982 when no sample was taken. The desired sample size was
100 individuals per month, and this minimum was taken in 32 of the 40 months. It proved
impossible to obtain sufficient rodents in May and June of 1982 and 1983, months when
BULLETIN FLORIDA STATE MUSEUM VOL 34(6)
population levels were at their minima, and in January, February, and April 1982--an
exceptionally dry year which drastically reduced rodent numbers. The maximum number of P.
natalensis examined in any one month was 515 in August 1984.
Sampling technique.-- Standard line trapping with single traps set 1-2 m apart, 300-500
traps per night, was the usual method employed, normally for three nights per site. Grid
trapping when done used 300 traps on a 1 hectare (ha) grid, with three traps set per point. Points
were at 10 m intervals. The trap types employed were small and medium, collapsible Sherman
live traps, Woodstream snap-type rat traps, metal snap traps, and "Little Nipper" mouse traps.
The latter proved to be the trap of choice, being sensitive enough to take mice and shrews of 3 g
weight, simple to set, and cheap enough to be regarded as expendable. The standard bait was
dried coconut. The sampling standard chosen to compare rodent abundance from period to
period was based upon a "Little Nipper" line (LN line) of 75 traps set and checked personally by
me. The remaining traps were set by field assistants. The LN line was set adjacent to the other
lines on every occasion on which samples were taken; when grids were used, an LN line was set 10
m from one of the grid borders, for comparison. In estimating abundance from the LN line,
results from the first two nights only were considered. These were corrected by subtracting both
the number of positive traps containing other small mammal species from the total set, and one-
half the number of closed but negative traps from the total (Hanney 1965), to provide a net
number of traps set, against which the number of traps positive for P. natalensis could be
compared. During the 40-month study period, 90,246 traps were set on the university campus,
and another 48,818 set elsewhere in the country, for a total of 139,064 trapnights. This does not
include the effort made during the mark and release study done for 13 months on the campus
(Tang Christensen unpubl.).
Examination of material-- Live animals were anesthetized with ether or chloroform, bled
by cardiac puncture, and brushed for ectoparasites, then examined as were the trap-killed
specimens. Head-body lengths (HBL) and tail lengths were always recorded, and often the ear
and hind foot lengths as well. Animals were weighed either by Pesola spring balances or a
Mettler automatic balance. Sex was determined. Females were examined externally to determine
perforation of the vagina, and teat number was occasionally recorded. Upon dissection the
uterus was examined for placental scars, and embryos present were counted for each limb of the
uterus. The crown-rump of one representative embryo of each litter was measured. Lactation,
when indicated by presence of turgid, milk-filled mammary glands, was noted. The male
reproductive status during the first 18 months of the study was recorded simply as scrotal or
abdominal testes. During the last two years, testes were examined for condition of the
epididymal tubules, these being recorded as visible or not. Initially, the presence of motile sperm
in the epididymal tubules was determined microscopically, until the correlation with visible
tubules was established. The weight of one testis was recorded for approximately 50 males per
month during the last 13 months of the study. Specimens of taxonomic value were prepared
either as round or flat skins, and skulls were saved. Approximately 50 individuals were
karyotyped from marrow smears, prepared two hours following injection IP with 0.05% colchicine
at 0.01 ml/g weight. Smears taken from the femur were immersed before drying in 0.75%
aquaeous KCI for 15 min, placed in Carnoy's fixative for 15 min, and air-dried. The smears were
stained immediately with 5% Giemsa at pH 7.2 for 20 min, washed in tap water and air-dried.
Prior to injection with colchicine, a sample of cardiac blood was obtained for electrophoresis.
Hemoglobin electrophoretic patterns were prepared by the Helena system for 150 P. natalensis
from the campus population, for comparison with samples from elsewhere in Tanzania. Data
were recorded on examination sheets and then transferred to keysort cards. Meteorological
records were obtained from the University Meterological Station for the period 1970-1985.
Study areas.-- The campus of Sokoine University of Agriculture is located on the western
edge of Morogoro, Morogoro Region, Tanzania (0651'S, 37*38'E), at an elevation of 650 m on
the northern slope of the Uluguru Mountains. Very little of the original vegetation remains at
low elevations in the Uluguru Mountains or in their vicinity, and random deforestation continues
toward the peaks despite their designation as a forest reserve. Along the south edge of the
campus on the slopes of the mountain range is a belt of secondary forest which contains remnants
254 TELFORD: MULTIMAMMATE RAT POPULATION BIOLOGY IN TANZANIA
of the savanna woodland association. Protected by the Division of Forestry of the university, it
has been used for experimental studies on introduced vegetation. A total of 22 sites of 1 ha. or
less in various parts of the campus were selected for sampling (Fig. 1). Selection was based upon
present or previous agricultural use, and related primarily to the cultivation of maize, which is the
major crop grown both on campus and in Morogoro Region. Study sites were classified as
follows: (1) growing maize, which sometimes had scattered beans or cassava intermixed with it;
(2) post-harvest maize, for approximately two months following the harvest, consisting of a thick,
high stubble that provides both abundant food and cover for rodents; (3) fallow field, where
evidence of maize cultivation during the previous season remained--post-harvest maize was
classed as fallow field two months following harvest; (4) old fallow field, where maize had not
been grown the preceding season, but which had supported a crop within two years; and (5) grass
habitat, which could be divided into two distinct types--that which succeeds old fallow field, and
that which had no recent record of cultivation but which was cropped regularly by machinery to
provide fodder for dairy cattle.
RESULTS AND DISCUSSION
Land-use cycle.-- Interpretation of the results presented here requires a
description of the land use cycle and climatic factors. The cultivation of maize
in the area depended strongly upon the timing and quantity of rainfall.
Farmers attempted to plant two crops each year, one before the "long rains"
and a second crop to take advantage of the "short rains." The long rain crop
usually was planted between February and May and represented the primary
agricultural effort of the year, furnishing about 90 percent of the annual maize
production. This crop was harvested from late June until August, and fields lay
fallow until late October. If short rains came, then planting on a considerably
reduced scale took place from late October into December. Harvest of this
crop, however, was unpredictable and generally of low yield, due both to
erratic and insufficient rainfall, and the difficulty of planting at the time of year
when rodent populations were at maximum density. Fallow fields sometimes
were burned prior to both planting periods, depending to some degree upon
the density of second growth and the availability of mechanical cultivators.
Except for the use of mechanized equipment, maize cultivation on the
university campus reflected traditional timing and practice of the Morogoro
area.
Climatic factors.-- During the 15-year period 1970-1985, the maximum
temperature recorded at the university campus was 34.1C in December 1974,
and the minimum was 14.00C in July 1973. The recorded maximum and
minimum during the 40 months of the study were 33.7TC and 14.20C. Maxima
remained relatively constant from August through April, 28-33, but then
declined slightly until August when they rise (Fig. 2). Minima remained at 20*
or above from December until May, and varied from 14 to 20 from June to
December.
520m
Morogoro
S/ A
a":
"a
Wa
'1
Cf
C8
S 15 Km
C- --
256 TELFORD: MULTIMAMMATE RAT POPULATION BIOLOGY IN TANZANIA
200 35
150 30
mm ..
100 .....25
..- :: ... . ...:::::::::::::::::::.:: : : < 0
0 15
SON DJ F MAM J J A
Figure 2. Climatic data for Morogoro, 1971-80. Lines indicate mean maxima and minima for
months September-August. Shaded area represents mean monthly precipitation for the same
period.
1971-80 -@- 1983-84 ----- 1982-83 --..-
35-T
.---- _,
25-
15 20
OCT DEC FEB APR JUN AUG
Figure 3. Monthly mean temperature maxima and minima at Morogoro, September-August in
1981-82 and 1982-83, in comparison to the averages for the preceding decade.
BULLETIN FLORIDA STATE MUSEUM VOL 34(6)
1971-80 ---- 1981-82
1984-85 -**--
DEC
AUG
Figure 4. Monthly mean temperature maxima and minima at Morogoro, September 1983-
August 1984 and September 1984-February 1985, in comparison to the averages for the
preceding decade.
1971-80 ----
1981-82 -
OCT DEC FEB APR JUN AUG
Figure 5. Monthly total precipitation at Morogoro, September 1981-August 1982 in comparison
to the average for the preceding decade.
200-
150-
100-
258 TELFORD: MULTIMAMMATE RAT POPULATION BIOLOGY IN TANZANIA
The first year of the study (Fig. 3) was notably warmer than the previous
10-year average from January through March, with maxima for those months
exceeding 330, about two degrees higher than average.
Other years more closely resembled the 1971-80 average (Figs. 3, 4).
The curve of mean monthly precipitation for the 10 years preceding the
study, 1971-80 (Fig. 2), shows two peaks: the maximum, an average of 184 mm
in April, and a second, somewhat lower peak of 126 mm in January. Minimum
levels occur from June to October, with the least rain in August, 6.2 mm. As
will be seen later, the critical period for rainfall for the rodent population is
from November through March; this is described in detail below, for each year
of the study showed significant differences from the pattern formed by the 10-
year average.
In 1981-82 (Fig. 5), rainfall was normal from September through
November, slightly below normal in December, less than half that expected for
January, and virtually absent in February, when only 4.4 mm were recorded.
The rainfall peak for the year, in April, was only 97.2 mm, slightly over half
that expected, and only 2 mm more than had been received in December.
In 1982-83 (Fig. 6), precipitation was double or greater than the 1971-80
average from September through December, and then decreased sharply to
produce a very dry January. Rainfall remained below normal until May, then
exceeded the expected in May, becoming normal thereafter. The heavy rain
from October through December had important consequences for the rodent
population that year.
In 1983-84 (Fig. 7), rainfall was below normal from September through
November but then rose sharply to levels about one-third above normal for
December and January. February and March were similar to the average, but
precipitation in April was again one-third more than expected, reaching nearly
300 mm in that month. The remainder of the year was normal. Heavy rain in
January, followed by a normal February and March appears to be the critical
environmental influence for the rodent population of that year.
In 1984-85 (Fig. 8), rainfall was normal in September and October, but in
November more than doubled the expected amount. Precipitation decreased
sharply in December to about half the average and continued to decline in
January, when only 10.8 mm was received, in comparison to the 126 mm
expected from the 10-year average. The study ended in February, which had a
greater than normal amount of rain.
The hypothesis advanced in this paper is that the events observed in the
rodent population studied can be explained as the result of annual variation in
the timing and level of precipitation during the critical period from November
through February of each year. This variation is summarized thus:
1981-82: January and February had extremely low
rainfall.
BULLETIN FLORIDA STATE MUSEUM VOL. 34(6)
1971-80 --*--
1982-83 --
OCT DEC FEB APR JUN AUG
Figure 6. Monthly total precipitation at Morogoro, September 1982-August 1983 in comparison
to the average for the preceding decade.
1971-80 --*--
1983-84 -
Figure 7. Monthly total precipitation at Morogoro, September 1983-August 1984 in comparison
to the average for the preceding decade.
OCT DEC FEB APR JUN AUG
260 TELFORD: MULTIMAMMATE RAT POPULATION BIOLOGY IN TANZANIA
1982-83: October, November, and December had
extremely high rainfall.
1983-84: December and January had exceptionally
high rainfall.
1984-85: November had very heavy rain, followed
by very little in December and January.
Population Biology ofPraomys natalensis at Morogoro.
Identity of the population studied.-- Karyotype studies elsewhere in
Africa have shown the presence of two karyotypes in Praomys populations: in
Sierra Leone (Robbins et al. 1983) and Ivory Coast (Bellier 1975), diploid
numbers of 32 and 38 have been have been distinguished as Praomys
(Mastomys) huberti and Praomys (Mastomys) erythroleucus, respectively
(Robbins et al. 1983). In southern Africa, a 32 chromosome form is
considered to be Praomys (Mastomys) natalensis, and a species with 36
chromosomes is thought to represent Praomys (Mastomys) coucha (Green et
al. 1980; Lyons et al. 1977; Gordon 1978;, and Lyons et al. 1980). However,
Hubert et al. (1983), without clearly stating their justification for their action,
have assigned the name natalensis to the 36 chromosome species, apparently
on the basis of a karyotype obtained by Matthey (1954) from a single specimen
sent him from Johannesburg. They thus ignored the opinions based upon far
more extensive studies of the South African populations by Lyons et al. (1977),
Gordon (1978), Green et al. (1980) and Lyons et al. (1980). While P. huberti
evidently does occur in East Africa, at least in Somalia (Capanna et al. 1982),
the status of Praomys (Mastomys) natalensis populations in Kenya, Tanzania,
and Malawi has not been resolved. Accordingly, the position taken here will be
that of Smithers (1983) for the South African populations: the Morogoro
shamba rats should be referred to as Praomys (Mastomys) natalensis sensu
latu.
A total of 35 multimammate rats from the university campus population
at Morogoro were karyotyped: all showed a diploid number of 32
chromosomes. Scattered samples from other localities had the same diploid
number.
Reproduction.-- Praomys natalensis is an annual species, reproducing
during the first and only year of its life, with parous females disappearing from
the population before the next breeding season begins (Fig. 9).
Mark and release studies on the campus population showed that females
do not survive to enter a second breeding season, and very few males live over
one year (Tang Christensen pers. comm.). This is consistent with the estimate
BULLETIN FLORIDA STATE MUSEUM VOL 34(6)
S 1971-80 --*--
1984-85 --
OCT DEC FEB APR JUN AUG
Figure 8. Monthly total precipitation at Morogoro, September 1984-February 1985 in
comparison to the average for the preceding decade.
OCT JAN OCTDECJAN OCT JAN OCT DEC
1981-82 1982-83 1983-84 1984
Survival of post-reproductive female Praomys natalensis from October to January
Shaded areas represent the proportion of post-reproductive females in the total
200-
150-
100
50'
Figure 9.
1981-85.
sample.
262 TELFORD: MULTIMAMMATE RAT POPULATION BIOLOGY IN TANZANIA
given by De Witt (1972) of a longevity in the wild of 339 days for P. natalensis
in South Africa. The main reproductive season begins in April of each year,
when female young of the preceding season appear with perforate vaginae
(Fig. 10).
The first litter is born in late April or early to mid-May, followed by
another three litters at approximately monthly intervals, with reproduction
ending in August (Fig. 11). It is possible that some females could produce
more than four litters in those years when conditions favor reproduction from
January onward, but survival of a given female through the entire season is
probably unlikely. Very few, if any, young are born from September through
December in field situations, but reproduction is apparently continuous in
Morogoro Region among those P. natalensis inhabiting houses or other
structures where food and cover are available.
Females produce, on the average, 11.7 embryos per litter, with a range of
3-23 seen in utero (Table 1). Females may show as many as 24 teats. Those
females smaller than 105 mm HBL appear to produce 1-2 embryos fewer than
those 105 mm and larger. Maximum litter size was seen in the females which
were 120-129 mm HBL, averaging 12.0-12.6, while in the largest and probably
oldest females, those over 129 mm HBL, slightly smaller litters were found,
averaging 11.8 (Table 1).
Little annual variation was seen in mean litter size compared with female
HBL, except in 1983 when females of 115-119 mm produced two more
embryos per litter than in the previous year (Table 2).
A comparison of mean litter size with embryo size indicates that perhaps
two embryos are lost during gestation, with the number actually born probably
averaging around 10.6 per litter (Table 3).
Maximum litter size occurs from May through July; early and late season
litters are 1-2 embryos fewer (Table 4).
Embryo resorption in 1982 and 1983 was highest in August, but in 1984
when reproduction was continuous from February through August, resorbing
embryos were seen in each month at similar levels (Table 5). In terms of the
total number of embryos produced by the females examined, the proportion
resorbed was only 0.4-3.2 percent.
Reproduction appeared to be more frenzied in 1982, when the population
was recovering from very low density probably caused by the severe drought in
preceding months. Females with either large or small embryos could be found
simultaneously lactating, implying that another litter was started immediately
after parturition (Table 6). In 1983 no females with embryos over 25 mm
showed lactation. The proportion of females with small embryos that were
also lactating decreased significantly in 1984, the year when reproduction was
continuous from February through August.
BULLETIN FLORIDA STATE MUSEUM VOL. 34(6)
GRAV./LACT. -
PERF. U
POST-REP.
NON-REP. E
Figure 10. Reproductive condition of female P. natalensis in March and April of 1982-84.
Categories are gravid or lactating, perforate, post-reproductive, and non-reproductive.
MJ JL AU SP MJ JL AU SP MJ JL AU SP
1982
1983
GRAV./LACT. 0
PERF. ED
POST-REP. ]
NON-REPROD. D
1984
Figure 11. Reproductive condition of female P. natalensis from May to September 1982-84.
100-r
264 TELFORD: MULTIMAMMATE RAT POPULATION BIOLOGY IN TANZANIA
TABLE 1. Variation in litter size compared with size of gravid female Praomys natalensis.
Female HBL Mean no. embryos Range
(mm) 1 SE (N)
< 95 10.5 2.5 (2) 8-13
95-99 9.3 1.5 (3) 7-12
100-104 9.60.9 (13)* 6-17
105-109 11.00.6 (25) 6-17
110-114 11.30.4 (53)* 5-19
115-119 11.60.3 (77)** 3-20
120-124 12.0 0.3 (85) 7-23
125-129 12.60.4) (65)** 7-21
> 129 11.80.3 (104)** 4-18
total sample 11.70.1 (427) 3-23
*, ** indicate comparisons of means, each significant at P < 0.05
TABLE 2. Annual variation in mean litter size correlated with head-body length of gravid
Praomys natalensis.
Mean no. embryos
+1 SE (N)
Female HBL
(mm) 1982 1983 1984
110-114 10.90.8 (18) 11.10.8 (13) 11.70.7 (20)
115-119 10.6-0.7 (22)* 12.70.7 (23) 11.50.4 (32)
120-124 11.90.5 (25) 12.110.7 (28) 11.9+0.5 (31)
125-129 13.50.9 (16) 12.8 0.7 (20) 12.0 0.5 (27)
> 129 13.5 1.0 (8) 11.70.5 (39) 12.00.4 (54)
* P < 0.05
BULLETIN FLORIDA STATE MUSEUM VOL. 34(6)
TABLE 3. Litter size of Praomys natalensis in comparison with embryo size.
Embryo crown-rump length (mm)
S1 SE (N)
Year < 10 10-19 > 19
1982 12.6-+0.5 (51)* 11.10.4 (32) 10.30.9 (16)
range (3-20) (7-16) (5-18)
1983 11.9+0.4 (72) 12.30.4 (56)* 10.70.6 (22)
range (4-23) (5-18) (5-15)
1984 12.10.3 (93) 11.80.4 (43)* 10.70.4 (36)
range (7-19) (8-18) (7-14)
1982-84 12.20.2 (216) 11.80.2 (131)* 10.60.3 (74)
range (4-23) (5-18) (5-18)
* P < 0.05
TABLE 4. Monthly and annual variation in mean litter size of Praomys natalensis on the
University Campus, Morogoro.
Mean 1 SE (N)
Month 1982 1983 1984 1982-84
Feb 10.70.3 (36)
Mar-Apr 9.7 (3) 10.80.4 (33) 10.70.4 (36)
May-Jun 11.70.4 (34) 11.40.8 (17) 12.90.5 (36) 12.10.3 (84)
Jul 13.20.6 (31) 13.00.4 (43) 12.60.6 (39) 12.00.3 (113)
Aug 10.50.5 (42) 11.50.3 (86) 11.20.5 (28) 11.10.2 (151)
May-Aug 11.60.3 (104) 11.00.3 (140) 12.30.3 (103) 11.70.1 (431)
266 TELFORD: MULTIMAMMATE RAT POPULATION BIOLOGY IN TANZANIA
TABLE 5. Monthly and annual variation in intrauterine mortality ofPraomys natalensis.
Litters Embryos
Year No. % w/resorbing % Embryos No. % Resorbing
Months embryos resorbing
1982
May-Jun 34 0.0
Jul 31 0.0 -
Aug 42 2.4 25.0 445 0.9
1983
Mar-Apr 3 0.0 -
May-Jun 17 0.0 -
Jul 31 3.2 15.4 561 0.4
Aug 86 26.7 11.9 1020 3.2
1984
Feb 36 8.3 9.1 388 0.8
Mar-Apr 33 15.2 10.5 362 1.7
May-Jun 36 13.9 8.7 470 1.3
Jul 39 12.8 14.9 499 1.6
Aug 28 7.1 8.3 316 0.6
TABLE 6. Proportion of gravid females simultaneously lactating.
% Gravid females lactating
Year # Gravid
Months Females < 25 mm > 25 mm
1982
May-Jun 31 38.7 3.2
Jul 31 16.1 6.5
Aug 42 21.4 4.8
1983
May-Jun 17 41.2 0.0
Jul 43 14.0 0.0
Aug 80 35.0 0.0
1984
Mar-Apr 33 3.0 0.0
May-Jun 36 2.8 0.0
Jul 39 0.0 0.0
Aug 28 3.6 0.0
BULLETIN FLORIDA STATE MUSEUM VOL 34(6)
Testes of adult males regress from September through November; no
young males produce sperm during that period (Fig. 12). Spermatogenesis
begins in December, continuing into January, and by February virtually all
males are reproductively capable. In those years when young are born in
January or February, they become mature by April when the main
reproductive period begins.
Abundance.-- In three of the four years, trap success was at maximum
following reproduction, in September-December (Fig. 13). Density declined
thereafter in each year studied, but the decline was sharper from September-
October to January-February 1981-82, the dry year, than in 1982-83 and 1984-
85, both considerably wetter years. Density in the September to December
period of 1983 was much greater than in the other years, and a "crash"
occurred in late December-early January (Fig. 13): the grid estimate taken
from 12-16 December was 769 per ha, while that obtained in an adjacent field
24-27 January, five weeks later, was 56 per ha. Migration from the adjacent
fields into the trapped field may have slightly reduced the numbers in adjacent
fields, but data from subsequent monthly catches substantiate an abrupt drop
in population density at that time of year. The abrupt decline in density was
reflected in the trap line results for the same areas, which showed a drop from
57 percent in December to 13 percent positive in January. The maximum
density recorded by grid estimates was 1125 per ha in October 1984. It
probably was higher at that time in 1983, before the crash, but no grid data are
available. Maximum trap success from line trapping was observed in
November 1983, when 68.3 percent of traps were positive for two nights'
trapping (Fig. 13).
In relative abundance, P. natalensis comprised nearly 87 percent of the
9306 rodents and insectivores trapped on campus during the program (Table
7). The shrew Crocidura hirta was next most abundant, followed by the striped
grass mouse, Lemniscomys griselda. The other seven species taken were far
less common, although there were certainly more Tatera spp. and Mus
minutoides available than trap results indicate. These are underrepresented,
probably because Tatera could escape from "Little Nipper" traps unless
seriously injured, and because of their size Mus may not be as trappable as
other species. Dasymys incomtus was virtually restricted to deep grass, Acomys
spinosissima was both seasonal--found in April only each year--and limited to
grass areas, while Pelomys fallax was seen only in a very wet area in June 1984,
following the excessive rainfall (nearly 300 mm) of April. No permanent
streams flow through the campus, and the common name, creek rat, indicates
its preferences. Rattus rattus was common in campus houses, but only one was
taken in a field trap. The tiny, unidentified shrew appears to be truly rare, but
again, may not be trapped easily due to small size.
268 TELFORD: MULTIMAMMATE RAT POPULATION BIOLOGY IN TANZANIA
60 77
0.8
0.&6 54 s 71 43101
0.4- 138 73
0.2
DEC FEB APR JUN AUG OCT DEC
1983 1984
VISIBLE N
NOT VIS.E3
Figure 12. Mean testis weight of P. natalensis correlated with visibility of epididymal tubules
from December 1983 to December 1984.
1981-82 -----
1982-83 --
1983-84 .
1984-85 ---
..T'"
SEP-OCT JAN-FEB
MAY-JUN
Figure 13. Trap success by bimonthly periods, corrected, for P. natalensis in the study areas,
September 1981-February 1985.
BULLETIN FLORIDA STATE MUSEUM VOL 34(6)
TABLE 7. Relative abundance of rodents and insectivores on the university campus, Morogoro,
Tanzania, from 29 July 1981-20 February 1985.
Species # Trapped % Total catch
Praomys natalensis 8059 86.6
Crocidura hirta 811 8.7
Lemniscomys griselda 249 2.7
Tatera 'robusta" 84 0.9
Mus minutoides 63 0.7
Dasymys incomtus 21 0.2
Acomys spinosissima 7 < 0.1
Crocidura sp. indet. 6 < 0.1
Rattis rattus 4 < 0.1
Pelomys fallax 2 < 0.1
TABLE 8. Seasonal variation in average relative abundance of rodents and insectivores in
cultivated vs. uncultivated habitats.
Mean % of total catch
Jul-Dec 81-84 Jan-Apr 82-85 May-Jul 82-84
Species Maize Grass Maize Grass Maize Grass
P. natalensis 94.4 66.6 83.0 65.2 64.8 62.2
range 92-97 23-93 80-87 56-72 56-75 50-79
L. griselda 1.4 18.4 1.8 8.8 5.4 9.4
range 1-3 1-57 0-4 2-19 0-8 2-24
T. 'robusta" 0.4 3.1 0.8 4.3 4.0 2.0
range 0-1 0-9 0-2 0-14 1-8 0-6
M. minutoides 0.3 1.1 0.7 0.8 1.1 0.5
range 0-4 0-3 0-12 0-3 0-3 0-1
C. hirta 3.5 13.4 13.5 19.2 23.8 24.6
range 2-4 4-31 10-18 5-30 9-29 12-45
D. incomtus 0.03 0.1 0.2 0.9 0.4 1.0
A. spinosissima 1.0 -
R. rattus 0.02 -
P. fallax 0.4
Crocidura sp. 0.04 0.08 -- 0.3
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
BULLETIN FLORIDA STATE MUSEUM VOL 34(6)
TABLE 9. Annual variation in relative abundance of rodents and insectivores in cultivated vs.
uncultivated habitats.
Traps
positive* % Total catch
Habitat Lemnis- Crocidura other
Year # % Praomys comys Tatera Mus hirta spp.
UNCULTIVATED
1981 36 8.7 58.3
1982 23 4.0 21.7
148 8.8 66.2
33 3.9 57.6
1983 824 17.6 92.1
505 9.7 71.7
96 5.9 50.0
1984 504 22.0 93.1
126 3.2 55.6
213 17.7 78.9
1985 190 9.1 67.4
CULTIVATED
1981 556 32.4 97.3
1982 896 24.6 93.9
334 8.5 79.9
138 2.8 74.6
1983 2046 38.0 92.7
365 15.6 86.8
36 3.5 55.6
1984 1786 27.2 93.8
361 9.5 79.8
187 8.0 64.2
1985 298 16.5 85.6
2.8 2.8 30.6
8.7 0 13.0
14.2 2.7 5.4
6.1 0 12.1
0.4 0.4 4.0
2.0 0.4 19.0
0 0 44.8
0 0.2 5.2
0.8 0 22.2
0 1.4 16.9
0 0 30.0
0 0 1.6
1.1 0.3 3.6
2.4 1.2 12.0
8.0 0 8.7
0.2 0.3 4.3
0.8 0.3 10.1
2.8 0 33.3
0.2 0.4 4.3
0 1.1 17.5
1.1 3.2 29.4
0 0 14A
* uncorrected, first two nights; the sequence of trapping periods is:
July-December January-April May-July.
272 TELFORD: MULTIMAMMATE RAT POPULATION BIOLOGY IN TANZANIA
PAROUS 9
MATURE C J]
NON-REPROD. O
1981-82
Figure 14. Praomys natalensis population structure on
October 1981-September 1982.
the university campus, Morogoro,
Figure 15. Praomys natalensis population structure on the university campus, Morogoro,
October 1982-September 1983.
OCT DEC FEB APR JUL SEP
1982-83
BULLETIN FLORIDA STATE MUSEUM VOL 34(6)
100-r
50+
211 228 3541
OCT
DEC
K4-+K{
FEB APR
1983-84
SEP
PAROUS
MATURE CO E
NON-REP.
Figure 16. Praomys natalensis
October 1983-September 1984.
population structure on the university campus, Morogoro,
100T
50
OCT NOV DEC JAN FEB
1984-85
PAROUS $ N
-MATURE d E]
NON-REP. M
Figure 17. Praomys natalensis population structure on the university campus, Morogoro,
October 1984-February 1985.
LJ,
274 TELFORD: MULTIMAMMATE RAT POPULATION BIOLOGY IN TANZANIA
50+
OCT DEC FEB APR
1981-82
JUL SEP
REPROD. U
POST-REP.E
NON-REP. [:
Figure 18. Reproductive condition of the female cohort from the university campus P. natalensis
population, October 1981-September 1982.
SREPROD.
POST-REP.
FEB APR
1982-83
NON-REP. [I
Figure 19 Reproductive condition of the female cohort from the university campus P. natalensis
population, October 1982-September 1983.
. . . . . .
II
BULLETIN FLORIDA STATE MUSEUM VOL 34(6)
REPROD.
50 POST-REP.
NON-REP. Q
OCT DEC FEB APR JUL SEP
1983-84
Figure 20. Reproductive condition of the female cohort from the university campus P. natalensis
population, October 1983-September 1984.
REPROD. U
POST-REP.E3
NON-REP. O
1984-85
Figure 21. Reproductive condition of the female cohort from the university campus P. natalensis
population, October 1984-February 1985.
276 TELFORD: MULTIMAMMATE RAT POPULATION BIOLOGY IN TANZANIA
TABLE 10. Proportion of total female sample post-reproductive or reproductive October 1981-
February 1985.
% Total females reproductive or post-reproductive
Month 1981-82 1982-83 1983-84 1984-85
October 24 14 21 56
November 24 20 26
December 15 11 12 5
January 13 7 9 40
February 6 38 69 33
March 4 38 75
April 0 22 64
May/June 90 79 61
July 75 48 79
August 60 91 51
September 23 61 30 *
* sample taken from grass habitat only
TABLE 11. Proportion of total female sample gravid or lactating October 1981-February 1985.
% Total females gravid or lactating
Month 1981-82 1982-83 1983-84 1984-85
October 1 6 0 0
November 1 0 0
December 0 0 0 0
January 0 0 0 9
February 0 1 50 0
March 0 0 10
April 0 9 33
May/June 76 77 37
July 64 44 49
August 50 83 25
September 5 0 0
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
278 TELFORD: MULTIMAMMATE RAT POPULATION BIOLOGY IN TANZANIA
SPARSE THICK
rIE"AQ CTIIDDQ I 1 CTIIDDl
POST-REP. -
NON-REP. r-
4 1 22
Figure 22. Structure of the female cohort from the university campus P. natalensis population in
October 1984 from three study sites which differed in quality of cover and, presumably, available
food.
FIELD GRASS FIELD GRASS FIELD GRASS
S- .................
DEC JAN FEB FEB
1982
MAR MAR
1983
REPROD.
POST-REP.
NON-REPROD.
Figure 23. Structure of the female cohort from the university campus P. natalensis population
collected from fallow maize fields or uncultivated grass areas from December 1982-March 1983.
100-T
50+
U I I
BULLETIN FLORIDA STATE MUSEUM VOL. 34(6)
REPROD.
POST-REP. 0
NON-REP. E
9 8 3
Figure 24. Structure of the female cohort from the university campus P. natalensis population
collected from post-harvest (PH) maize fields (sites 8 & 9) or uncultivated grass areas (site 3) in
August 1984.
FIELD GRASS FIELD FIELD GRASS
1982 1983 1984
1985
REPROD. $
POST-REP.
MATURE O0
NON-REPROD.
Figure 25. Praomys natalensis population structure in fallow maize field or uncultivated grass
microhabitats in January of each year, 1982-85.
279
280 TELFORD: MULTIMAMMATE RAT POPULATION BIOLOGY IN TANZANIA
samples (Tables 13, 14) demonstrated that mean HBL and mean weight of
female P. natalensis were greater in the critical December-February and
March-April periods of 1983-84 than at comparable times in 1981-82 and 1982-
83. This probably reflects an increased food supply despite greater population
density during the exceptionally heavy rainfall of December 1983-January 1984.
The data also could indicate survival of the larger members of the cohort, with
smaller individuals having greater mortality or being forced to emigrate from
the study areas.
CONCLUSIONS
The data obtained during this study suggest the following conclusions:
1) In dry years, reproduction in either uncultivated or
cultivated habitats begins in April and ends in August.
Populations begin reproduction in those years at minimal
density levels.
REPROD. I
POST-REP. El
NON-REP. D
1982 1983 1984 1985
Figure 26. Structure of the female cohort from the university campus P. natalensis population in
fallow maize field or uncultivated grass microhabitats in January of each year, 1982-85.
TABLE 12. Average trap success forPraomys natalensis by habitat and season.
% Positive traps in
Season Maize fields Uncultivated grass
July-December 31.2 17.1
January-April 12.7 7.3
May-June 4.0 5.4
BULLETIN FLORIDA STATE MUSEUM VOL 34(6)
TABLE 13. Mean head-body length of female Praomys natalensis compared with rainfall, 1981-
85.
Mean head-body length (mm)
1 SE Mean rainfall (mm)
Months 1981-82 1982-83 1983-84 1984-85 1971-80
October 103.7+ 1.7 99.8 1.1 102.4 1.8 99.8 1.8
No. exam. (96) (84) (72) (72)
Nov.-Dec.
rainfall 147.1 332.4 159.3 183.6 158.6
Dec.-Feb. 100.60.8 102.20.9 110.20.7 108.00.7
No. exam. (140) (189) (290) (283)
Jan.-Feb.
rainfall 44.4 70.2 298.7 169.5 212.1
Mar.-Apr. 104.90.8 105.90.7 117.81.3
No. exam. (88) (176) (164)
Mar.-Apr.
rainfall 162.3 218.2 408.4 -307.8
May-June 118.01.1 120.41.3 121.01.1
No. exam. (59) (24) (71)
May-June
rainfall 88.7 155.2 86.2 -102.1
2) Off-season breeding may occur in those years with heavy
short rains, its timing and duration dependent upon onset
and duration of the rains. Little or no off-season breeding
occurs, apparently, in uncultivated habitats in years with
heavy short rains. When the short rains are heavy, the
population enters the main breeding season at much
higher density levels than in dry years (Table 15).
3) The population levels attained following the main
breeding season, i.e. from September through December,
are probably correlated with the occurrence of off-season
breeding earlier in that year, which as indicated depends
upon heavy short rains 9 to 12 months earlier.
The data on reproduction agree with other studies, elsewhere in Africa.
Average litter size ranges from 9.5-12.1 (Brambell and Davis 1941; Chapman
282 TELFORD: MULTIMAMMATE RAT POPULATION BIOLOGY IN TANZANIA
TABLE 14. Mean weights of female Praomys natalensis compared with rainfall, 1981-85.
Mean weight (g) + 1 S.E. Mean
rainfall (mm)
Months 1981-82 1982-83 1983-84 1984-85 1971-80
October 27.81.1 28.21.0 28.21.3 24.11.4
No. exam. (96) (84) (70) (72)
Nov-Dec
rainfall 147.1 332.4 159.3 183.6 158.6
Dec-Feb 28.4 0.6 29.70.7 32.70.9 28.90.5
No. exam. (140) (189) (290) (282)
Jan-Feb
rainfall 44.4 70.2 298.7 169.5 212.1
Mar-Apr 30.1 0.5 31.5 0.6 38.9 1.3
No. exam. (87) (174) (164)
Mar-Apr
rainfall 162.3 218.2 408.4 -307.8
May-June 51.81.6 51.02.6 47.41.5
No. exam. (60) (24) (71)
May-June
rainfall 88.7 155.2 86.2 -102.1
TABLE 15. Summary of annual variation in rainfall and reproductive patterns, with trap
success prior to normal onset of Praomys natalensis reproduction in April
April trap
year rainfall pattern reproductive pattern success (%)
1981-82 < normal Dec.,very dry Jan.- no reproduction from Sept.
Feb. to April 5.0
1982-83 very wet Oct.-Dec., < normal 1 litter Jan., then none until
Jan.-Apr. April 7.6
1983-84 very wet Dec.-Jan., normal Feb.- continuous reproduction Jan.
April through August 34.4
1984-85 very wet Nov., then dry Dec.- 1 litter Jan., none in
Jan. February
BULLETIN FLORIDA STATE MUSEUM VOL 34(6)
et al. 1959; Hanney 1965; Coetzee 1965; Delany and Neal 1969). These other
averages, however, are based upon far fewer females examined: in the one
study where the number of P. natalensis was adequate (4636, Coetzee 1965),
only 481 females were dissected. Coetzee (1967) reported data on intrauterine
mortality thus: corpora lutea averaged 10.92, 9.47 embryos were implanted,
and 9.22 healthy fetuses were observed. In the present study, the average of
12.2 embryos less than 10 mm decreased to 11.8 between 10 mm and 19 mm,
and 10.6 in excess of 19 mm. Coetzee's (1965) average litter size of 9.5 may
indicate a slight difference in reproductive rate between South Africa and
Tanzania, or could indicate that the two populations are not conspecific.
Coetzee (1975) reported a low breeding rate during dry season (winter),
with a break in reproduction in September, while Delany and Neal (1969)
found reproductive females in Uganda from May to July and October to
December, commenting that "peak breeding season occurs mainly towards the
end of the rainy season and beginning of the dry season." Their samples were
taken from sites between 900 m and 1100 m elevation, where some rainfall
appears to occur in every month. In Malawi, Hanney (1965) examined 159
female P. natalensis without finding pregnant females from June to January.
Hubbard (1972) presented fragmentary data from an overall sample of 225
females taken in several Tanzanian localities (Muheza, Lake Manyara, Iringa,
Njombe, and Himo), reporting pregnant females in January, February, March,
May, July, September, and November, but the data are virtually meaningless
given the variety of rainfall patterns which occur over that broad area.
Some of the observations by Harris (1937) were based upon material
from Morogoro; He described P. natalensis as being least active from January
to May, with "The majority of the mice at this time appear to be young and in
good condition." Maturation of both sexes occurred about the beginning of
March, followed soon by pregnant females and young, while maximum
numbers were found in July and August. Although Harris studied fluctuations
in numbers for two years at Morogoro, from December 1931 to December
1933, he apparently did not publish quantitative data from his work nor a
detailed description of methods, which unfortunately prevents a direct
comparison with the present study.
The data of Chapman et al. (1959) from Rukwa in Mbeya Region are
based upon erratically collected material but, as presented by Coetzee (1975),
show a pattern of reproduction somewhat different to that found in Morogoro
during the present study. The percentage of juveniles in the population
reached maxima in May and June, remained high until October, then declined
rapidly through March, rising again from April. In Morogoro, the juvenile
cohort rose continuously relative to the adult component from July to
December of each year, as an overall pattern, though the rise began earlier in
1984 (April). The decrease in juvenile percentage found in Rukwa can be
attributed to possibly earlier maturation, beginning in October rather than
283
284 TELFORD: MULTIMAMMATE RAT POPULATION BIOLOGY IN TANZANIA
December as in Morogoro, with those juveniles found from November through
February representing reproduction comparable to that following heavy short
rains in Morogoro. The differences might also indicate a species difference in
reproductive pattern. Sample sizes reported by Chapman et al. (1959) are
inadequate to draw a meaningful comparison with the Morogoro study.
There is agreement between conclusions presented here and those of
other authors concerning the role of precipitation on population dynamics.
Coetzee (1975) thought that the cold winters in South Africa might act toward
controlling density, with the concomitant influence of sparse plant cover during
late winter providing less protection against predators. Cold is not a factor in
tropical Africa, except perhaps with montane rodent populations, but certainly
the lack of ground cover observed during the very dry period of November-
April 1981-82 contributed to greater mortality and, thus, lower density going
into the main reproductive season of 1982. And cover was distinctly better
during the following two wetter years. Delany and Neal (1969) suggested that
rainfall might be the most important factor governing rodent breeding in
Uganda, through its indirect effect upon food availability. Coetzee (1975)
linked the abundance of food to population explosions, a conclusion also
suggested by Smithers (1971), who found the number of pregnant female P.
natalensis in Botswana to be very low during the last two years of a four year
drought. Following the end of the drought, a massive explosion took place. In
Coetzee's opinion, population explosions might be linked with the abundance
of food. He listed the following factors as important regulators of P.
natalensis density: (1) a high reproductive rate due to large litter size and short
intervals between litters, (2) the age at first litter, and (3) the duration of
breeding period, which is largely controlled by the rainy season with its indirect
influence over food supply. An optimum food supply can lead to a population
explosion which would disrupt the normal social structure, thus influencing
litter size and interval, the size of neonates, and survival rate. In South Africa,
high density coincides with cold weather, scarcity of food, lack of cover and
increased predation, all of which can lead to a population crash.
The present study reinforces Coetzee's conclusions. Although perhaps
important only in areas with Morogoro's climatic pattern, it is likely that the
influence of precipitation is most critical during the short rain period, setting
the stage, as it were, for the density threshold upon which the P. natalensis
population enters the main breeding season in April of each year. If the short
rains fall far below the average, density will decrease to minimal levels between
January and April, and population levels will increase thereafter at a rate
proportionate to subsequent precipitation levels. With a succession of
favorable short rains, within two years the population can "explode" as was
observed in 1983-84, followed by a "crash," as seen in January 1984. That this is
a recurrent phenomenon is supported by Harris (1937) in his Morogoro study:
"The most striking feature [of fluctuations in population density] was a fall
BULLETIN FLORIDA STATE MUSEUM VOL 34(6)
from a catch of 170 mice in December 1931, to 20 in January 1932." This
reduction in catch by 88 percent compares well with the grid estimate of
December 1983 of 769/ha which dropped to 56/ha. in January 1984, a decline
of 93 percent. Harris attributed the rodent "plague" of 1930-32, with peak
abundance in 1931, to migration of rodents into the Morogoro area from
Kimamba, over 40 miles to the east, rather than to a result of "an abnormal
increase of the local population." Without more detail of his sampling
procedures, this conclusion is arguable. As closely as the university campus
population was monitored during the 40-month period, there was no
suggestion during 1982-83 that numbers were "explosively" increasing, although
abundance was clearly higher following the 1982 short rains, and off-season
breeding occurred in January 1983. The less than obvious rise to "crash" level
through the influence of higher precipitation level on food and cover, with
increased survival and extended reproductive period, is a simpler and more
likely explanation of high density than is migration from distant populations.
Implications for control.-- Given the high cost to a fragile economy of
comprehensive rodent control programs, it is essential that control be rational
to achieve maximum benefit for both agriculture and public health. An
attempt to significantly decrease rodent numbers during the post-harvest
period when maximum density is reached is far less effective and considerably
more expensive than it would be to take preventive measures when populations
are at minimum density. As this coincides with the pre-reproductive period in
Morogoro, from January to April (disregarding off-season breeding in years of
excessive rainfall), each rodent removed from the breeding population
represents a far more significant result than one removed following harvest,
because of the very real possibility that an individual female can produce 50
offspring between April and September. By concentrating rodent control
activities in fields between January and the onset of the long rains in April,
limited resources will have maximum effect, and both germination and harvest
success should be improved. Coupled with encouraging farmers to clear away
stubble immediately after harvest and improvement of grain storage structures,
as pointed out by Harris a half-century ago, effective rodent control could be
realized. Systematic sampling programs in climatically differing areas can
indicate proper timing for rodenticide application, i.e. in the period
immediately preceding onset of the main reproductive period, probably
predictable by close study of rainfall patterns.
286 TELFORD: MULTIMAMMATE RAT POPULATION BIOLOGY IN TANZANIA
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