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Can. J. Zool. 76: 1153–1164 (1998) © 1998 NRC Canada
1153
Population density, survival, and rabies in
raccoons in an urban national park
Seth P.D. Riley, John Hadidian, and David A. Manski
Abstract: Density and survival of a raccoon (Procyon lotor) population in Rock Creek Park, an urban national park in
Washington, D.C., were estimated using mark–recapture and radio-tracking over an 8-year period following the appearance of
the mid-Atlantic States (Mid-Atlantic) rabies epizootic. Raccoon density ranged from 333.3 to 66.7/km2, with an overall park
estimate of 125/km2. This density places the Rock Creek population within the range of other urban and suburban populations
and is many times greater than raccoon densities reported from other habitats. Density was particularly high in one thin spur of
parkland with the smallest ratio of area to urban edge. Raccoon survival rates were high except among juveniles during the
rabies epizootic. Data on rabies prevalence from Washington, D.C., indicate a cycle with peaks in 1983 during the initial
epizootic and again in 1987 and 1991, a pattern similar to that seen in other carnivores and in rabies models. We found evidence
of decreased raccoon density during and after the 1987 rabies resurgence relative to the years following the original epizootic,
when rabies prevalence was low. While hunting and trapping represent a major mortality factor for many rural raccoon
populations, urban and suburban populations and protected populations may frequently be subject to epizootics of diseases
such as canine distemper and rabies, even years after initial contact with a disease.
Résumé : La densité et la survie d’une population de ratons-laveurs (Procyon lotor) ont été estimées par capture–recapture et
par radiotélémétrie dans le parc Rock Creek, un parc national urbain de Washington, D.C., au cours d’une période de 8 ans
consécutive à l’éclatement de l’épidémie de rage dans les états du milieu de la côte atlantique. La densité des ratons-laveurs
s’échelonnait de 333,3 à 66,7/km² pour une densité globale de 125/km² dans tout le parc. Cette densité place la population de
Rock Creek au même rang que les autres populations urbaines et suburbaines et est beaucoup plus élevée que la densité des
ratons-laveurs d’autres habitats. La densité s’est avérée particulièrement élevée dans une projection étroite du parc où le rapport
surface/périmètre de la zone urbaine était faible. Sauf chez les jeunes, les taux de survie des ratons-laveurs sont demeurés
élevés au cours de l’épidémie. Les données sur l’évolution de la rage au sein de la population de ratons-laveurs de Washington,
D.C., indiquent que la fréquence de la maladie a atteint un sommet en 1983 au début de l’épidémie, un autre en 1987 et un autre
en 1991, suivant les mêmes tendances que chez les autres carnivores et conforme aux modèles théoriques. La population de
ratons-laveurs a subi un déclin durant la résurgence de la maladie en 1987 et par la suite, comparativement aux densités
enregistrées après l’épidémie originale alors que la fréquence des cas était faible. La chasse et le « trappage » constituent des
facteurs de mortalité importants pour les populations rurales de ratons-laveurs, mais les populations urbaines et suburbaines ou
les populations protégées sont souvent sujettes aux épidémies de maladies comme la maladie de Carré ou la rage, souvent
même plusieurs années après le premier contact avec la maladie.
[Traduit par la Rédaction]
Introduction
The study of urban wildlife ecology is a growing but still
neglected field despite the rate of urbanization throughout
North America and other parts of the world. An understanding
of wildlife ecology in urban and suburban areas and in adja-
cent nature preserves is crucial to the continued persistence of
wild populations in the face of human impacts (Murphy
1988). The raccoon (Procyon lotor) is a common urban spe-
cies that thrives in many habitats throughout North America
(Kaufmann 1982), including in and around Washington, D.C.
The 1983 arrival of the mid-Atlantic States (Mid-Atlantic)
rabies epizootic in raccoons in Washington, D.C., raised inter-
est in raccoon ecology and disease ecology in urban areas,
particularly because of the high densities of both humans and
raccoons and the increased possibility of transmission of dis-
ease from raccoons to humans or domestic animals (Jenkins et
al. 1988). Researchers in urban and suburban areas have
found dense populations of raccoons (Schinner and Cauley
1974; Hoffman and Gottschang 1977; Slate 1980; Jacobsen
1982; Rosatte et al. 1990), and dense populations are more
likely to be subject to epizootics of contact diseases such as
rabies and canine distemper and may be more likely to con-
tinue to harbor a disease after the initial epizootic. In this
study we wanted to increase our understanding of the popula-
tion dynamics of this important urban species and of urban
Received 19 September, 1997. Accepted 7 January, 1998.
S.P.D. Riley,1 J. Hadidian,2 and D.A. Manski.3 Center for
Urban Ecology, National Park Service, National Capital Region,
1100 Ohio Drive SW, Washington, DC 20242, U.S.A.
1Author to whom all correspondence should be sent at the fol-
lowing address: Department of Environmental Science and
Policy, University of California, One Shields Avenue, Davis,
CA 95616, U.S.A. (e-mail: spriley@ucdavis.edu).
2Present address: The Humane Society of the United States,
2100 L Street NW, Washington, DC 20037, U.S.A.
3Present address: Acadia National Park, P.O. Box 177,
Bar Harbor, ME. 04609
1154 Can. J. Zool. Vol. 76, 1998
© 1998 NRC Canada
wildlife in general by measuring the density and survival of
raccoons in Rock Creek Park, an urban National Park in
Washington, D.C., by tracking rabies and measuring the
impact of the disease on raccoon density and survival.
Because urban habitats are drastically altered, fewer spe-
cies survive in them, but those that do may benefit from more
exclusive use of available resources. Mammals other than
raccoons also exist at high densities in urban areas, including
small mammals (Babinska-Werka et al. 1981) and other car-
nivores such as red foxes (Harris 1981). High urban densities
have also been reported for certain bird populations (Nuor-
teva 1971; Emlen 1974; Vale and Vale 1976; DeGraaf and
Wentworth 1981; Beissinger and Osborne 1982). Dense pop-
ulations of urban wildlife may result from an increased avail-
ability of resources such as food, refuges, or den sites. The
distribution of resources in the landscape may also influence
wildlife densities in developed areas. Concentration of forag-
ing or denning habitat in restricted natural areas can lead to a
higher expressed density of animals in urban habitats (Vale
and Vale 1976; Matthiae and Stearns 1981; Dickman 1987;
Dickman and Doncaster 1987).
Dense wildlife populations are more likely to be impacted
by diseases transmitted by contact and may be more likely to
maintain the disease. The Mid-Atlantic rabies epizootic in
raccoons began in the late 1970s in Virginia and reached
Washington, D.C., in late 1982 with 5 cases, peaking in 1983
with 158 cases (Jenkins and Winkler 1987). The disease may
have particularly impacted high density urban populations,
such as those in Baltimore and Washington, D.C. (Winkler
and Jenkins 1991). Anthony et al. (1990), using vehicle fatal-
ities as a population index, note a decline in raccoons in Balti-
more following the rabies epizootic.
Epizootic rabies has the potential to affect survival rates in
raccoons. A few researchers have measured raccoon sur-
vival, although generally with an interest in the effects of
human harvesting, which can significantly reduce survival
rates (Fritzell and Greenwood 1984; Clark et al. 1989; Has-
brouck et al. 1992). Brown et al. (1990) reported raccoon sur-
vival before and after harvest season during the onset of the
rabies epizootic in Pennsylvania, but they found no rabies
mortality. A raccoon population in rural Virginia experi-
enced increased mortality during the epizootic (Seidensticker
et al. 1988). Raccoon survivorship and mortality factors may
be different in urban settings, where recreational hunting and
commercial trapping are generally prohibited but the proba-
bility of being struck by a vehicle, trapped as a nuisance ani-
mal, or infected by an epizootic disease such as rabies or
canine distemper may be greater. Epizootics of these diseases
have been studied in urban raccoon populations (e.g., Bigler
et al. 1973; Roscoe 1993), although raccoon survival was not
directly measured in these studies.
Raccoon rabies cycled upwards again in Virginia in 1986,
perhaps because of an increase in susceptible animals after
the epizootic (Torrence et al. 1992; Winkler and Jenkins
1991). Torrence et al. (1992, p. 374) note that “The fact that
the positive percent of rabid raccoons was still decreasing in
1989 may mean that rabies will continue to decline to an
endemic level and then disappear, or assume a cyclic pattern
over a longer period of time than was analyzed in this study,”
and Roscoe (1993) found 4-year cycles of canine distemper in
raccoons. Mathematical models of rabies in red foxes produce
cycles every 3–5 years, and density and rabies prevalence in
red foxes in Europe and Canada cycle with the same period
(Anderson et al. 1981). Seasonal mating, reproduction, and
dispersal, which are thought to lead to rabies cycles in foxes
(Anderson et al. 1981), are also characteristic of raccoons.
However, raccoons exhibit a different immune response to
rabies than red foxes, both in their response to different types
of vaccines (Rupprecht et al. 1986) and in the ability of some
raccoons to develop some immunity to the disease (Carey and
McClean 1983). A model of raccoon rabies that includes a
class of animals which survives and develops natural immu-
nity to rabies produced cycles at 4- to 5-year intervals but
with smaller amplitudes in subsequent peaks and shorter
dampening times than the red fox models (Coyne et al. 1989).
Coyne et al. (1989) speculate that no cycling in disease preva-
lence or raccoon density should be detectable in the field.
We hypothesized that raccoons in Rock Creek park would
exist at high densities relative to populations in rural areas,
both because of plentiful food and denning resources and
because the park represents a concentrated source of resources
adjacent to less hospitable urban habitats, thus perhaps elevat-
ing raccoon density in the park. We hypothesized that survi-
vorship would be lower in Rock Creek raccoons during the
epizootic because of increased mortality from rabies. We also
measured rabies prevalence in Washington, D.C., raccoons
for 12 years, starting with the epizootic in 1983, to look for
long-term trends and for evidence of cycling. Finally, we
hypothesized that raccoon density would be affected by any
post-epizootic increases in rabies prevalence.
Materials and methods
Study area
The study was conducted at four sites in Rock Creek Park (Fig. 1), a
710-ha National Park located within metropolitan Washington, D.C.
(38°57′N, 77°02′W). The park was established in 1890 by an Act of
Congress that expressly prohibited any “injury or spoilation” of ani-
mals and other natural resources. Approximately 85% of the park
consists of natural areas of mixed mesophytic forest with a deciduous
overstory dominated by tulip poplar (Liriodendron tulipifera), oak
(Quercus spp.), and beech (Fagus grandiflora).
For each of the four study sites, Piney Branch, Hazen, Colorado
Avenue, and H-2 (Fig. 1), we estimated the density of the human
population and housing units in the area surrounding each site from
census tract data (Anonymous 1988a, 1988b). We also calculated the
total area of each site and the length of its urban edge with a
planimeter, and we then calculated the interior to edge ratio. The
Hazen study area is the smallest (30 ha) and thinnest area, with an
interior to edge ratio of 12.7 m2/m. Hazen also has the highest adja-
cent human population density, 45.9 individuals/ha, and housing unit
density, 35.5 units/ha. The other spur of parkland, Piney Branch, is
larger at 61 ha, with an interior to edge ratio of 16.6 m2/m and adja-
cent human and housing unit densities of 25.2 and 11.2/ha, respec-
tively. The Colorado Avenue and H-2 areas, both part of the main
body of the park, cover 80 and 56 ha and have much larger interior
to edge ratios, 57.3 and 61.5 m2/m, respectively, than the two spurs.
The Colorado Avenue and H-2 sites have similar demographic fea-
tures to Piney Branch, with adjacent human densities of 25.2 and
28.2/ha and adjacent housing unit densities of 11.2 and 11.8/ha,
respectively.
We measured raccoon density at the Piney Branch, Hazen, and Col-
orado Avenue sites. We chose the Piney Branch and Hazen areas
because they are thin strips of parkland surrounded by urban area and
the Colorado Avenue area because it forms part of the main body of
Riley et al. 1155
© 1998 NRC Canada
Fig. 1. Study areas in Rock Creek Park.
1156 Can. J. Zool. Vol. 76, 1998
© 1998 NRC Canada
the park and served as a comparison for the other two areas. We treated
these three sites as separate units for population analyses because,
although they shared some borders, they are separated by roads and a
large stream (see Fig. 1) and because only 10 of 129 raccoons recap-
tured during the study were recaptured in a neighboring site.
We measured raccoon survival at the Piney Branch and H-2 sites.
The Piney Branch site was chosen for intensive radio-tracking work
at the beginning of the study, and a later study of raccoon family rela-
tionships at the H-2 site provided survivorship data from long after
the original epizootic.
Trapping protocol
We livetrapped raccoons to obtain data for mark–recapture estimates
and to radio-collar animals for survival estimates. We set live traps
(Tomahawk No. 207, Tomahawk Live Trap Company, Tomahawk,
WI 54487, U.S.A.) for 5 consecutive nights during each trapping
period. We used 10 sites in Hazen (0.32/ha), 12 sites in Piney Branch
(0.20/ha), and 15 sites in Colorado Avenue (0.19/ha). Two traps were
set at each site to allow for multiple captures. We baited traps in late
afternoon or early evening and examined them at first light. We took
all captured raccoons to a quarantine facility for examination unless
they had been previously captured and examined during the trapping
period. We immobilized raccoons with ketamine hydrochloride
(Ketaset, Bristol Laboratories, Bristol-Myers Co., Syracuse, NY
13221, U.S.A.) at dosages of 10–12 mg/kg (Bigler and Hoff 1974) or
a mixture of ketamine and xylazine hydrochloride (Rompun, Mobay
Corp., Shawnee, KS 66201, U.S.A.) at 5:1 and 7:1 ratios at the same
dose rate. All subjects were ear-tagged in both ears with Monel No. 4
metal ear tags (National Band and Tag Co., Newport, KY 41072,
U.S.A.) and tattooed on the chest with a unique number. We deter-
mined the age and sex of each animal. We classified males with a
fully extrudable baculum measuring 95 mm or larger as adults
(Sanderson 1961) and females with teats of 5 mm or larger as adults
(Steuwer 1943). We fitted animals chosen for radio-tracking studies
with a radio collar (Telonics, Inc., Mesa, Ariz., U.S.A., or Advanced
Telemetry Systems, Isanti, Minn., U.S.A.). Following recovery we
released animals at their respective capture sites in the afternoon or
evening of the same day.
Data analysis
Population density
We estimated population size at each study site using mark–recapture
techniques (Otis et al. 1978; Seber 1982; Nichols 1992). We trapped
for 11 periods in the Piney Branch area, 7 in the Hazen area, and 4 in
the Colorado Avenue area. We defined four seasonal periods: winter
(January–March), spring (April–June), summer (July–September),
and fall (October–December). Sampling intensity varied between
study sites because we were particularly interested in long-term data
for the Piney Branch area and because trapping was conducted for
purposes other than population estimation alone, particularly early in
the study.
We estimated population size using the program recapture, which
employs a modified version of the Jolly–Seber model (Seber 1982)
for open populations. The model modifications account for animals
known to have died during the study (tag returns), eliminate negative
birth and immigration values and survival rates greater than 1, and
allow for periods during which no influx or outflow occurs (Buck-
land 1980, 1982). Of available population estimation techniques, the
Jolly–Seber model is considered the most appropriate for raccoon
populations (Hallet et al. 1991). We were able to estimate population
size for 16 of the 22 trapping periods, since open-population models
such as the Jolly–Seber model do not generate estimates for the first
or last period of any single analysis. In order to take both capture
probability and survivorship into account in looking for sexual differ-
ences, for each trapping period we constructed a 2 × 2 contingency
table of the numbers of males and females captured in that period
which were later recaptured or were never seen again (J.D. Nichols,
personal communication).
We converted population size to density by dividing each
population-size estimate by the size of the area of parkland in which
we trapped. We had attempted, through the number and placement of
trap sites, to effectively sample each study site. Since we knew from
radio-tracking data that many raccoons had home ranges which
included both parkland and adjacent neighborhoods, this estimate
may overestimate the density of raccoons using a larger area that
includes urban habitats. Ultimately, however, we are interested in the
density of raccoons that use the park, so this density estimate seemed
most applicable.
Survival
We measured raccoon survivorship using radio-tracking data col-
lected in 1983–1984 in the Piney Branch area and in 1989–1990 in
the H-2 area. We estimated survival on the basis of radio-days of sur-
vival (1 radio-day is 1 day on which one raccoon was being radio-
tracked) with the program micromort (Trent and Rongstad 1974;
Heisey and Fuller 1985). Survival is defined as the probability of a
radio-collared animal surviving from the beginning through the end
of a specified time period. We used the “bias-corrected” estimates of
survival computed by micromort because the maximum-likelihood
estimator for survival over periods longer than 1 day has a small bias
directly related to increasing interval length, decreasing sample size,
and decreasing daily survival rate (Heisey and Fuller 1985). We also
estimated cause-specific mortality rates, or the probability of dying
from a specific cause over a certain time interval, for rabies and non-
rabies mortality.
Locations were obtained on each radio-collared animal at least
three times per week. All radio collars included a motion sensor or
“mortality mode,” so that if the radio did not move for 12 h, the pulse
rate of the signal doubled. Therefore, when an animal died, we were
able to recover the body within 2 days. Although Heisey and Fuller
(1985) suggest computing a minimum estimate of survival based on
the assumption that each animal which disappears has died, we com-
puted maximum estimates of survival based on the assumption that
animals whose signals were lost were still alive. Using maximum sur-
vival estimates provides a more conservative test for disease effects.
Maximum estimates also seem more reliable because a number of ani-
mals whose signals were lost were later known to be alive and because
radio-collared raccoons exhibited long-range dispersal movements.
All animals that died and were recovered were necropsied at the
National Zoological Park’s Department of Pathology. Brain tissue
was removed from all subjects and tested for rabies by the District of
Columbia Department of Human Resources with the use of fluores-
cent rabies antibody tests (Abelseth and Trimarchi 1983).
We computed survival rates for the entire 1983–1984 telemetry
study, for each season, for the period when rabies prevalence was
high (August 1983 to March 1984), and for the remaining 9 months
when rabies prevalence was lower (April 1984 to December 1984).
For the 1989–1990 study we computed survival for the whole study
period (April 1989 through April 1990) for the adults, and for July
1989 to April 1990 for adults and juveniles because no juveniles were
radio-collared before July, and for each season.
Between May 1983 and October 1984 all captured raccoons, and
therefore all radio-collared raccoons, were experimentally immu-
nized (Jenkins et al. 1988) using a killed rabies vaccine (IMRAB,
Mieureux Laboratories, Paris, France). This procedure probably
affected survival during the initial rabies epizootic, although we can-
not quantify these effects. Animals followed in the 1989–1990 study
of raccoon family relationships in H-2 were not immunized.
Rabies
Data on the number of positive raccoon rabies cases and the total
number of raccoons tested for rabies each month from 1983 through
1994 were obtained from District of Columbia’s Animal Control
Riley et al. 1157
© 1998 NRC Canada
Division of the Department of Public Health (T. Harper, personal
communication). This agency has routinely tested all suspect rac-
coons submitted to it since the rabies epizootic reached Washington,
D.C., in the fall of 1982.
Disease incidence, or the number of positive cases of a disease, is
an uncertain measure of disease prevalence because of the often large
variation in the numbers of animals tested (MacInnes 1987; Torrence
et al. 1992). We used the ratio of positive rabies cases to the number
of animals tested as a measure of rabies prevalence that reduces the
impact of this variation in sample size (Winkler and Jenkins 1991).
We used a 3-month running average of this ratio to reduce the impact
of any single month that might also constitute a small sample (Fig. 2).
We used spectral analysis to test for cycles in raccoon rabies preva-
lence (Shumway 1988).
Statistical tests
We compared estimates of population size and survivorship between
different time periods, such as periods of high versus low rabies inci-
dence, different age and sex groups, and different study areas, using z
tests for simple comparisons (Rosner 1982) and the χ2 test of Sauer
and Williams (1989) for more complex comparisons using the pro-
gram contrast (Hines and Sauer 1989). Fisher’s exact tests and χ2
tests were used to look for sexual differences in recapture probability
and Yates’ correction was used with χ2 with 1 degree of freedom
(Rosner 1982). We decided that three estimates of population size
with coefficients of variation of 1.00 or more (Table 1) were too
imprecise and they were not used in comparisons.
Results
During the study we captured 386 different raccoons 525
times. Of these, 57 (14%) were recaptured at least once and
30 (8%) were recaptured two or more times. We captured
significantly more males (223 animals caught 295 times)
than females (163 animals caught 230 times) (χ2= 9.02, p <
0.005, df = 1). The overall sex ratio (M:F) of animals trapped
was 1.37:1. Only 1 trapping period of the 19 that we could
test was associated with a significant difference in recapture
probability between males and females (χ2= 5.25, p < 0.05,
df = 1; 16 Fisher’s exact tests, 3 χ2 tests), so we used popula-
tion estimates that combined the sexes.
Population density
We estimated population density for 16 trapping periods over
the three different study sites (Table 1). Density estimates
ranged from a high of 333.3 raccoons/km2 during August and
November 1985 in the Hazen area to a low of 67/km2 in
March 1988 for the Piney Branch population.
We found a significantly higher density of raccoons in the
Hazen study area than in the Piney Branch (χ2 = 11.07, two-
sided p < 0.001, df = 1) or Colorado Avenue (χ2= 10.03, two-
sided p < 0.001, df = 1) area. Densities did not differ signifi-
cantly between the Piney Branch and Colorado Avenue study
sites (χ2= 0.31, two-sided p = 0.58, df = 1). These compari-
sons are affected by the fact that we did not always sample
the three study areas during the same periods, in particular we
sampled different sites during different periods of rabies
prevalence (see “Rabies and population density” below). We
sampled the Piney Branch area during the 1984–1985
decrease in rabies after the initial epizootic, as well as during
and after the first post-epizootic rabies peak in 1986–1987.
Fig. 2. Three-month running average of ratio of number of rabies cases to number of animals tested, for raccoons in Washington, D.C. (February
in all years).
1158 Can. J. Zool. Vol. 76, 1998
© 1998 NRC Canada
We sampled the Hazen area predominantly during the 1984–
1985 period (the September 1987 estimate has a high coeffi-
cient of variation and is not useful for comparisons), while the
Colorado Avenue area was sampled only during 1986–1987.
A comparison of Piney Branch and Hazen using only com-
mon sampling periods (January 1984, August and November
1985, and April 1986) still reveals a significantly higher rac-
coon density in the Hazen area (χ2 = 6.68, p = 0.01, df = 1). A
similar comparison is not possible for Hazen and Colorado
Avenue or for Piney Branch and Colorado Avenue (the
December 1986 Piney Branch estimate has a high coefficient
of variation), although the latter comparison showed no dif-
ference even with all estimates included and would not be
expected to change.
Seasonal averages varied significantly between years for
Piney Branch (fall, χ2= 26.4, p < 0.001, df = 2; winter, χ2 =
21.92, p < 0.001, df = 3). We examined our most consistently
sampled study area, Piney Branch, for seasonal variation in
raccoon density and found that although mean density from
July to December (131.6/km2) was higher than mean density
from January to June (88.5/km2), the difference was not sta-
tistically significant at the 0.05 level (χ2= 2.04, one-sided p =
0.08, df = 1). The average density of Rock Creek raccoons
from mark–recapture estimates was 125/km2.
Rabies and population density
The Mid-Atlantic rabies epizootic moved into the Washing-
ton, D.C., area in the fall of 1982 (Jenkins and Winkler 1987),
with an initial epizootic peak occurring in the spring and sum-
mer of 1983. The ratio of rabies cases to animals tested shows
that the initial epizootic was followed by a drop in rabies
prevalence through late 1984 and 1985, followed by peaks in
late 1986 and 1987 and again in late 1991 and early 1992
(Fig. 2). Spectral analysis of these data confirms the cycle and
shows statistically significant cycling (χ2 = 34.25, 7 point
smoothing, p < 0.005, df = 8) with a period of 41 months
(0.0245 cycles per point, 1/0.0245 = 40.8 months), or about
3½ years.
We used the data from the Piney Branch area to test for
changes in population size relative to rabies prevalence. Pop-
ulation size from January 1984 to April 1986 (n = 4 esti-
mates) was significantly higher than from December 1987
through March 1988 (n = 2 estimates; χ2= 8.86, p < 0.005,
df = 1). The higher population estimates between January
1984 and April 1986 coincided with a period of low rabies
prevalence after the epizootic, and the lower population esti-
mates between late 1987 and early 1988 occurred after the
rabies resurgence in 1986–1987 (Fig. 2).
Survival
We radio-tracked 49 animals (33 adults and 16 juveniles)
between August 1983 and February 1985 at the Piney Branch
study site and 19 animals (10 adults and 9 juveniles) between
April 1989 and April 1990 at the H-2 study site. Seven adult
radio-collared raccoons died in the Piney Branch area during
1983–1984. One animal tested positive for rabies, two were
killed by cars, three were trapped and euthanized as nuisance
animals, and one drowned. The much higher mortality (11 of
16 animals) occurring among juveniles during the same period
was partly attributable to deaths from rabies (4 animals), but
also included 1 animal that was hit by a car, 3 that were
trapped and euthanized as nuisances, 2 that died of gastro-
enteritis, and 1 that died of unknown causes. In the 1989–1990
study, two adults died, a male of pneumonia and a female of
unknown causes, and one juvenile male died of rabies. In the
1983–1984 study, radio signals were lost for 8 of 49 animals
(2 juvenile females, 2 adult females, and 4 adult males). In the
1989–1990 study, signals from 7 of 19 radio-collared animals
were lost, but 4 animals were later recaptured or resighted and
so were known to have been alive at least through the end of
the study in April 1990.
For the 1983–1984 study, we combined juvenile males and
females into one group for computing survivorship because
of small sample sizes for each sex. Survival of adult males
was generally higher than that of adult females, but the differ-
ence was only significant for one of six seasons (spring 1984,
z = 2.06, two-sided p < 0.05) and over the period from April
to December 1984 (z= 2.15, two-sided p < 0.05). Moreover,
z values were inflated because only two adult males died
during the study, both in winter 1984, resulting in a survival
variance of 0 for all other periods. Survival was therefore
computed for a single adult class. The sexes were also
combined for the 1989–1990 study because sample sizes
were small (2) for the adult male and juvenile female
groups, and for purposes of comparison with the 1983–1984
data.
For adults, survival during the height of the initial rabies
epizootic (the 9-month period between August 1983 and
Table 1. Population size estimates, 95% confidence intervals,
coefficients of variation, and density estimates from mark–recapture
data for raccoons in the Piney Branch, Melvin Hazen Park, and
Colorado Avenue study areas in Rock Creek Park, Washington, D.C.
Raccoon density
Mean 95% CI CV (no./km2)
Piney Branch
Jan. 1984 90 49–154 0.32 147
Aug. 1985 105 67–162 0.25 172
Nov. 1985 103 44–146 0.27 169
Apr. 1986 65 28–131 0.45 106
Dec. 1986 60 19–265 1.20 98
Dec. 1987 45 26–76 0.27 76
Mar. 1988 40 20–62 0.28 65
Dec.–Jan. 1988 66 24–103 0.29 109
Jan. 1990 53 10–330 1.92 87
Melvin Hazen Park
Jan. 1984 95 28–193 0.54 313
Aug. 1985 100 60–151 0.26 333
Nov. 1985 100 45–138 0.26 333
Apr. 1986 77 27–131 0.35 256
Sept. 1987 60 11–191 1.00 200
Colorado Avenue
Oct. 1986 82 28–164 0.43 102
July 1987 66 18–115 0.42 83
Riley et al. 1159
© 1998 NRC Canada
March 1984) was 0.830 and was not significantly different
than survival over the next 9 months (0.850) (z= 0.185, one-
sided p = 0.425) or for an equivalent period, July through
April, in 1989–1990, during which survival was actually
lower (0.728) (z = 0.590, one-sided p = 0.288) (Table 3).
Juvenile survival (0.254) during the 9-month epizootic period
was significantly lower (z =1.73, one-sided p < 0.05) than
during the next 9 months (0.640) and during the same period
in 1989–1990 (0.825) (z= 3.01, one-sided p < 0.005).
Discussion
Raccoon density in Rock Creek Park was from twice to more
than 100 times that reported for the species in non-urban hab-
itats and was consistent with the few estimates published for
other urban and suburban raccoon populations (see Table 2).
Raccoon populations vary considerably in size, however,
and high densities have been reported outside of urban areas
(see Table 2; Twichell and Dill 1949; Lehman 1977),
although the highest recorded density (Twichell and Dill
1949) was determined by an intensive search of den sites and
hand capture of animals, not by mark–recapture estimation.
Urban environments likely provide resources, such as food
and den sites, and conditions, such as greater protection from
exploitation, that directly benefit and support dense raccoon
populations.
Higher raccoon densities in urban areas are likely often
related to greater survivorship. Where they are hunted or
trapped, human harvest is a dominant cause of death for rac-
coons (Atkeson and Hulse 1953; Sanderson 1961; Johnson
1970), representing up to 78% of mortality (Clark et al.
1989). Starvation and malnutrition in late winter and early
spring, perhaps compounded by the effects of parasites and
disease, have also been cited as significant mortality factors
for rural raccoons (Mech et al. 1968), which can lose from 19
to 50% of their body mass over the winter (Steuwer 1943;
Table 2. Estimates of raccoon density from mark–recapture studies.
Raccoon density
(no./km2) Urban/rural Mortality sourcesaSource
250.0 Rural Protected Twichell and Dill 1949
125.0 Urban Rabies and distemper This study
111.1 Urban Distemper Schinner and Cauley 1974
66.7 Suburban Unknown Hoffman and Gottschang 1977
55.6 Suburban Distemper Slate 1980
55.6 Urban Rabies and distemper Rosatte et al. 1990
55.6bRural Protected and distemper Lehman 1977
43.5cRural Protected and distemper Hable et al. 1992
35.7 Urban Unknown Jacobsen 1982
27.0 Rural Unknown Jacobsen 1982
17.5 Rural Harvested Urban 1970
17.5 Rural Harvested Hasbrouck 1991
17.2 Rural Harvested Sonenshine and Winslow 1972
13.9 Rural Harvested Moore and Kennedy 1985a
12.8 Rural Harvested Slate 1980
12.7 Rural Harvested Kennedy et al. 1991
11.8 Rural Harvested Johnson 1970
11.8 Rural Harvested Lehman 1984
11.8cRural Harvested Perry et al. 1989
10.5dRural 5 harvested, 2 protected Leberg and Kennedy 1988
8.0 Rural Harvested VanDruff 1971
6.5cRural 1 harvested, 1 protected Nixon et al. 1993
5.9 Rural Protected and distemper Rabinowitz 1981
5.7 Rural Protected and distemper Keeler 1978
5.6 Rural Harvested Lehman 1980
3.6 Rural Harvested Nottingham et al. 1982
1.4 Rural Protected and distemper Mech et al. 1968
0.9 Rural Harvested Orloff 1980
aMortality sources indicates whether the population is harvested or protected and other major sources of mortality
mentioned in the reference.
bAveraged from counts in several years.
cAveraged from two different sites.
dBased on an average of density estimates from seven sites in western Tennessee. The estimate for a protected
National Park area was 5.5 ha per raccoon.
1160 Can. J. Zool. Vol. 76, 1998
© 1998 NRC Canada
Mech et al. 1968; Johnson 1970; Moore and Kennedy 1985b).
Raccoon populations in urban areas are free from intense har-
vest pressure and may also benefit from stable food and den-
ning resources that mitigate the effects of severe winter
conditions. Urban and suburban animals lost 16% of their
body mass in Ohio (Hoffman 1979) and 10% in Rock Creek
(J. Hadidian, unpublished data) over the winter.
Food resources for Rock Creek raccoons are abundant both
within and adjacent to the park, although we do not have
explicit geographic data on these resources. We know from
scat analyses (J. Hadidian, unpublished data) that these ani-
mals rely on a variety of food items, such as earthworms and
insects, from January until late April. During the rest of the
year they focus on abundant fruits, both native and ornamen-
tal, including (in seasonal order) mulberries, cherries, wild
cherries, raspberries, grapes, peppervine, and persimmons.
They also eat acorns in the late fall and by November and
December, palpation of the hind pelvic region of Rock Creek
raccoons reveals significant fat reserves. Oak trees and fruit
trees and bushes exist in the park but also abound in yards and
gardens outside the park. Garbage and feeding by humans
represent supplemental food sources that may be situationally
important, although we think that most Rock Creek animals
do not rely on them. The abundant fruit and acorn resources
available in and around the park may contribute to the high
density of Rock Creek raccoons.
A concentration of resources in small areas of natural habi-
tat may also contribute to high densities of urban raccoons.
Spatial models in ecology often assume that density does not
change with the size of suitable habitat (e.g., MacArthur and
Wilson 1963; Hanski 1991), although some models explicitly
vary habitat quality, which can lead to differences in density
(e.g., Pulliam 1988; Doak 1995). We speculate that the pres-
ence of small or fragmented natural areas surrounded by less
suitable denning habitat may lead to larger concentrations of
raccoons. This effect may be particularly pronounced if food
resources are distributed more widely than dens, as is proba-
bly the case in Rock Creek. Matthiae and Stearns (1981, p.
62) note that forest islands in urban areas act as refuges for
raccoons and support “numbers greater than carrying capac-
ity.” Our highest raccoon density occurred in the Hazen site,
an area distinguished by having both the smallest ratio of
interior area to surrounding urban edge and the highest
human population density outside. Multi-resident apartment
complexes also surround the Hazen area to a greater extent
than other study sites, considerably reducing the number of
chimneys and yard trees frequently used as den sites in resi-
dential areas (Hadidian et al. 1991). The extremely high pop-
ulation density found by Twichell and Dill (1949) may also
have been the result of a concentration of resources, since by
their description they collected raccoons from a thin strip of
ideal riparian habitat.
Table 3. Survival rates and mortality rates from rabies and from other sources for raccoons in the Piney Branch and
H-2 study areas of Rock Creek Park, Washington, D.C.
No. of No. of Rabies mortality Other mortality
Age-class days radio-days Survival rate rate rate
Piney Branch
August 1983 – December 1984
Adults (n = 33) 519 10612 0.71±0.09 0.04±0.04 0.25±0.09
Juveniles (n = 16) 519 3076 0.16±0.10 0.32±0.13 0.49±0.13
August 1983 – March 1984
(during rabies epizootic)
Adults (n = 32) 244 5372 0.83±0.08 0.04±0.04 0.13±0.07
Juveniles (n = 16) 244 1716 0.26±0.12 0.32±0.13 0.40±0.14
April 1984 – December 1984
(after rabies epizootic)
Adults (n = 23) 275 5240 0.85±0.08 0.00±0.00 0.15±0.08
Juveniles (n = 7) 275 1360 0.64±0.19 0.00±0.00 0.33±0.19
H-2
April 1989 (adults) and
July 1989 (juveniles) –
April 1990
Adults (n = 10) 395 2562 0.73±0.16 0.00±0.00 0.26±0.16
Juveniles (n = 9) 304 1717 0.83±0.15 0.16±0.15 0.00±0.00
Note: Rates were determined from radio-collared raccoons and are given as the mean ± SD. Number of days is the number of
calendar days during the period in question, and number of radio-days is the cumulative number of days that different animals in
each age-class were followed during the period: 1 radio-day = one radio-collared raccoon followed (and alive) for 1 day (note that
the survival rate does not always equal 1 minus the sum of the mortality rates) because using the bias-corrected survival rate
slightly decreases the survival rate estimate; see Methods).
Riley et al. 1161
© 1998 NRC Canada
A prerequisite for dense populations of carnivores is the
ability to tolerate more conspecifics in less space, a situation
that can lead to behavioral changes. Dense populations of red
foxes in urban areas in England exhibit a variable social
structure that allows for high densities (MacDonald 1981).
Though Fritzell (1978) reported territoriality among male rac-
coons in a low-density population in North Dakota, our pre-
liminary analysis of telemetry data for Rock Creek raccoons
indicates extensive overlap of home ranges for both males
and females. Daytime resting sites, particularly den trees, are
also shared, both simultaneously and on different days (Had-
idian et al. 1991). Barash (1974) found that raccoons caged
next to their neighbors from the wild showed fewer signs of
aggression than those caged next to totally foreign animals.
Perhaps raccoons in high-density populations can adjust their
social relationships to allow many adults of both the same and
different sexes to coexist without constant conflict.
Epizootic diseases such as rabies and canine distemper
may be more easily transmitted in populations where individ-
uals are closely associated. These diseases are the most fre-
quently reported causes of mortality in high-density
populations of raccoons and in unharvested populations
(Table 2). Of the eight mark–recapture studies with the high-
est reported densities, all six that mention population regula-
tion cite disease as a major factor. In addition, disease,
especially canine distemper, had a significant impact on rac-
coon populations in every study conducted in protected areas
(Mech et al. 1968; Cauley and Schinner 1973; Hable et al.
1992; Hoff et al. 1974; Lehman 1977; Keeler 1978; Slate
1980; Rabinowitz 1981; Rosatte et al. 1990; Roscoe 1993).
Although some investigators of harvested populations men-
tion disease (e.g., Johnson 1970), none of them count disease
as a major contributor to raccoon mortality. Because urban
and suburban raccoon populations are both dense and pro-
tected, it is not surprising that all four of the urban and sub-
urban studies that discuss sources of mortality cite disease as
a significant one. Rabies epizootics in urban raccoons were
first reported more than 20 years ago in Florida, with high
concentrations of raccoons being particularly susceptible
(Bigler et al. 1973).
Our direct measurement of survival in urban raccoons
during and after a disease epizootic revealed that survivor-
ship of adult raccoons in Rock Creek was high and constant
both in 1983–1984 and 1989–1990, and specifically during
the epizootic in the summer and fall of 1983. The juvenile
survival rate during the same epizootic period was signifi-
cantly lower, more than a third of the deaths being directly
caused by rabies. We attribute the high survival rate of
adults during the epizootic in part to the efficacy of the
immunization program. Young raccoons may be less able to
mount an immune response after immunization than older
animals, as is the case in dogs (Bunn 1991; S. Jenkins, per-
sonal communication). Had we not immunized all radio-
collared animals, the adult survival rate may well have also
been lower in 1983–1984 than in 1989–1990. However,
Brown et al. (1990) followed two groups of adult raccoons,
one immunized and one not, along the “leading edge” of the
Mid-Atlantic rabies epizootic in Pennsylvania and although
harvest mortality was high, they recorded no rabies mortal-
ity, even in the group that had not been immunized. Immuni-
zation was also not a definitive means of avoiding the
disease in our study, as one immunized adult died of rabies.
Seidensticker et al. (1988) found a lower survival rate in rac-
cons during the rabies epizootic in Virginia, although they
did not test the difference statistically or provide the ages of
radio-collared animals. Survival of adult raccoons in Rock
Creek appears similar to that in areas with little or no exploi-
tation (Mech et al. 1968; Fritzell and Greenwood 1984) and
higher than that found in intensively harvested populations
(Glueck 1985; Clark et al. 1989; Brown et al. 1990). The
juvenile survival rate in Rock Creek during the epizootic
was considerably lower than that found in other studies,
regardless of exploitation level.
The tendency of disease to replace human harvest as a sig-
nificant cause of mortality in urban or protected raccoon pop-
ulations raises the question of whether harvest mortality is
additive or compensatory. Some authors believe that harvest
mortality is additive in raccoons (Clark and Fritzell 1992;
Nixon et al. 1993). The patterns seen in raccoon mark–
recapture studies (Table 2) do not imply that all nonharvested
raccoon populations do, or will, undergo epizootics of rabies
or distemper. However, protected, urban, and high-density
populations should more often endure these epizootics, and
epizootic disease may represent another form of additive
mortality in raccoons, particularly in young animals.
Raccoon rabies remains endemic in the Washington, D.C.,
area and in fact the disease appears, through early 1994, to be
on a fairly regular cycle (Fig. 2). The 3½-year cycle seen in
the Washington, D.C., rabies data is comparable to the 2- to
3-year cycles seen in the percentages of raccoons testing pos-
itive in five counties in Virginia (Torrence et al. 1992), the 4-
year interval between an initial distemper epizootic and sub-
sequent concurrent rabies and canine distemper epizootics in
Florida raccoons and gray foxes (Urocyon cinereoargenteus)
(Hoff et al. 1974), the 4-year cycles of canine distemper in
raccoons in New Jersey (Roscoe 1993), and the 3- to 5-year
cycles of red fox density and rabies prevalence in Europe and
Canada (Anderson et al. 1981). Using their model of raccoon
rabies with a class of immune animals, Coyne et al. (1989)
speculate that no cycling in disease prevalence or density
should be noticeable in the field for raccoons, and that popu-
lation density is less likely to fall below the threshold level
for disease maintenance, thereby permitting the disease to
persist in the population. We were able to see cycles in rabies
prevalence in our data, contrary to the model. However, the
period of the cycles we found is consistent with models of
disease dynamics in foxes and raccoons, and rabies in Wash-
ington, D.C., and the southeastern U.S.A. in general does
appear to have become endemic, as predicted by the raccoon
model.
Unfortunately, we do not have data on population density
from before the epizootic in order to measure the initial
impact of the disease on raccoon abundance. Nevertheless, a
significant drop in the density of the Piney Branch population
occurred between the period 1984–1985, when rabies preva-
lence was low, and the period after late 1986 – early 1987,
when disease prevalence increased, indicating that rabies may
have seriously impacted the population 4 years after the ini-
tial epizootic. Other areas of the Mid-Atlantic and northeast-
ern U.S.A and eastern Canada can expect the rabies virus to
maintain itself in raccoon populations and perhaps to impact
raccoon density long past an original epizootic.
1162 Can. J. Zool. Vol. 76, 1998
© 1998 NRC Canada
Acknowledgments
We thank Rock Creek Park Superintendents Georgia Ellard
and Roland Swain for permission to conduct this research, as
well as for encouragement and support throughout the period
of the study. We are especially indebted to Bob Ford, Rock
Creek Park Resource Manager, for both the support and the
assistance without which this study could not have been con-
ducted. John Seidensticker of the National Zoological Park
provided important advice and assistance during the early
phases of the study, as did Suzanne Jenkins of the Virginia
State Department of Health. Richard Montali of the Depart-
ment of Pathology provided important advice and assistance,
especially in support of animal necropsies. Tina Harper,
Chief of D.C. Animal Control, kindly provided the data and
records kept on animal submission and rabies cases kept by
her agency. Dianne Ingram assisted in both the trapping and
radiotelemetry programs. We thank Chris Nice, Jim Nichols,
Ben Sacks, two anonymous reviewers (twice!), and especially
Martha Hoopes, Mikaela Huntzinger, and Dirk VanVuren for
comments on earlier drafts of this paper. Jim Nichols and
Stephen Buckland provided advice on population estimation
and Bob Shumway provided analysis and assistance with the
rabies time series data.
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