HARVARD UNIVERSITY
Library of the
Museum of
Comparative Zoology
MC2
OCCASIONAL PAPERS LIBRARY
of the MAY OAjooq
MUSEUM OF NATURAL HISTORY
The University of Kansas
Lawrence, Kansas UNIVERSITY
NUMBER 130, PAGES 1-33 APRIL 27, 1989
COMPARATIVE ECOLOGY AND
SOCIAL INTERACTIONS OF NORWAY RAT
(RATTUS NORVEGICUS) POPULATIONS
IN BALTIMORE, MARYLAND
By
G. E. Glass1, J. E. Childs1, G. W. Korch1
and J. W. LeDuc2
Despite its world-wide distribution and significance as an agricultural pest
and reservoir of important human pathogens, there have been few recent
ecological studies of wild Norway rats (Rattus norvegicus). This is particu-
larly true in the United States, where little has been done since the early work
by Davis and coworkers (Davis etal. 1948; Davis and Hall 1948, 1951;Emlen
etal. 1949; Davis 1949, 1951 a-c, 1955). Recent studies have tended to focus
on single populations. Farhang-Azad (1977a, b), and Farhang-Azad and
Southwick (1979) examined rat populations and their parasites in the Balti-
more City Zoo from 1972 to 1974. Stroud (1982) studied the population
dynamics of sylvatic R. norvegicus in rice fields in Northern California, and
Recht (1982) used radiotelemetry in suburban California to demarcate the
fine structure of home ranges, activity patterns, and movements. One of the
few comparative studies was conducted by Blanchard et al. (1985) who
examined wounding among rats in different Hawaiian cane fields.
Results from these and other studies (e. g. Bishop and Hartley 1976;
Robitaille and Bovet 1976; Hartley and Bishop 1979; Pye and Bonner 1980)
show similar trends in many aspects of rat ecology but differ in some
'Department of Immunology and Infectious Diseases, The Johns Hopkins University, 615
N. Wolfe St., Baltimore, Maryland 21205.
2United States Army Medical Research Institute of Infectious Diseases, Fort Detrick,
Frederick, Maryland 21701.
2 OCCASIONAL PAPERS MUSEUM OF NATURAL HISTORY
significant respects. It is often difficult to determine if the differences simply
reflect the various methodologies used or genuine differences in the popula-
tions. For example, Davis et al. (1948), using a mark-recapture approach,
reported that the home range diameters for rats in urban Baltimore rarely
exceeded 25 meters. Using radiotelemetry, Recht (1982), on a University
campus, and Taylor (1978), in an agricultural setting, obtained home range
diameter estimates of 150-600 m. The size structures of rat populations also
differ markedly. Blanchardetal. (1985) reported that 25%of their populations
in Hawaii were under 110 g and less than one percent exceeded 250 g. In
contrast, Davis (1951b) noted that 26% of his Baltimore population were less
than 150 g but 63% exceeded 250 g. Farhang-Azad and Southwick (1979)
reported an intermediate weight structure in their population, with 39%
exceeding 250 g and 42% less than 150 g. While some of these differences
may be due to geographic variation in genetic composition, even on a local
scale populations may vary in average size by as much as 57% (Davis 195 lc),
suggesting that much of the variation is due to phenotypic plasticity. Dif-
ferences in annual population dynamics, sex ratios, reproduction, and recruit-
ment patterns are also apparent when various studies are compared (Da-
vis 1949; Leslie et al. 1952; Farhang-Azad and Southwick 1979; Stroud
1982).
Although Norway rats are widespread in urban centers of the world, most
recent studies of rat ecology have focused on rural populations. The few urban
studies have been in settings resembling rural habitats, e. g. parks or suburbs
(see Recht 1982), rather than in densely populated residential centers.
Simultaneous comparative studies of rats from widely varying habitats within
the same circumscribed region are especially rare. In mis paper we compare
patterns of population dynamics, growth, survival, body size distributions,
reproduction, movement, and social interactions of Norway rats from several
types of habitats in Baltimore to document the extent and potential causes of
differences in the population ecology of this species.
MATERIALS AND METHODS
Trapping. R. norvegicus individuals were trapped from nine sites
throughout Baltimore City (Fig. 1) from April 1980 through June 1986.
Efforts were concentrated in two periods; April through December 1980
(11.9% of captures), and January 1984 until June 1986 (82.1% of captures).
Two areas, Cherry Hill (CH) and Western Run (WR), are parklands (Plate I).
CH is primarily grassland dominated by reed (Phragmites communis), aster
{Aster multifloria) and goldenrod (Solidago sp.). WR is riparian habitat
dominated by maple (Acer rubra) and elm, (Ulmus americana). Abundant,
free-flowing water is available at both sites. Human habitation is located
across a mowed lawn and street approximately 0.3 km from the trap locations
URBAN ECOLOGY OF NORWAY RATS
WR
WG
CV
RH
PP
WA
Fig. 1. Locations of the nine study sites within Baltimore City. Abbreviations for the sites
are given in the text. CH and WR were parkland sites. PP (PP1, 2, and 3) and WA were low SES
areas, RH and CV transitional areas and WG a moderate SES area.
at WR and is separated by an intervening, heavily traveled highway at least
0.8 km from trapping sites at CH. These distances are sufficient to minimize
the potential contact with commensal rats (Schroder and Hulse 1979).
The seven remaining areas are urban residential sites. Rats were trapped
in the central alleys and backyards of attached row houses that run the length
of each block (Plate I). Residential sites differ in the economic status of the
inhabitants (Olson 1976; Taylor et al. 1979). Winston-Govane (WG) is a
moderate socio-economic status (SES) area with a 100% occupancy rate in
predominantly single family dwellings. Home ownership is common (>70%).
Little refuse was available at WG for forage, and rats found harborage under
shrubs and hedges. Four sites, Washington (WA), Bradford, Port and Chase
(three adjacent blocks designated PP 1 , PP2, and PP3), are low SES areas. The
turnover of the human population was frequent at all low SES sites; 90% of
the population rented space in dwellings (Taylor et al. 1979). Refuse was
discarded in the alleys and rats relied on it for forage (Plate I). At WA only 28%
of the houses were occupied and refuse was deposited at only a single corner
4 OCCASIONAL PAPERS MUSEUM OF NATURAL HISTORY
of the alley. Occupancy at all PP sites was high (>90%) but at PP3 half of the
perimeter was occupied by an open, grass covered, schoolyard.
Charles Village (CV) and Reservoir Hill (RH) are transitional areas, with
a combination of single family and multi-residential housing. Areas behind
dwellings in low SES and transitional neighborhoods were typically covered
by concrete, although some yards contained small gardens and earth. Rats
found harborage under the concrete slabs at these sites (Plate I) and occasion-
ally infested the basements of houses.
Rats were trapped using 12.5 X 12.5 X 40 cm wire mesh single-capture live
traps baited with peanut butter. In residential areas 30 traps were placed along
runways or adjacent to burrow entrances. Traps were spaced at 7-10 m and
most yards were sampled. Traps were placed in the same locations during
each trapping period. Traps at CH were placed in a 0.5 ha, 49 station (7 X 7)
grid with traps spaced at 10 m. At WR 30 traps were scattered in sites near
burrows or runways along the water course. Traps were baited and set prior
to sunset. Initially, traps were collected at 0 1 00 hrs, following the major peak
of rat activity (Calhoun 1962; Takahashi and Lore 1980). After November
1984 traps were checked at 0100 hrs but left open throughout the night and
were collected shortly after sunrise. Capture locality, to household address, or
grid station was recorded for each rat
A mark-release study was undertaken at CH, WG, WA and PP3 to provide
information on growth rates, survival, and movements. Tagging began in July
1984 at WG, in October 1984 at CH and PP3, and in June 1985 at WA.
Captured animals were marked with serially numbered ear tags and held for
several hours to ensure recovery from anesthesia before release. Recaptures
of marked animals were relatively uncommon during portions of the mark-
release study so that statistical estimates of rat population sizes were unsat-
isfactory. Instead, indices of population size (rats/trap night) were obtained
(Emlen et al. 1949).
Data Collection and Analysis. Animals were transported to the labora-
tory and anesthetized with a 1:10 solution of xylazine and ketamine HC1,
injected intramuscularly. While anesthetized, animals were examined for
wounds, which were scored on a 5 point qualitative scale (Glass and Slade
1980). Animals with no wounds were scored as 0, minor injuries such as tail
wounds were scored as 1. Small bites on the flanks, near the base of the tail
or shoulders were 2. Larger wounds (>0.5 cm) were scored as 3 or 4 depending
on the degree of damage to underlying tissue. The scoring scheme was similar
to Farhang-Azad and Southwick's (1979) except that their sixth category
(fatal wounds) was not used. Differences in wounding rates between habitats,
seasons, sexes and weight classes (see below) were tested using hierarchical
log-linear models (Everitt 1977). Analyses were begun using main effects and
statistical significance of the parameters were used as the criterion to infer
their importance. The addition of higher order terms was used to test for
URBAN ECOLOGY OF NORWAY RATS 5
interactions among main effects.
Body weight was recorded to the nearest gram, and after July 1984,
standard measurements were recorded to the nearest mm. Associations
between body weightand length for males and females from each habitat were
examined using Principal Components Analysis (PC A). Differences in these
relationships were tested by Analysis of Covariance.
External reproductive condition (testes scrotal or nonscrotal; vaginal
orifice perforate or imperforate, and lactating or not) was noted. Internal
reproductive data were recorded for sacrificed animals after giving anesthesia
to excess. Testicular length and width and the presence of epididymial
convolutions were noted for males. Epididymial convolutions were consid-
ered evidence of spermatogenesis (Davis and Hall 1948; Jameson 1950;
Batzli and Pitelka 1971). Testis volume was calculated assuming each testis
was a prolate spheroid (V = 4/3 k (Testis width/2)2 (Testis length/2)). Litter sizes
were recorded for females.
Data on behavioral interactions, foraging and movements by individuals
were gathered by direct observation at CV, RH, PP2 and WA (which were lit
by street lamps). Observations were made by two individuals using an
automobile as a blind. Animal movements and interactions could be observed
to a distance of 20-25 m (Childs 1986). Two types of records were made of
interactions. The first involved counts of the number of rats present in the
alley during a five minute interval and their spatial grouping. Rats within 1 m
of each other were considered to be 'together.' Secondly, the types of
behaviors made by focal individuals were recorded. Behaviors were classi-
fied as locomotory, agonistic (chasing, fighting and jumping), following
(including mating), and foraging. Ten minutes were allowed to elapse
between intervals to reduce the dependence of observations.
Following snowfalls, active burrows and runways were identified by
tracks in the snow. Runways and movements were recorded on census bureau
plats of each site. During direct observations, each animal's location was
noted at 5 second intervals and the path marked as the straight line distance
between points. Length of travel was estimated as the straight line distance
along the long axis of movement, corresponding to the method of calculating
movements from the trapping data.
All data were analyzed using an IBM 370 system and 1984 S AS software.
All values reported in the text are mean ± one standard deviation, except as
noted.
RESULTS AND DISCUSSION
Population Dynamics and Size Structure. A total of 760 rats were
captured 913 times during 8440 trap nights. Of these, 388 were necropsied
and 372 were marked and released. Six hundred and twenty-three were
6 OCCASIONAL PAPERS MUSEUM OF NATURAL HISTORY
captured in residential sites while 1 37 were caught in parklands. Rat captures
peaked in late autumn 1984, declined over winter, and reached minimal
numbers in early spring 1985 (Fig. 2). At most sites populations increased in
late spring 1985 and rose towards peak levels in autumn before declining
again in the winter of 1985-86. The 1985-86 decline was not as precipitous
as in 1984-85. The populations reached minimal numbers in spring 1986.
in
>0.10, interac-
tion; Table 1). Rats in the 200-299 g weight class predominated in parklands
(Fig. 3), and the absence of animals in the largest weight categories was the
primary reason for the smaller size of parkland rats. Trends in body length
mirrored those observed with body weight (Table 1). Because of the homo-
geneity of body length/body weight relationships across groups, weight was
used to assign each animal to one of six 100 g weight classes up to 500 g and
only patterns based on body weight are discussed.
50 i
40
30 -
20 ■
~ 10 H
>
o
z
Ui
o
UJ
&
URBAN
(n-352)
??
URBAN
( n-331)
50 -
a
PARKLAND
40 -
(n«99)
30 -
20 -
10 ■
??
PARKLAND
(n.68)
<1
3 4 ^5 <1 1
( MASS CLASS (g/x100)
4 £5
Fig. 3. The proportion of male and female R. norvegicus in 100 g weight classes from
residential and parkland sites. Males from both sites were significantly larger than females and
residential rats larger than parkland rats. Note the near complete absence of parkland rats >400 g.
8 OCCASIONAL PAPERS MUSEUM OF NATURAL HISTORY
Table 1. Summary statistics (£ ± sd; n) for body weight (g) and body length
(mm) of 853 captures for male and female Rattus norvegicus from urban residen-
tial and parkland sites. Males are significantly larger than females, in both areas.
Residential rats are significantly larger than parkland rats (2 way ANOVA).
Habitat
Body Weight
Body Length
Residential
male
female
325.3 ± 150.0; 353
294.1 ±148.1; 332
217.3 ±37.0; 247
206.3 ±34.8; 21 8
Parkland
male
female
232.4 ± 90.2; 99
211.3 + 88.9; 69
207.0 ±30.5; 99
202.2 ±28.5; 69
Overall, the proportions of each weight class contributing to the popula-
tion varied monthly (Fig. 4). Rats in weight classes >400 g were most
abundant, ranging from a low of 18% in March to a high of 47% in June. The
other classes tended to make up more variable portions of the population.
Animals in the smallest weight class (<100 g) were present, in low numbers,
throughout the year, making up 5-15% of the population. They had abimodal
abundance, peaking in the winter-early spring (February-April) and again in
the autumn (October) the months surrounding peak reproduction (see below).
June was the only month in which they were not captured.
The intermediate weight classes (1 00-299 g) were most abundant in late
summer and early autumn . The gradual increase in the 1 00- 199 g weight class
during late autumn probably reflects the growth of weanlings from the
secondary reproductive peak in early autumn. Rats in the 300-399 g class
usually represented 15-23% of the population but reached maximal abun-
dance of 37-38% in March and June prior to the onset of the two reproductive
peaks (see below).
Seasonal patterns in the size structures of residential and parkland popu-
lations differed. Residential populations maintained stable size structures
between seasons, while parkland populations varied widely (Fig. 5). The
major source of variation was in the abundance of sexually immature animals.
The smallest weight classes (<100 g and 100-199 g) in parklands, reached
maximal abundance in the autumn and winter, when they represented >50%
of the populations. In the summer their prevalence was minimal (<10%) while
in the spring, their high frequency (37%) was caused by the abundance of rats
in the smallest weight class (< 100 g). The largest weight classes (300-499 g)
were the only groups with a nearly constant abundance throughout the year
(15-18%).
Norway rat populations, in a variety of habitats, undergo annual fluctua-
tions with a major peak in the late spring or late autumn, followed by a trough
u
c
URBAN ECOLOGY OF NORWAY RATS
dd R. norvegicus
99 R. norvegicus
>500 g
400-499
300-399
200-299
100-199
<100 g
M
M J J A S
Time (Months)
N
Fig 4. Monthly distribution of weight classes for male and female R. norvegicus through the
year. Animals less than 200 g were considered sexually immature.
during the winter or early spring (Davis 1951a; Bishop and Hartley 1976;
Hartley and Bishop 1979; Farhang-Azad and Southwick 1979; Stroud 1982).
Two studies reporting major peaks in late spring also observed secondary
peaks in late autumn (Farhang-Azad and Southwick 1979; Stroud 1982).
Populations with spring peaks derived most of their numbers from juveniles
10 OCCASIONAL PAPERS MUSEUM OF NATURAL HISTORY
H d Residential
□ 9 Residential
| d Parkland
I 9 Parkland
c
a>
o
i-
a>
a.
Spring
Summer
tL
y
=
i
\uti
1
mn
5
Mass Class (g x 100)
Fig. 5. Seasonal comparison of weight classes for male and female R. norvegicus from
residential and parkland sites. Weight classes are <1 = 0-99 g, 1 = 100-199 g, 2 = 200-299 g,
3 = 300-399 g, 4 = 400-499 g, and 5 = >500 g.
entering the population. Stroud (1982) noted two spring peaks due entirely to
the entry of juveniles into the trappable population. Farhang-Azad and
Southwick (1979) also found that a large portion of their population (70% of
their sample) were juveniles (<200 g) during the spring peak, and that these
rats recolonized burrow systems abandoned during the winter. These popu-
lations differed from those described by other workers in that much of the
habitat was unused when population numbers were low. It may be the
availability of resources at some sites, such as parklands, permits extensive
recruitment from burrows where rats overwintered. In other, more stable
populations, such as those in residential areas, much of the available area
remains occupied through the winter and recruitment is reduced.
Differences in the annual changes in weight structure of the parkland and
residential populations support this interpretation (Fig. 5). At residential sites
the size structure remained constant throughout the year and the populations,
URBAN ECOLOGY OF NORWAY RATS
11
though fluctuating, remained relatively more stable in numbers. In parklands
only the proportion of large adults (>300 g) remained relatively constant,
possibly because they could occupy the few available overwintering sites.
The remainder of the population was highly influenced by seasonal factors.
Growth Rates and Survival. If differences in population dynamics and
size structure reflected responses to differences in habitats, this should be
evident in the growth and survival of individuals. Growth rates for rats were
obtained from the changes in body weight of 96 animals caught at least one
month after being tagged. Overall, rats less than 200 g showed extremely
rapid growth (Fig. 6), often exceeding 60 g/month. The rate of increase
N = 9
32
20
22
o
E
DC
5
o
o
70 1
60
45-
30-
25-
20-
15-
10-
5-
0
-5-
-10-
-15-
-20-
-25-
<1
i
2
>5
Mass Class (x100g)
Fig. 6. Mean growth rates and 95 % Confidence Interval (g/month) for each weight class
calculated from recapture data. Sample sizes are shown above each class. Weight classes are
defined in Fig. 5.
12 OCCASIONAL PAPERS MUSEUM OF NATURAL HISTORY
slowed in the 200-299 g class (30.6 g/month) and nearly ceased in the >300
g weight classes (9.5 g/month). Variation in growth rates increased in the
largest weight class. Overall growth rates (Fig. 6) were similar to those
reported by Calhoun (1962) and Bishop and Hartley (1976).
Sample sizes were too small to examine monthly differences in growth
rates between habitats. Instead, data were grouped by habitat and season for
the combined sexes. Seasons were categorized as either warm (May-October;
12°-23°Cmean monthly temperature; NOAA 30 year average for Baltimore)
or cool (November-April; 1°-8°C). Weight classes with similar growth rates
were grouped into four categories (<200 g, 200-299 g, 300-399 g, >400 g).
There were highly significant (/5<0.01) two-way interactions for habitat x
season and habitat x weight class, indicating that growth patterns differed
between residential and parkland populations (Table 2). Rats from residential
sites grew faster than parkland rats regardless of weight class, but particularly
among <200 g animals, where they outgrew parkland rats by an average of
29.4 g/month during the warm season and 57.1 g/month in the cool season.
Parkland rats in the 300-399 g class, on average lost weight, regardless of
season, while residential rats continued to grow (19.8 g/month). During the
cool season residential rats grew more rapidly than parkland animals, but
differences were not statistically significant during the warm season. Overall,
parkland rats grew more slowly during the cool months than during the warm
season (F=4.26, />=0.05), while residential rats by contrast grew significantly
faster during the cool season (f=6.43, F<0.025).
Survivorship, measured between first and last captures of marked animals,
for rats recaptured at least once (including within the same trap period),
showed rapid exponential declines in both habitats. (In P=- 0.40t + 0.10,
^=0.98, residential; In P=- 0.30t - 0.20, r2 =0.97, parkland; Fig. 7). Survival
of tagged animals was somewhat, but not significantly, better in parklands
Table 2. Growth rates (g/month) oiRattus norvegicus (X ± se; n), during warm
(May to October) and cool (November to April) seasons, in residential and
parkland sites.
Mass Class (g)
Season
<200
200-299
300-399
>400
Warm
Residential
66.6±5.8;18
43.5 ± 7.4; 7
2.3 ±12.0; 7
4.4 ±8.7; 9
Parkland
37.2 ±7.4; 7
34.1 ±13.8; 4
-12.0 ± 8.0;2
—
Cool
Residential
81.2±8.0;11
57.0 ±7.0; 2
37.3 ±17.7; 7
22.0 ±10.4; 4
Parkland
24.1 ±11.6; 5
8.2 ±4.5; 7
-8.3 ±23.0; 6
—
URBAN ECOLOGY OF NORWAY RATS
13
O.OCHU
• Residential
x Parkland
-0.50
-1.00-
>
£ -1.50
CO
o
>»
= -2.00-
A
CO
n
o
-2.50-
-3.00-
-3.50
6 7 8
Time (Months)
11 12
Fig. 7. Survival time of marked animals on the study sites. Survival followed exponential
decays in both populations. The half-life and 90 % turnover time are indicated by dashed lines.
(Smirnov test T, = 0.098; P>0.\0). Median life expectancy after first capture
was 7 weeks in parkland and 8 weeks in residential populations. There was
a 90% population turnover by six months in residential areas and by 7.25
months in parklands. Despite the rapid loss of marked animals, 1 3 individuals
lived in excess of six months, three survived at least nine months, and one
animal was alive when the study ended, a year after marking. Six of the eight
14 OCCASIONAL PAPERS MUSEUM OF NATURAL HISTORY
residential and 2 out of 5 parkland rats exceeded 250 g when first captured.
Given the above growth rates, these animals were probably three to five
months old at first capture and at least a year old at the time of last capture.
Our median survival time of two months is similar to that reported by other
workers. Bishop and Hartley (1976) and Stroud (1982), who studied popula-
tions in agricultural areas similar to our parkland, found median survival rates
of 1-3 months and maximal survival of 10-12 months, while Davis (1948,
1951 b, c) examined a highly provisioned population with the same results.
Thus, survivorship of rats seems to remain fairly constant under a wide range
of conditions.
Differences in the size structure of local rat populations are not due to
differential mortality but to differences in growth rates of individuals as well
as possible underlying differences in migration, and reproduction (see be-
low). Assuming average growth rates (Table 2), and an initial size at weaning
of 40 g (Frazer 1977), residential rats reached 200 g (approximate sexual
maturity) within three months, whereas it took parkland rats more than twice
as long to reach the same size. As a consequence, for a given size, parkland
rats tended to be chronologically older than residential rats. Evidence indi-
cates that population dynamics are causally related to individual growth rates
by the timing of reproductive maturity (see Reproduction). If so, growth rate
variation may explain spatial differences in Norway rat population dynamics.
Differences in growth rates may be due to the availability of high energy
resources (i. e. refuse) in residential sites. Other workers have also noted that
rats from areas with high resource abundance are significantly larger than
animals from low resource areas (Leslie et al. 1952). Schein and Orgain
(1953) estimated that in low SES neighborhoods there was approximately 2.5
times the amount of refuse necessary to maintain resident rat populations and
that removal of refuse significantly reduced the population size (Orgain and
Schein 1953). Davis (1949) raised animals from rural and urban areas in the
laboratory and found that, with equal resources, there were no differences in
growth rates or body sizes despite the observed differences among field
populations. This suggests that much of the body size variation among local
populations is due to phenotypic plasticity in growth rates.
Seasonal differences in growth rates between habitats may be associated
with the timing of resource abundance. Grasses, forbs and arthropods were
the major food items for parkland rats and were particularly abundant during
the warm season. Refuse was available to residential rats the entire year but
did not spoil as quickly during the winter. Rotten foods are generally avoided
by rats (Schein and Orgain 1953) so the amount of available forage during the
warm season may be less than during the cool season. In addition, population
densities were higher during the warm season and access to food may have
been restricted by dominant population members (see below) (Orgain and
Schein 1953).
URBAN ECOLOGY OF NORWAY RATS
15
Sex Ratios. Overall, 53.1% of the rats were males, which does not differ
significantly from unity (%^3.23; \df, P>0.05). However, there were differ-
ences in sex ratios across habitats. In residential sites males made up 51.5%
(n=683) of the population, while they comprised 59.3% (n=167) in the
parklands (tf=5.15\ ldf, /><0.025). Seasonal trends were similar in both areas
with an excess of males in late winter and early spring (Fig. 8), followed by
an increase in the proportion of females, with a peak in June. During the
remainder of the year, the sex ratio showed a consistent, but nonsignificant,
excess of males.
80-
70-
60-
(0
50--
40-
30-
20
T
J F M AMJ J A SOND
Time (Months)
Fig. 8. Monthly changes in the proportion of trapped male R. norvegicus. Numbers in
brackets are sample sizes and vertical bars are standard errors.
16 OCCASIONAL PAPERS MUSEUM OF NATURAL HISTORY
Females predominated in smaller weight classes (<300 g) in residential
sites, whereas there was an excess of males in the larger classes (x^B.87,
/><0.025) (Table 3). In parklands, females also appeared to dominate the
smallest weight class while males were at greater frequencies in the remaining
groups, but the differences were not significant (x^S.98, P>0.10).
Table 3. Proportion of male Rattus norvegicus by weight class, in populations
from residential and parkland study sites. Numbers in parentheses are sample
sizes.
Habitat
<100
Body Weight (g)
100-199 200-299 300-399
400-499
>500
Residential
Parkland
0.44(70) 0.48(144) 0.39(95) 0.57(160) 0.59(153) 0.56 (66)
0.38(16) 0.57(52) 0.64(67) 0.57(30) 1.00 (3) —
The excess of males in parkland populations supports the view that they
represent 'colonizing' populations. Male rats tend to disperse farther and
more frequently than females (Calhoun 1962). The seasonal trends are
consistent with the overall pattern of a minor excess of males, except during
late winter and spring (Fig. 8), when a large excess of males was followed by
an excess of females. This pattern was also observed by Kendall (1984) and
may be a trapping artifact; pregnant females reduce activity outside the
burrow late in gestation, and males become more active while searching for
mates (Calhoun 1962). The peak in male abundance corresponded with the
peak in spring reproduction (Fig. 9) and more females were trapped as
reproduction declined during early summer.
Most studies (e. g. Davis 1951; Davis and Hall 1951; Bishop and Hartley
1976; Stroud 1982) have observed an excess of males although Leslie et al.
(1952), and Farhang-Azad and Southwick (1979) reported a higher propor-
tion of females. The latters' trapping methods were more effective at sam-
pling animals remaining near burrows and may account for the observed
differences. Reasons for the weight associated changes in sex ratios, particu-
larly in the residential populations, are unclear. Some workers have shown
that at or shortly after birth the sex ratio is slightly, but consistently, skewed
towards males. Leslie et al. (1952) reported 51.9% of 509 offspring taken
from nests were males. Farhang-Azad and Southwick (1979) also noted that
51% of animals < 50 g were males. But Bishop and Hartley (1976) and Davis
(1951c) reported shifts towards an excess of males in larger weight classes,
while females predominated in weight classes less than 200 g. Males
reportedly grow more quickly than females (Calhoun 1962), so such a pattern
is expected. In our samples, males weighed 10% more than females (Table 1)
but we could not show sexual differences in the growth rates with our
relatively small samples.
URBAN ECOLOGY OF NORWAY RATS
17
0-....0 9 9
A- ---A Pregnant
0)
o
■a
o
100-
90-
80-
70
60-
50-
40-
30-
20-
10-
V /
J ' F ' M
A M J
S O N D
Time (Months)
Fig. 9. Monthly changes in the proportion of reproductive animals trapped. Open circles
(females) and closed circles (males) are percentages of perforate and/or lactating females and
scrotal males observed. Triangles represent the proportion of pregnant females determined from
necropsy.
Reproduction. External examination indicated reproduction occurred in
males from all weight classes and during every month of the year. Overall,
84.0% (n=424) of males had scrotal testes when captured. There was a
significant difference in the proportion of scrotal males among weight classes
(subdivided into <100 g, 100-199 g, > 200 g; %2 = 183.56; 2df, F<0.0001).
Males less than 200 g had descended testes less often than larger males. Males
less than 100 g were occasionally scrotal (9/32) but six of these were 92-99 g,
and the three others had unconvoluted epididymides when necropsied. There
was probably no active spermatogenesis in these individuals (see below). A
greater proportion of the 100-199 g class males from residential sites were
scrotal (61%) than were their parkland counterparts (33%) (%2=6.20; Idf,
P<0.025). There were no significant differences between residential and
parkland rats in any other weight category.
18 OCCASIONAL PAPERS MUSEUM OF NATURAL HISTORY
The frequency of scrotal males varied seasonally (Fig. 9), decreasing in
late autumn and early winter and peaking in late winter and spring. This was
due to annual changes in the proportion of smaller individuals and in their
reproductive status in the population. Nearly all males > 200 g (99.0%) were
scrotal, while smaller males, which represented an increasing fraction of the
population in autumn and early winter (Fig. 4), were frequently nonscrotal
during these seasons. As small individuals disappeared or matured in late
winter and early spring, there was a concommittant rise in the proportion of
scrotal males. Smaller males were the only group to show seasonality in
reproductive condition. Although 81% (n=26) captured between June and
August had descended testes, only 50% (n=24) were scrotal in the spring, and
only 30.4% (n=69) were scrotal during the autumn and winter.
Necropsies of 97 (23 parkland, 74 residential) males complemented the
results obtained from external examination. Among males greater than 200 g,
convoluted epididymides were present in all scrotal individuals and both
nonscrotal males examined (Fig. 10). Only 3/n of the males <200 g had
convoluted epididymides and all had scrotal testes. Of the remaining 14
nonreproducuve males, six were scrotal.
Overall, males had a mean testicular volume of 1.53 ± 0.71 cc. Testis
volume increased as a function of body weight up to approximately 300 g and
then leveled off at approximately 1.7 cc. Males less than 200 g had testicular
volumes less than 0.6 cc. The relationship between body weight and sperma-
togenesis held for both parkland and residential rats. Despite the greater age
of 200 g parkland rats, they failed to become sexually mature until they
reached this weight (Fig. 10).
External examination suggested reproduction among females of every
weight class during every month of the year (Fig. 9). Eighty-two per cent
(«=382) of females had perforate vaginal orifices or were lactating. Females
less than 100 g were rarely perforate (u/46) and never lactating. Of 95 females
between 100-199 g, 66.3% had perforate vaginal orifices but none were
lactating or pregnant. All but three females greater than 200 g were perforate
(n=241) and 37.1% were lactating. Lactating females were rarely observed
during the late autumn and winter. Lactation peaked in April and May and
again in September and October (Fig. 9).
Females at residential sites showed external evidence of reproduction at a
larger average body weight than parkland females. Only 16% of residential
females < 100 g were perforate while 56% of parkland females from this class
were perforate (%2=6.17; Idf, P<0.025). The proportion of residential females
between 100-199 g with perforate orifices was also lower than in parklands
(63% vs 77%), but the difference was not significant (x2=2.62; Idf, P>0.25).
The three nonperforate females >200 g were from residential sites.
Necropsy data from 193 females supported observations based on external
examination. No female less than 200 g (n=60) was pregnant, regardless of
URBAN ECOLOGY OF NORWAY RATS
19
3.0
2.4
o
I 1.8
O
>
= 1.2
to
a)
0.6-
0-
*. ••
.%:.
A* I *
•••
i
000° °A O
100
200 300
Body Mass (g)
400
500
600
Fig. 10. Relationship between testis volume and body weight for 97 R. norvegicus. Closed
circles represent residential males with convoluted epididymides and open circles represent
residential males with unconvoluted epididymides. Closed triangles are parkland males with
convoluted epididymides and open triangles are parkland males with unconvoluted epidi-
dymides.
vaginal condition. Overall, 42.1% (aj=133) of females >200 g were pregnant
(Fig. 11). The proportion of pregnant females between 200-499 g was
relatively constant (200-299 g,41.7%; 300-399 g, 37.8%; 400-499 g 33.3%)
X2=0.5 1; 2df, P=0.98). Among females >500 g 70.8% (n=24) were pregnant.
The proportion of pregnant animals among 200-399 g parkland rats was
significantly higher than among residential rats. Eighty percent (ai=10) of
parkland females were pregnant compared to 30.8% (n=52) of residential
females (%2=9.03; Idf, P<0.005). This accounts for the slightly higher
frequency of pregnancy among 200-399 g rats, inasmuch as no parkland
female exceeded 400 g. The prevalence of pregnancy in residential rats did not
approach that of parkland rats unul residential females exceeded 500 g.
Pregnant females were found throughout the year, although reproduction
decreased during the summer and winter (Fig. 9). Two breeding peaks were
noted, one in April and the other in September. Between these times breeding
was minimal in June.
The peaks in pregnancies were due to seasonal breeding in <400 g females.
Rats in larger weight classes tended to be pregnant throughout the year at a
relatively constant rate while females <400 g were pregnant primarily
between May and September. Only 3 of 14 of females less than 400 g were
pregnant between October and April while 21 of 47 were pregnant between
May and September (%2=3.81; Idf, P=0.05). In larger weight classes the
20
OCCASIONAL PAPERS MUSEUM OF NATURAL HISTORY
O
c
0)
3
200-99g
(N = 24)
300-99g
(N=37)
400-99g
(N=48)
>:500g
(N = 24)
T 1
4 6 8 10 12 14
t
Litter Size
Fig. 1 1 . Distribution of litter sizes among four female weight classes. Females <200g were
never pregnant. Arrows indicate the median litter size for each weight class.
URBAN ECOLOGY OF NORWAY RATS 21
comparable values were 11 of 25 and 22 of 47 (x2=0.60; idf, P>0.50).
Seasonal pregnancy patterns did not appear to differ between habitats, though
sample sizes were too small to statistically evaluate the data.
Litter sizes also followed a weight-specific pattern. Mean litter size was
10.49 ± 0.45 (Jc± se, n=57; range 1-16). Litter sizes increased with female
body weight, up to 500 g, and then declined (Fig. 1 1). Despite this, the largest
litters were found among females in the 300-399 g weight class. Females
>500 g had a broad range of litter sizes and, consequently, reduced average
litter size. The small litters in this class tended to be found in females with all
embryos in a single uterine horn. Usually, the other horn was reduced and
thread-like. Median litter sizes were the same for 200-399 g females from
both residential and parkland populations (median=10).
Sexual maturity appears to be size — rather than age-dependent. In our
populations 200 g seems to represent a physiological threshold necessary for
reproduction. Males have convoluted epididymides only after reaching 200
g. Although residential males are scrotal at lower weights they apparently
cannot breed. These results are consistent with Davis and Hall's (1948)
observations. They noted that males became scrotal (at 105 g) well in advance
of producing spermatozoa (200 g). They also reported there was no seasonal
testicular regression so males were reproductive throughout the year, but
smaller males tended to be scrotal more often during the summer than the
winter.
Similarly, females do not become pregnant until they reach at least 200 g,
despite having perforate vaginal orifices. These results support Davis' (1949,
1951c) conclusions that rats in rural areas mature more slowly than those in
urban environments. Recent studies examining the effects of exercise and diet
on the onset of puberty in laboratory rodents support the hypothesis that body
weight rather than age is the critical factor in reproductive maturity (Perrigo
and Bronson 1983; Bronson 1987). Comparisons of the age and size at first
ovulation for ad lib fed rats and slowly growing, exercising rats showed a
delay of nearly 2.5 months in onset among exercising rats until they reached
the same weight as control rats (106 vs 1 12 g). This size threshold for maturity
profoundly affects population dynamics, because the slower growth rates of
parkland rats mean fewer animals survive to reproduce (Slade et al. 1984;
Sauer and Slade 1987). As a result, migration may become a relatively more
significant factor in maintaining these populations.
The size-dependent relationship between litter size and body weight
observed in this study is typical of rats (Davis 1951c; Leslie et al. 1952;
Farhang-Azad and Southwick 1979; Hartley and Bishop 1979). In most
populations litter size is an increasing function of female body size. In a few
studies a small proportion of females < 200 g were pregnant (Leslie et al.
1952; Farhang-Azad and Southwick 1979) but none were observed in this
study. Although females < 200 g have been considered reproductively mature
22 OCCASIONAL PAPERS MUSEUM OF NATURAL HISTORY
by some workers (e. g. Leslie et al. 1952; Farhang-Azad and Southwick 1979)
because of perforate vaginal orifices or the presence of corpora lutea, they
contributed little to these populations' reproductive outputs. Determining the
size at which females reach sexual maturity has a significant impact on
estimated fecundities. For example, Davis (1951c) considered females < 200
g to be sexually immature and calculated a mean litter size of 10.1 young/
female, close to our value of 10.5. Farhang-Azad and Southwick (1979) used
all females > 139 g (based on corpora luteal counts) in their estimates of
average litter size (8.7). However, when their data are stratified by the
female's weight class (their Table III), their medians and quartiles do not
differ from ours. Similarly, Leslie et al. (1952) noted that altered criteria for
sexual maturity produced large differences in productivity estimates within
the same population.
The bimodal, seasonal pattern of reproduction (Fig. 9) is characteristic of
most rat populations, with both midsummer and winter declines in fecundity
(Davis 1951c; Davis and Hall 1951; Leslie etal. 1952; Calhoun 1962;Bishop
and Hartley 1976; Pye and Bonner 1980; Stroud 1982). Leslie et al. (1952)
noted peaks in late spring and autumn. Davis (1951c) observed a decrease
from a peak pregnancy rate of 45% in March-April to a low of 21 % in early
summer followed by a secondary peak in autumn. Farhang-Azad and South-
wick (1979) described bimodal reproductive peaks in April, and September
or October.
Our data indicate that seasonal reproduction occurred primarily in smaller
rats (<400 g) while larger animals bred throughout the year. Davis and Hall
(1951), and Leslie et al. (1952) also noted seasonal breeding by smaller
animals, while larger females bred throughout the year. The midsummer
breeding depression may be due to the large influx of nulliparous young
females born in the spring (Fig. 4) that have reached sexual maturity but have
not become pregnant, while the second autumn reproductive peak occurs as
they are recruited into the breeding population. The winter decline in breeding
may represent a delay in the maturation, because of environmental condi-
tions, that often characterizes the autumnal cohort of young rodents (Calhoun
1962).
The proportion of pregnant females remained relatively constant with
body size, ranging from about 35^0%, except among females >500 g, which
were nearly always pregnant. These results are similar to those of Leslie et al.
(1952) and Davis (195 lc) who reported pregnancy rates of 37.9% and 30.4%,
respectively. Median litter size increased with female weight up to the largest
weight class (>500 g) and then declined, suggesting that, despite the high
pregnancy rate, these females may be approaching reproductive senescence.
Davis (1951c) also found a decline in litter sizes among very large females,
although Leslie et al. (1952) observed the decrease only among their 'non-
rick' females. Farhang-Azad and Southwick (1979) reported that litter size
URBAN ECOLOGY OF NORWAY RATS 23
continued to increase even in their largest animals. However, in their study the
proportion of pregnant females in this class declined dramatically compared
to the rest of the population.
Movements. We recaptured 107 of 372 (29%) marked rats 153 times.
Distances between adjacentcaptures were significantly greater in parkland (3c
= 24.7 ± 4.55 m; 95% Confidence Interval, n=64) than in residential areas (3c
= 13.5 ± 5.17 m, n=77). Most distances between recaptures were relatively
short, particularly in residential areas (Fig. 12). Residential rats were recap-
tured 27.3% of the time at the same location. In contrast, in the parkland only
7.8% of ratrecaptures were at the same trap sites. There were several instances
of relatively long-distance movements. Two rats moved more than 85 m in
residential sites, spanning the trapping areas.
Movements based on tracking after snowfall, in alleys, were significantly
longer than those based on capture data (21.6 ± 3.61 m, ai=39). These
movements were direct, going from a burrow or brush pile, along fence rows
and other cover, to a food source and returning. There was little evidence of
exploratory behavior or other movements. Data from direct observations of
individuals were not significantly different in the mean distance traveled,
compared with tracking, but were slightly longer (22.4 ± 7.33 m; /i=73) and
there was more exploratory behavior (see below). These data probably under-
estimate the real distance travelled because of difficulties in observation.
Some rat movements were much farther than indicated by the mean
distance travelled. Four individuals traversed the length of the alleys during
a single foray — a distance of approximately 90 meters. One individual
travelled 165 m, during 15 minutes, moving the length of the block, crossing
a side alley, the main street, and moving part way down the next block before
it was lost going under a fence. Rats crossed major thoroughfares, although
levels of movement were lower than within alleys. During 21 hours of
censusing at one site, a mean of 0.5 rats/hour crossed the street, while an
average of 41.5 rats/hour traversed the alley. However, these rates varied
depending on the levels of rat and human activity.
The movement data shed some light on differences noted by other workers.
Distances traveled in residential areas (based on trapping (Fig. 12) were short,
typically less than 15 m. This matches the results of Davis et al. (1948) who,
using various methods, found 96.3% of movements were less than 25 m.
Visual observations and tracking in snow indicate that trapping results
underestimate the linear distance rats move, and fail to reveal the tendency for
rats to traverse entire alley systems. As such, they support Taylor's ( 1 978) and
Recht's (1982) criticisms that trapping seriously underestimates rat move-
ments. However, observations and tracking also indicate most movements in
residential areas are short and remain within alleys while crossing major
streets is relatively rare. As such, residential rats are probably restricted to
much smaller areas than rats in parkland environments.
24 OCCASIONAL PAPERS MUSEUM OF NATURAL HISTORY
Residential
N = 77
Parkland
N = 64
6 >7
Distance (x 10m)
6 >7
Fig. 12. Distributions of distances between recaptures for residential and parkland rats.
Median distances are indicated by arrows. Parkland rats moved significantly farther than
residential rats.
Even if trapping underestimates movements, comparisons of parkland and
residential sites show that rats move farther in rural than in urban areas (Fig.
12). Other workers have noted that individual movements are labile and
frequently a function of the distance between the burrow site and food. If food
is readily available rats move very little while other individuals in the same
population may move considerable distances to forage (Davis et al. 1948;
Taylor 1978; Hardey and Bishop 1979; Pye and Bonner 1980). If food
availability is a major determinant of the distance rats travel, movements in
urban areas may be shorter than in rural settings due to the presence of refuse.
URBAN ECOLOGY OF NORWAY RATS
25
Most rats in our residential sites had to move less than 20 m from their burrows
to reach food. Parkland rats relied on grasses and arthropods for food.
Grasses, in particular, are nutritionally inferior to the refuse available in
residential areas (Schein and Orgain 1953) and may require parkland rats to
move further or more often to obtain resources.
Wounding. Wounding status was scored for 702 rats. Patterns of wound-
ing showed significant sexual, seasonal and weight based differences (Fig.
13). Fifty-two percent of residential males, 42.3% of residential females, and
25.0% and 9.4% of parkland males and females, respectively, were wounded.
There was no statistically significant difference in wounding patterns be-
tween residential and parkland rats (x2=6.01; Adf, P=0.20) after sexual,
seasonal and size factors were considered. The apparently lower prevalence
of wounding in parklands appears to be due to differences in weight structure.
Therefore, both habitats were combined for further analysis. The effects of the
remaining variables on wounding were mutually independent, as indicated by
the lack of statistically significant pairwise and higher order terms.
Males had significantly more severe wounds than females (x2^ 12.98; Adf,
/^O.Ol). Severe wounds (grades 3^4) were found on 15.9% of males, but only
10.5% of females were seriously wounded. Wounding prevalence increased
>
o
z
UJ
o
LU
DC
oV
6 29 38 21
CSSSSSg
PARKLAND
??
22 20 13
m mm
< 1 1 2 >3 <1
RESIDENTIAL
27 58 35 71 70 24
> 3
??
GRADE 4
GRADE 3
GRADE 2
GRADE 1
GRADE 0
32 69 46 48 41 23
£ 5
MASS CLASS (X 100 9>
Fig. 13. Frequencies of wounding for male and female rats from residential and parkland
sites stratified by weight class. There were no significant differences in wounding rates between
habitats within sexes or weight classes. Weight classes are defined in Fig. 5.
26 OCCASIONAL PAPERS MUSEUM OF NATURAL HISTORY
steadily with weight in both sexes. Prereproductive (<200 g) individuals had
lower wounding rates than sexually mature rats. Two males < 200 g («=121)
had body wounds and only six had minor tail wounds. Nine females < 200 g
(n= 133) had body wounds. Both sexes had approximately equal wounding in
the 200-299 g weight class (Fig. 13). Wounding increased dramatically
among males in the larger classes so that 84.2% of males >500 had body
wounds and more than half had grade 4 wounds. In contrast, less than half of
all females in the largest weight classes had body wounds and only two had
grade 4 wounds. Instead, most (60.7%) were tail or minor rump wounds.
Seasonally, serious wounding occurred at significantly higher rates during
the summer and autumn (37.2% of animals) than during the winter and spring
(10.1%) (x^ 10.85; Adf, T^O.03). Particularly among females, wounding
was nearly absent during the spring, and was elevated during the summer
among animals in the 300-499 g weight classes (1 of 30 versus 12 of 16
animals).
The choice of 200 g as a criterion for characterizing mature animals is
consistent with the data on wounding patterns. Adult rats are rarely aggressive
towards sexually immature individuals. However, as they approach sexual
maturity, aggressive encounters with socially dominant animals increase
(Calhoun 1962; Blanchard et al. 1985). In our study, animals < 200 g were
rarely wounded and were never observed involved in prolonged fights (see
Behavioral Interactions). Their wounds were rarely severe.
At about 200-299 g wounding increased in both frequency and severity.
Wounding increased dramatically in larger males, presumably due to inter-
male conflict for access to breeding females (Calhoun 1962). Although none
of the attacks could be directly ascribed to this, only a few fights involved
access to food or other identifiable resources. Severity of wounding became
so widespread in large males (Fig. 13) that it represented a significant risk to
individuals (Calhoun 1962; Farhang-Azad and South wick 1979). Unlike
Blanchard et al. (1985), we noted a strong sexual difference in wounding
severity consistent with description from enclosed populations (Calhoun
1962). The decrease in wounding among males during the winter may
represent decreased intermale conflict as female reproduction and population
size declines overwinter.
Causes of wounding in females have received less attention in the litera-
ture. However, despite its lower severity than in males, wounding occurred in
a significant proportion of the females (Fig. 13). Male rats are, generally,
unaggressive towards females (Calhoun 1962; Blanchard et al. 1984).
Dominant females reportedly attack unknown males and may be bitten in
return, but this is rare (Blanchard et al. 1984), implying that most wounds are
due to interactions with other females. Increased wounding rates during the
summer may represent increased conflict among reproductive females as
populations increased from winter and spring lows. This is supported by the
URBAN ECOLOGY OF NORWAY RATS
27
restriction of increased wounding rates to the 300-499 female rats, the major
reproductive classes.
Behavioral Interactions. Observations were made at four residential
sites, and involved 964 observations of 1 130 rats in 92 hours. Rats were seen
as lone individuals in 87.0% (n=839) of all observations, in pairs in 10.6%
(n=105) and in trios or larger groups in 2.1% (n=20) of the cases (Table 4).
Table 4. Behaviors and group size of Ran us norvegicus viewed in alleys.
Number of Rats Observed*
Behavior
1
2
3
>4
Total
Locomotor
817
10
9
3
839 (87.0)
Foragingb
21
4
1
2
28 (2.9)
Agonisticc
—
78
3
—
81 (8.4)
Folio wingd
—
13
2
1
16 (1.7)
Total
838(87.0)
105(10.9)
15(1.4)
6 (0.6)
964
Observations of >1 rat includes only animals seen together as a group.
bIncludes feeding, drinking, and food carrying.
Includes chasing, fighting, leaping, and displacing,
deludes passive following and mating.
Overall, 87.0% (n=838) of all behaviors were classified as locomotory
(Table 4). Locomotory behaviors usually involved single rats moving across
an alley (a distance of 3>-A m), and were typically of short duration (<3
seconds). Movements across alleys were rapid, involving a 'galloping' gait
(Ewer 1971; Recht 1982). However, on occasion, animals would amble and
pause for bouts of grooming which typically lasted 0.5-3 minutes. Move-
ments, along the entire alley (above), took 5-20 minutes, and periods of rapid
movement were interspersed with pauses, grooming, and exploration of the
immediate surroundings.
Agonistic encounters were the most commonly observed interactions
(Table 4). Chasing was the most common agonistic behavior (ai=28 pairs) and
involved rapid movements, without prolonged contact, accompanied by
frequent squealing that could be heard from a distance of 20 m. Chases lasted
from a few seconds to more than a minute. Short duration chases usually
terminated with the chased animal escaping into a burrow. Longer chases
usually involved access to a resource, such as food, that neither animal would
abandon. The pursuing animal was in all cases the larger of the pair. One 254
g female rat maintained sole access to a food pile for more than 90 minutes
by repeatedly chasing four small juveniles that approached her while she fed.
Fighting («=21 observations) and displacing (n=20) occurred at similar
28 OCCASIONAL PAPERS MUSEUM OF NATURAL HISTORY
frequencies during agonistic encounters. Fighting involved actual physical
contact, squealing, tumbling, and apparent biting; it frequently terminated in
chases. Fights normally lasted less than 10 seconds, although one lasted more
than a minute with periods of rest interspersed between fighting. Submissive
behaviors, other than escapes, were never observed. Displacing behavior
occurred when a rat approached a second individual and the second rat
abandoned his/her location and left the area. The movements of both animals
were unhurried and the approaching rat did not chase the displaced animal.
Jumping or leaping behavior was seen on five occasions. Typically, one rat
would leap with all four legs off the ground as a second animal approached.
This behavior is similar to Robitaille and Bovet's (1976) 'startling jump,'
except that touching by the second animal did not always occur. Leaping is
reportedly an escape from aggressive behavior (Ewer 1971; Robitaille and
Bovet 1976; Hart 1982). The jumping animal immediately fled the area, often
precipitating a chase by the other rat
Following behavior was differentiated from agonistic behavior, especially
chases, by the slowness of the gait and the absence of squealing. Seven of the
16 following observations involved pairs of large adults. In each case
illumination was sufficient to establish that the following animal was a male
and the leading animal probably a female (indicated by large size and the
absence of scrotal testes). This form of following behavior may be related to
courtship (Calhoun 1962; Ewer 1971; Hart 1982), but mating was only seen
once. It involved a single male following at a female's flank and mounting
twice. Each mounting was followed by the male licking his genitals (Ewer
1971; Dewsbury 1975).
Following was also observed among small rats. Five pairs of juveniles
(<100 g) emerged from burrows and jointly explored areas adjacent to the
burrows. Groups of three or more rats formed the remaining observations. In
all cases the animals were small (<100 g) and engaged in exploratory
behavior. This observation was supported by trapping. In three instances we
captured three individuals in a single trap and all the animals were <100 g.
This could only have occurred if the animals were following very closely.
Foraging behavior involved actual feeding, drinking or carrying of objects
in the mouth. Most observations of foraging (21 of 28) involved single rats
(Plate I, Table 4) but occasionally, large groups of rats (6 or 7) fed from a single
refuse pile. Generally, there were no overt interactions among individuals at
such times, but five chases resulted from individuals attempting to preempt
food. Rats were most frequently observed feeding on and carrying refuse
items, although animals also ate fallen mulberry fruits. Foraging on garden
plants and grasses was indicated by piles of clipped vegetation at burrow
entrances. Rats drank from pools of water that accumulated in the alleys
during rainstorms or following human activity (car washing, gardening, etc.).
Drinking, or at least the lowered posture associated with it (Ewer 1971),
URBAN ECOLOGY OF NORWAY RATS 29
occurred for several minutes without interruption.
Observations indicate that away from burrow systems most rats acted as
solitary individuals with little direct social interaction (Table 4). Most
interactions that did occur were agonistic, and while most did not involve
actual physical contact, fighting occurred in a high proportion of cases. Fights
were of short duration and typically occurred between nearly equally sized
animals. Small rats were never seen fighting, although they were occasionally
chased. These data are consistent with Robitaille and Bovet's (1976) obser-
vations of rat interactions at a landfill, where rates of agonistic encounters
were high. Also consistent with their observations was the absence of
submissive behaviors (other than retreating from chases, and displacement),
that characterize interactions among laboratory rats (e. g. Hart 1982). This
lack of complex submission behaviors may be due to the inability of
subordinate rats to escape under laboratory conditions, or because more
elaborate submissive behaviors only occur among members of the same
burrow, which we could not observe. Escape may be the only submissive
response between rats from different burrows. The wounding data are also
consistent with the interpretation that complex appeasement behavior may
not occur among free-ranging rats.
CONCLUSION
This study suggests that many of the reported differences in the character-
istics of rat populations primarily reflect plastic phenotypic responses to the
environments; methodological differences among workers are of secondary
importance. Direct comparisons of population composition, movements, and
survival and growth rates from different habitats within the same geographic
area show the broad range of responses of a very labile species. Individuals
in parkland populations grow more slowly, are smaller sized, mature later,
live at lower densities, are more vagile, but live as long as rats from residential
sites. Many of these factors may be directly related to the growth rates of
individuals and as such may reflect variation in resource availability and
quality. Future studies should focus on quantifying the availability and
distribution of resources in differing habitats and relating them to the
dynamics of the populations.
ACKNOWLEDGMENTS
Financial and logistical support for this study was provided by contract
D AMD-894-C-40 1 5 to J . E. Childs. The views of the authors do not purport
to reflect the position of the Department of the Army or Department of
Defense. We wish to thank the people of Baltimore, especially the residents
of our study areas for their understanding and patience. We appreciate the
30 OCCASIONAL PAPERS MUSEUM OF NATURAL HISTORY
assistance of the Department of Parks and Recreation for providing permis-
sion to use various city locations for our study. Animals were handled in
accordance with NIH Guidelines "Guide for the Care and Use of Laboratory
Animals" and were trapped under permits from the Maryland Department of
Natural Resources. David E. Davis, Robert D. Holt, and two anonymous
reviewers read and greatly improved earlier drafts of this paper.
SUMMARY
Norway rat (Rattus norvegicus) populations were studied at nine sites in
the city of Baltimore by a combination of removal and mark-recapture
methods. Trapping locations varied in the density and socioeconomic status
of human habitation, ranging from rural, parkland habitat to high density, low
income, urban areas. Rat population sizes varied annually with major peaks
of abundance in autumn. Parkland numbers were smaller than in residential
areas, except in spring when an influx of animals resulted in a temporarily
large population. Populations in parklands appeared to be more influenced by
local movements of animals than by in situ recruitment. Rats moved nearly
twice as far (3c=25 m) between recaptures in parklands as in residential areas
(3c=14 m). Direct observation of residential rats showed movements were
primarily restricted to a single alley system and generally involved move-
ments from a burrow to a food source (i. e. refuse). Animals in parklands were
significantly smaller than those in residential sites. Survival rates of marked
animals did not differ among sites, but residential rats grew more rapidly and
stopped growing at larger body weights than parkland rats. Rats did not
become sexually mature until a body weight of 200 g, based on necropsy data,
in both habitats. Litter size averaged 10.5 young/pregnant female and 42% of
females were pregnant. Pregnant rats were more common in parklands than
in residential areas. Mean litter size was maximal for female body weights of
about 300 g. Reproduction was seasonally bimodal with peaks in spring and
autumn. Bimodality was due to seasonal breeding in smaller animals;
individuals >400 g bred at a constant rate year round. Wounding followed
seasonal, sexual and weight-specific patterns. Sexually immature animals
were rarely wounded. Wounding increased with size for both sexes but was
more severe among males than females. Wounding tended to decrease during
the late winter when populations and reproductive rates were at their lowest.
Females 300-499 g had their highest rates of wounding in late summer as
population sizes and reproduction began to peak. Wounding rates were
consistent with observations of behavioral interactions. More than 16% of all
behaviors were agonistic, and actual Fighting represented more than 5% of
recorded observations. Many of the differences reported in rat population
biology from throughout the species' range were observed within the various
populations in Baltimore and may be due to phenotypic responses to resource
availability.
URBAN ECOLOGY OF NORWAY RATS 31
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URBAN ECOLOGY OF NORWAY RATS
33
Plate I. Characteristic urban habitats trapped for R. norvegicus within Baltimore included
urban parkland at CH (top) and urban residential sites at PP (middle) and RH (bottom). Note
large mounds of dirt excavated from under concrete slabs that mark the entrances to rat burrows
(arrows) in residential areas. Large quantities of refuse were available for foraging in some low
SES residential areas (middle and bottom). Foraging by rats and cats were common at these
locations (bottom). Note the smaller rat underneath the garbage near the cats as well as the larger
individual to the left (arrow). A kinked tail with several scars are visible on the larger rat.
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