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Analysis of the impact of trap-neuter-return programs on populations of feral cats



To evaluate 2 county trap-neuter-return (TNR) programs for feral cat population management via mathematical modeling. Theoretical population model. Feral cats assessed from 1992 to 2003 in San Diego County, California (n = 14,452), and from 1998 to 2004 in Alachua County, Florida (11,822). Data were analyzed with a mathematical Ricker model to describe population dynamics of the feral cats and modifications to the dynamics that occurred as a result of the TNR programs. In both counties, results of analyses did not indicate a consistent reduction in per capita growth, the population multiplier, or the proportion of female cats that were pregnant. Success of feral cat management programs that use TNR can be monitored with an easily collected set of data and statistical analyses facilitated by population modeling techniques. Results may be used to suggest possible future monitoring and modification of TNR programs, which could result in greater success controlling and reducing feral cat populations.
opulations of feral cats are large, have high intrinsic
rates of growth, and are highly adaptable to differ-
ent and sometimes harsh habitats. Feral cats often are
regarded as pests on the basis of their predatory habits
and the negative effect they may have on wildlife pop-
They may function as hosts for diseases and
vectors that can infect humans, domestic animals, or
; yet, colonies of feral cats often are main-
tained through feeding and care by people who have
strong affection for these cats.
There have been many attempts to eradicate pop-
ulations of feral cats or to regulate their population
sizes at low numbers. Such projects have included
intentional release of panleukopenia virus, poisoning,
predator introduction, euthanasia, and neutering.
Often, despite intense effort, attempted control pro-
grams fail because growth rates within the population
do not decline or because of additional recruitment of
cats into the population, although some programs have
successful reduction in feral populations
with humane trapping programs. The general public
often finds extermination programs for feral cats unac-
ceptable, yet also often is intolerant of cat predation on
wildlife. It has proven difficult to assess program suc-
cess; theoretical models would be helpful to guide
interpretation of data from control programs and to
provide motivation for changes that could increase
Feral cats are territorial animals, and their highest
potential for population increase occurs when popula-
tions are low. The maximum per capita rate of increase
is the maximum mean number of female cats produced
annually from each female cat, including the cat and its
female kittens. A cat population size tends to increase
until a carrying capacity is reached. This carrying
capacity depends mainly on food and appropriate area
for territories. After the carrying capacity has been
reached, density dependence forces the per capita
growth rate to drop to 0. Matrix methods are used to
study the sensitivity of long-term population growth
rates to perturbations in survivorship and fecundity
and have been used to evaluate feral cat population
By use of a logistic (Ricker) model to lower
feral cat populations, 2 general approaches are possi-
ble: the carrying capacity can be decreased (eg, by dis-
couraging public feeding of feral cats), or the maxi-
mum per capita rate of increase can be lowered (eg, by
increasing mortality rate
or by neutering female cats).
For feral cat populations to decline, the maximum per
capita rate of increase needs to decrease to < 0.
Temporarily lowering the population size below the
carrying capacity yields no long-term population
reduction if this is not accomplished. The cat popula-
tion will simply increase back to carrying capacity.
The objective of the study reported here was to use
data from 2 trap-neuter-return (TNR) programs to
evaluate development and implementation of models
that could determine program success and calculate
the rate of neutering needed to decrease the feral cat
Materials and Methods
Modeling—Statistical analyses and modeling were per-
formed with computer software.
For all statistical tests, a
value of P < 0.05 was considered significant. Cat population
regulation was modeled on the basis of a Ricker model:
= e
where R
is an annual population multiplier or net funda-
mental reproductive rate, r
is the maximum per capita
rate of increase, N
is the population size at time 1, and K
is the carrying capacity. If R
= 1, the net annual growth of
JAVMA, Vol 227, No. 11, December 1, 2005 Scientific Reports: Original Study 1775
Analysis of the impact of trap-neuter-return
programs on populations of feral cats
Patrick Foley, PhD; Janet E. Foley, DVM, PhD; Julie K. Levy, DVM, PhD, DACVIM; Terry Paik, DVM
Objective—To evaluate 2 county trap-neuter-return
(TNR) programs for feral cat population management
via mathematical modeling.
Design—Theoretical population model.
Animals—Feral cats assessed from 1992 to 2003 in
San Diego County, California (n = 14,452), and from
1998 to 2004 in Alachua County, Florida (11,822).
Procedure—Data were analyzed with a mathematical
Ricker model to describe population dynamics of the
feral cats and modifications to the dynamics that
occurred as a result of the TNR programs.
Results—In both counties, results of analyses did not
indicate a consistent reduction in per capita growth,
the population multiplier, or the proportion of female
cats that were pregnant.
Conclusions and Clinical Relevance—Success of
feral cat management programs that use TNR can be
monitored with an easily collected set of data and sta-
tistical analyses facilitated by population modeling
techniques. Results may be used to suggest possible
future monitoring and modification of TNR programs,
which could result in greater success controlling and
reducing feral cat populations. (
J Am Vet Med Assoc
From the Department of Biological Sciences, California State
University, Sacramento, CA 95819 (P. Foley); the Department of
Medicine and Epidemiology, School of Veterinary Medicine,
University of California, Davis, CA 95616 (J. Foley); the
Department of Small Animal Clinical Sciences, College of
Veterinary Medicine, University of Florida, Gainesville, FL 32610-
0126 (Levy); and 420 Dewane Dr, El Cajon, CA 92020 (Paik).
The authors thank Linda Louise Steenson and Dr. Karen Scott for
coordination of trapping.
Address correspondence to Dr. J. Foley.
05-04-0193.qxp 11/11/2005 12:55 PM Page 1775
the population r
is 0 (ie, the population size is multiplied
by 1.0).
To apply the model to TNR data, results from trapping
were inserted into the model as index values (linear multi-
pliers of the actual values) and interpreted with the assump-
tion that trapped cats represented some fraction of all cats in
the county; this fraction was divided into an index value (eg,
the index carrying capacity) to yield an estimated county-
wide value. The county-wide feral cat population size was
approximated; there were 1,040,149 households in San
Diego County in 2000, of which 8.9% of those interviewed
reported that they fed a mean of 2.6 feral cats/household.
Thus, a minimum county-wide estimate of feral cat popula-
tion size for 2000 was 240,690 feral cats. In Alachua County,
12% of interviewed households reported that they fed a mean
of 3.6 feral cats each. There were 84,963 households in 1999
and approximately 36,398 feral cats.
Estimates of feline population growth rate (R
) were
obtained from the trapped cat data. The R
was calculated as
= N
t + 1
and r
= lnR
where N
and N
t + 1
are indices of the actual population size,
equal to the total number of cats neutered at clinics for that
year. It was not necessary to estimate either K or N
because the growth rates describe population trajectories
independent of absolute or index values of population size
and carrying capacity. The regression of per capita growth
rate on population size provided the estimate of maximum
per capita rate of increase (y-intercept) and, for convenience,
an index of carrying capacity (x-intercept).
The actual car-
rying capacity was obtained by multiplying the index carry-
ing capacity by the estimated total feral cat population in that
county and dividing by the total cats trapped.
Program success was evaluated with several methods.
Evidence for density-dependent population regulation was
sought by plotting per capita growth rate as a function of year
to determine a significant reduction in per capita growth rate
as detected by a significant negative linear regression of per
capita growth rate on time. Similarly, evidence of reduced
fecundity was sought by use of linear regression for the pro-
portion of female cats pregnant when neutered over time.
The Malthusian parameter r
(maximum per capita rate of
increase) calculated for each county was used to obtain a
Malthusian multiplier, R
= e
Management of feral cat R
means getting a new value,
'. Population decline occurs when R
' is < 1.0; R
can be
written as the sum of survivorship (p) and offspring produc-
tion (R
– p). The critical fraction (s) of cats that would need
to be neutered in a population to induce a decline can be
obtained by solving the following equation:
1 = R
' = p + (R
– p)(1 – s)
to get
s =
– 1
– p
One can also approximate the proportion of cats that
must be neutered each year (M) to gradually reach M = s
neutered cats. Neutered cats accumulate in the population
because they survive at rate p from year to year. If the num-
ber of cats neutered annually is m and the program continues
many years, when neutered individuals are counted right
after neutering but before death,
M = m
= m
1 – p
To achieve the neutering level s = M/N, the annual neu-
tering rate s
must satisfy the following equation:
= s(1 – p)
1776 Scientific Reports: Original Study JAVMA, Vol 227, No. 11, December 1, 2005
Figure 1—Yearly distribution of all feral cats evaluated for neu-
tering in San Diego County (A) and Alachua County (B).
Figure 2—Monthly distribution of pregnant feral female cats
evaluated for ovariohysterectomy in San Diego County (1992 to
2003, all years summed [A]) and Alachua County (1998 to 2004,
all years summed [B]).
05-04-0193.qxp 11/11/2005 12:55 PM Page 1776
When survivorship (p) is close to 1.0, this is a much
lower burden for the neutering program. The calculation is
only approximate because N is not constant over the lifetime
of the neutering program, survivorship may differ between
neutered and non-neutered cats, and cats do not live indefi-
nitely. In the absence of field data, the annual survival rate
(p) can be estimated from the mean cat life span as follows:
p = 1–
mean life span
and if such data were available, the life span and annual sur-
vival rate should be estimated at low population sizes.
Data—Data from the Feral Cat Coalition were acquired
during a trapping program involving volunteers from across
San Diego County, California, from 1992 to 2003 and from a
similar program from 1998 to 2004 run by Operation Catnip
Inc in Alachua County, Florida. Cats were live-trapped,
transferred approximately once per month to participating
veterinary clinics, examined, vaccinated, surgically neutered,
and returned to their colonies after a short postoperative
recovery period. For each day that clinics were held, data
compiled included clinic number and date, location of the
clinic, number of males neutered, number of females
neutered, number of cats already neutered when trapped, and
total females subdivided into the categories pregnant and not
pregnant. Data regarding San Diego County demographics
were obtained from the California Department of Finance
and included number of humans in the county and number
of households. For Alachua County, demographic data were
obtained from the US Census Bureau. Data regarding cat
ownership, feeding of feral cats, approximate county-wide
cat numbers, and number of feral cats were obtained or cal-
culated from published surveys of San Diego and Alachua
County households.
JAVMA, Vol 227, No. 11, December 1, 2005 Scientific Reports: Original Study 1777
Figure 3—Monthly distribution of pregnant feral female cats
evaluated for ovariohysterectomy in San Diego County (1992 to
2003 [A]) and Alachua County (1998 to 2004 [B]).
Figure 4—Annual per capita feral cat population growth rate by
year for feral cats evaluated for neutering in San Diego County
(1992 to 2003 [A]) and Alachua County (1998 to 2004 [B]).
Figure 5—Regression of annual per capita growth (r) on annual
index population size (N) for feral cats evaluated for neutering in
San Diego County (1992 to 2003 [A]) and Alachua County (1998
to 2004 [B]).
05-04-0193.qxp 11/11/2005 12:55 PM Page 1777
Feral cat demographics—From 1992 to 2003,
14,452 cats were submitted as feral cats to veterinary
clinics in San Diego County for neutering (Figure 1;
data for 1992 represent only part of the year, when the
program began). Of these cats, 565 (4%) had already
been neutered; 14,129 surgeries were performed on
6,494 (46%) male and 7,635 (54%) female cats. The
number of cats neutered over the months of the year
did not vary significantly (P = 0.13), but the presence
of pregnant cats was strongly seasonal, with numbers
increasing in spring, compared with winter and fall
(Figures 2 and 3). Overall, 17.2% of trapped female
cats were pregnant.
In Alachua County, 11,822 cats were submitted for
neutering from 1998 to 2004 (Figure 1). Of these, 258
(2%) cats had previously been neutered; 11,564 surg-
eries were performed on 4,928 (43%) male and 6,636
(57%) female cats. Evaluation of pregnant cats revealed
a double peak, with increases in March and August
(Figures 2 and 3). Sixteen percent of trapped female
cats were pregnant.
Model results—Per capita growth rate in San
Diego County ranged from –0.58 to 0.30, with a
value of 0.25 for 2002 (Figure 4). Values for Alachua
County were similar. Regressing per capita growth
rate on population size yielded estimates of the
index carrying capacity (x-intercept) and maximum
per capita rate of increase (y-intercept) of 1,323 and
0.45 (P = 0.09), respectively, for San Diego County
and 1,855 and 1.41, respectively, for Alachua
County (P = 0.1; Figure 5). In the last year of data
for each county, the total numbers of trapped cats
were 1,514 (0.63% of the total estimated feral cats)
in San Diego County and 2,213 (9.6%) for Alachua
County. Thus, the county-wide carrying capacities
were estimated as 210,325 and 19,323 feral cats,
1778 Scientific Reports: Original Study JAVMA, Vol 227, No. 11, December 1, 2005
Table 1—Critical overall neutering rate required to bring growth rate in a feral cat colony to 1.0 for var-
ious growth rate, life span, and survivorship (p) estimates.
Mean life span (y)
Growth rate and p 1 2 3 4 5 6 7 8 9 10 11 12
p 0.00 0.50 0.67 0.75 0.80 0.83 0.86 0.88 0.89 0.90 0.91 0.92
1.0 0000000 0 0000
1.5 0.33 0.50 0.60 0.67 0.71 0.75 0.78 0.80 0.82 0.83 0.85 0.86
2.0 0.50 0.67 0.75 0.80 0.83 0.86 0.88 0.89 0.90 0.91 0.92 0.92
2.5 0.60 0.75 0.82 0.86 0.88 0.90 0.91 0.92 0.93 0.94 0.94 0.95
3.0 0.67 0.80 0.86 0.89 0.91 0.92 0.93 0.94 0.95 0.95 0.96 0.96
3.5 0.71 0.83 0.88 0.91 0.93 0.94 0.95 0.95 0.96 0.96 0.96 0.97
4.0 0.75 0.86 0.90 0.92 0.94 0.95 0.95 0.96 0.96 0.97 0.97 0.97
4.5 0.78 0.88 0.91 0.93 0.95 0.95 0.96 0.97 0.97 0.97 0.97 0.98
5.0 0.80 0.89 0.92 0.94 0.95 0.96 0.97 0.97 0.97 0.98 0.98 0.98
Table 2—Critical annual neutering rate required to bring growth rate in a feral cat colony to 1.0 for var-
ious growth rate, life span, and p estimates.
Mean life span (y)
Growth rate and p 1 2 3 4 5 6 7 8 9 10 11 12
p 0.00 0.50 0.67 0.75 0.80 0.83 0.86 0.88 0.89 0.90 0.91 0.92
1.0 0000000 0 00 0 0
1.5 0.33 0.25 0.20 0.17 0.14 0.13 0.11 0.10 0.09 0.08 0.08 0.07
2.0 0.50 0.33 0.25 0.20 0.17 0.14 0.13 0.11 0.10 0.09 0.08 0.08
2.5 0.60 0.38 0.27 0.21 0.18 0.15 0.13 0.12 0.10 0.09 0.09 0.08
3.0 0.67 0.40 0.29 0.22 0.18 0.15 0.13 0.12 0.11 0.10 0.09 0.08
3.5 0.71 0.42 0.29 0.23 0.19 0.16 0.14 0.12 0.11 0.10 0.09 0.08
4.0 0.75 0.43 0.30 0.23 0.19 0.16 0.14 0.12 0.11 0.10 0.09 0.08
4.5 0.78 0.44 0.30 0.23 0.19 0.16 0.14 0.12 0.11 0.10 0.09 0.08
5.0 0.80 0.44 0.31 0.24 0.19 0.16 0.14 0.12 0.11 0.10 0.09 0.08
Figure 6—Monthly distribution of pregnant feral female cats
evaluated for neutering in San Diego County (1992 to 2003 [A])
and Alachua County (1998 to 2004 [B]).
05-04-0193.qxp 11/11/2005 12:55 PM Page 1778
respectively. The calculated values for R
for each
county were 1.57 for San Diego County and 4.1 for
Alachua County.
Critical neutering rates depend on R
and sur-
vivorship (Tables 1 and 2). Reported
mean life spans
in feral cats range from 2 to 8 years. By use of a medi-
an life span of 5 years for San Diego County, the criti-
cal neutering fraction (s) would be approximately 71%
(94% for Alachua County). The needed annual neuter-
ing fraction (s
) was 14% for San Diego County and
19% for Alachua County. Hypothetical feral cat popu-
lations would decrease between these values.
To assess the success of the TNR program, data
were evaluated for density-dependent population regu-
lation and a significant reduction in the proportion of
female cats that were fertile. When per capita growth
rate was regressed on year, there were no indications of
a significant reduction in per capita growth rate (ie,
evidence for density dependence) in either of the coun-
ties (P = 0.24 and 0.1 for San Diego and Alachua coun-
ties, respectively; Figure 4). The proportion of preg-
nant females cycled annually, but an overall reduction
in either of the counties was not detected (Figure 6).
Feral and stray cats represent more than 40% of all
cats in the United States, are fed by an estimated 10%
to 20% or more of households, and are rarely
It is desirable to reduce feral cat popula-
tions because of welfare concerns for the cats, concern
about the effects of feral cats on vulnerable wildlife,
and public health considerations. The American
Association of Feline Practitioners supports appropri-
ately managed feral cat colonies, but that group’s posi-
tion statement indicates that the goal of colony man-
agement should be the eventual reduction of the
Additionally, feral cat colonies should not be
located near at-risk wildlife. Although several control
methods including TNR have been proposed and
implemented, assessment of their efficacy has typically
been missing or at most anecdotal. This is unfortunate
given the substantial investment of resources required
to run an effective program and the skepticism with
which TNR is regarded by many people.
Feral cat populations are extraordinarily capable
of reaching local carrying capacities as a function of
reproductive mechanisms that emphasize breeding
efficiency. These include induced ovulation, weaning
of kittens as young as 50 days old, an age of first repro-
duction as early as 8 months, and many (approx 130)
days pregnant per year.
Consequently, cats have
some of the highest maximum per capita rates of
increase among carnivores, estimated in 1 study
23.3%. Population sizes, home range size, and local
carrying capacity of feral cats all vary extensively,
depending on habitat type and availability of food and
safe den sites. Intrinsic control of feral cat populations
may occur by density-dependent mechanisms includ-
ing starvation, predation, control of reproductive suc-
cess, and disease. Although cats, particularly males, are
feral cat colonies receiving abundant
food supplementation may have a reduction in appar-
ent territoriality as cats co-occupy territories or
attempt to maintain small territories (sometimes
accompanied by stress and fighting).
The purpose of TNR programs is rarely articulated
in the language of population ecology but often is
motivated by an attempt to reduce population size (N
and per capita growth rate (r
) by reducing reproduc-
tion. Additional goals of TNR may include provision of
veterinary care and vaccines to reduce the threat of
feline and zoonotic diseases, improve the quality of life
of homeless cats, avoid euthanasia as a control method,
and, in some programs, reduce the population size.
In many TNR programs, including those described
here, direct assessment of possible changes in popula-
tion size is not possible because data collection and
population structure do meet assumptions of capture-
recapture or other similar methods of estimating pop-
ulation size. Although index values were necessarily
used for parameters because actual population counts
were not available or practical, the trajectories of pop-
ulations (whether or not populations were declining)
could be determined from calculation of maximum per
capita rate of increase without accurately detecting
population size or carrying capacity.
The models reported here also have the flexibility
of providing statistics that could be used to evaluate
success of control programs, methods for calculating
the fraction of cats that must be neutered to force pop-
ulation decline, and the annual neutering rate required
to eventually achieve the required neutered fraction.
The assessment statistics are R
(multiplier for the
maximum per capita rate of increase), which can be
calculated from the time series and, as a multiplier,
must be < 1.0 for the population to be in decline; the
proportion of cats that are pregnant, which should be
declining significantly in a successful program; and the
proportion of trapped cats that already are neutered,
which should increase. This last statistic was not eval-
uated in the data given here because the TNR programs
specifically avoided retrapping cats, which was unfor-
tunate because keeping account of previously ear-
tipped cats would have made the calculation of the
proportion neutered more accurate.
The present study yielded mixed results regarding
the success of large TNR programs in San Diego and
Alachua counties. Results of the programs had previ-
ously been summarized
regarding the number of cats
neutered, but the effect of neutering on the free-roam-
ing cat population had not been analyzed. Our analysis
indicated that any population-level effects were mini-
mal, with R
(the multiplier) ranging from 1.5 to 4,
which indicated ongoing population growth (similar to
values in previous studies), and critical needed values
of neutered cats (ie, the proportion of all cats that
needed to be neutered to reduce R
to < 1.0) of 71% to
94%, which was far greater than what was actually
achieved. There are several potential limitations to the
data; the net reproductive rate was estimated under the
assumption that trapping effort and efficiency were
unbiased across sites and trapping periods. Retrapping
success for feral cats probably was underestimated
because cats were marked after neutering by removal
of a small distal portion of the pinna and ear-tipped
cats usually were released from cages without count-
JAVMA, Vol 227, No. 11, December 1, 2005 Scientific Reports: Original Study 1779
05-04-0193.qxp 11/11/2005 12:55 PM Page 1779
ing. The estimate of total numbers of feral cats was
somewhat inaccurate because it was calculated from
general surveys of how many people feed how many
feral cats. However, this statistic was not used in the
model itself but rather provided an estimate of the cal-
culated proportion of all available feral cats that were
being neutered, to allow for interpretation of model
successes. The regression of per capita growth rate on
population size was not significant for either San Diego
or Alachua counties, possibly reducing confidence in
the estimate of population growth rates. However, this
was not surprising given that a time series of at least 20
years is typically required before such a regression is
found to be significant.
Nevertheless, the coefficient
of regression (y-intercept) still represented the maxi-
mum likelihood estimator for maximum per capita rate
of increase.
In some ways, results were similar to those
obtained in an earlier, stage-structured matrix model of
feral cat demographic features.
The matrix model
forced λ < 1, analogously with the Ricker model forc-
ing R
< 1, for the population to decline.
Implementation of the stage-structured model suggest-
ed that no plausible combinations of life history vari-
ables would likely allow for TNR to succeed in reduc-
ing population size, although neutering approximately
75% of the cats could achieve control (which is unre-
alistic), a value quite similar to results in the present
study. An important distinction between the 2 models
was the incorporation of density-dependent reduction
of fecundity and possible saturation of the population
with neutered cats in the present model.
Feral cat control programs are notoriously diffi-
cult, and in many cases, short-term control has been
followed by a long-term return to precontrol condi-
tions. Attempted control of a feral cat population on
Marion Island in the Indian Ocean had poor success
for many years.
The population size on the island was
estimated by use of a line transect at approximately
2,200 cats, and in 1979, virulent panleukopenia virus
was released on the island. Although in 1 study
it was
concluded that the population density of cats had
declined, this conclusion was based on questionable
statistical analyses. Within 5 years, intrinsic popula-
tion growth rates were reported to have increased 4
times, and although population sizes had supposedly
declined, predation on seabirds continued. Hunting
was instituted, and ongoing population estimates were
assessed by use of the highly biased index of cat sight-
The authors acknowledged that control (ie, sup-
pression) would only succeed with ongoing intensive
hunting. Feral cats have been eliminated from at least
48 islands, including Marion Island, primarily through
hunting (sometimes with dogs), trapping, poisoning,
and disease and typically on fairly small islands with
low cat density.
In contrast with hunting, disease, or other methods
of feral cat control that increase mortality rates, TNR
has the potential advantage of allowing niches to
become saturated with neutered individual cats. If, con-
currently with the reduction in maximum per capita
rate of increase, carrying capacity is reduced (typically
by reduction of food oversupplementation) and immi-
gration is controlled, there may be a humane, gradual
reduction in overall cat numbers. Future feral cat man-
agement programs could potentially achieve better suc-
cess with a few modifications of the TNR paradigm.
Despite the substantial expenditure of resources to
operate the 2 TNR programs described here, they prob-
ably were performed on too large a scale; many cats
were neutered, but this constituted a very small overall
proportion of the cats. Moreover, feral cats within a
county surely do not constitute a single population, fur-
ther diluting the enormous overall effort into numerous
smaller efforts with less impact. Trap-neuter-return pro-
grams should be focused on well-defined, preferably
geographically restricted, cat populations, rather than
diluting effort across multiple populations. In future
TNR studies, it would be helpful if trapping efforts were
standardized to allow for the least biased index esti-
mates of population size from trapping efficiency (catch
per unit effort
), although with such an intelligent
species, cats may modify behavior after experience with
the traps. If population growth actually is declining,
then per capita growth rate should decline consistently.
Also, retrapping statistics, which were not obtained in
these programs, are particularly valuable because they
allow for comparison of observed retrapped (neutered)
proportions with the critical proportions needed to
reduce R
to < 1.0.
Focused TNR programs have had some success. A
survey-based assessment
of TNR for small colonies
(mean, 7 cats) revealed moderate success, with reduc-
tion of mean colony size by as much as half. A two-
thirds reduction in population size was obtained in a
feral cat colony on a university campus where every cat
was specifically included in the census.
causes of loss from the population included euthanasia
of sick cats, adoption, and deaths (often vehicular trau-
ma), increases in population were attributable to immi-
gration but not births because virtually all resident cats
were neutered. For these programs, managers were
able to evaluate success because every cat could be
counted. In larger programs, such enumeration is
impossible and index-level assessment, such as that
described here, becomes necessary.
Statistical assessment of the impact of TNR pro-
grams on population size is critical to help gain credi-
bility for such programs. Because of the increasing will
to address humane, conservation, and public health
concerns associated with free-roaming cats, tools to
evaluate program success will increasingly contribute
to achieving management goals.
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JAVMA, Vol 227, No. 11, December 1, 2005 Scientific Reports: Original Study 1781
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... This dynamic has impeded scientifically-based efforts to manage cats and has instead promoted less effective strategies such as trap-neuter-release (TNR) programs. TNR has proved largely ineffective at stabilizing and reducing cat populations, particularly after accounting for reductions to a cat population due to other strategies, such as permanent removal of animals for adoption or euthanasia (e.g., Foley et al. 2005;Longcore et al. 2009;Crawford et al. 2019). What is needed is a return to science-based decision-making and to a perspective that values ecosystem health in the management of free-roaming cats. ...
... Although TNR has appeal to those wishing to avoid euthanasia as a solution, it is not effective at reducing feral cat numbers at scale. As shown by demographic modeling (Andersen et al. 2004;Foley et al. 2005), the proportion of the cat population that is needed to be spayed or neutered (in a population not supplemented with food) must exceed 70% or more to reduce cat numbers through decreasing births in a population. This decrease has only been achieved once at a meaningful scale (Gunther et al. 2022), where the authors noted the need to implement TNR at high intensity, sustain the effort over long time scales, and over all contiguous areas. ...
... In fact, this one example cost more than one million dollars ($US) over 9 years of TNR implementation (Gunther et al. 2022). Notably, two other large, intensive, and well-funded TNR efforts, conducted in California and Florida over eleven (1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003) and six years (1998)(1999)(2000)(2001)(2002)(2003)(2004), respectively, failed to reduce cat numbers (Foley et al. 2005). Feeding at TNR sites increases reproduction for unsterilized individuals, attracts new cats through immigration and abandonment, attracts wildlife seeking a food resource (e.g., skunks Mephitis mephitis, raccoons Procyon lotor, Virginia opossum Didelphis virginiana, grey fox Urocyon cinereoargenteus), and results in increased interactions with wildlife and subsequent opportunities for spreading disease. ...
Full-text available
Free-roaming domestic cats (i.e., cats that are owned or unowned and are considered ‘at large’) are globally distributed non-native species that have marked impacts on biodiversity and human health. Despite clear scientific evidence of these impacts, free-roaming cats are either unmanaged or managed using scientifically unsupported and ineffective approaches (e.g., trap-neuter-release [TNR]) in many jurisdictions around the world. A critical first initiative for effective, science-driven management of cats must be broader political and legislative recognition of free-roaming cats as a non-native, invasive species. Designating cats as invasive is important for developing and implementing science-based management plans, which should include efforts to prevent cats from becoming free-roaming, policies focused on responsible pet ownership and banning outdoor cat feeding, and better enforcement of existing laws. Using a science-based approach is necessary for responding effectively to the politically charged and increasingly urgent issue of managing free-roaming cat populations.
... Auch in ländlichen Gebieten kommen manchmal Kolonien verwilderter Katzen vor, was insbesondere dann problematisch ist, wenn sie sich in der Nähe oder innerhalb eines Schutzgebietes befinden (Foley et al., 2005). Centonze & Levy (2002) Ortschaften leben, um sich menschliche Nahrung zu erschließen (Biró et al., 2005) aber zusätzlich außerhalb der Siedlungen jagen (Woods et al., 2003). ...
... Die Größe der Territorien kann in Abhängigkeit der einzelnen Individuen und des Lebensraumes sehr stark zwischen unter 1 ha und über 600 ha variieren (Lüps, 2003 Tieren/km² beobachtet (Liberg et al., 2000). Der wichtigste limitierende Faktor, welcher so hohe Dichten zulässt, ist das Vorhandensein von Nahrung (Liberg et al., 2000;Lüps, 2003;Foley et al., 2005). Das Füttern der Hauskatzen macht sie weitestgehend unabhängig von natürlichen Nahrungsquellen (Lüps, 2003;Baker et al., 2008) (Lüps, 2003;Schnidrig, 2010). ...
... Levy, 2002;Coe et al., 2021). Das Ziel von TNR-Programmen ist, nach Ansicht von Befürwortern und Tierschützern, die so behandelten Populationen oder Kolonien durch Kastration zu verringern oder zumindest zu stabilisieren und richtet sich selten nach Belangen des Naturschutzes (Foley et al., 2005;Robertson, 2008). Im Vordergrund steht das Wohl des Einzeltieres (Foley et al., 2005). ...
Full-text available
The impact of domestic cats on vertebrates is now known globally - they are a major risk for endangered and threatened species. Hybridization of domestic and wild cats must also not go unnoticed, so there are already studies across Europe with differentiated results on this. Especially in the last decades, however, the domestic cat has become an increasingly popular pet throughout the western world, and its numbers continue to increase, which can lead to unnaturally high densities in settlements, for example. To summarize the current state of knowledge on this topic we supplemented the previously published report of Hackländer et al. (2014) with current data and literature. In particular, the topics of hybridization, potential management measures, legal framework and food analyses, which specifically address the impact of domestic cats on biodiversity, were considered in an expanded manner. The research revealed the need for action on the topic, which should not be underestimated, and the necessity of both the acceptance of personal responsibility and the consistent implementation of given political frameworks. The paper appeared in the BOKU Berichte zur Wildtierforschung und Wildbewirtschaftung and is available online. Translated with (free version)
... While TNR appeals to those seeking to avoid euthanasia as a solution, it is ineffective at reducing feral cat numbers at scale. In a population not supplemented with food, the percent of the cat population needed to be spayed or neutered must exceed 70% to reduce cat numbers through decreasing births in a population (Andersen et al., 2004;Foley et al., 2005). Such a large number has only been achieved once at a meaningful scale (Gunther et al., 2022), where the authors noted the need to implement TNR at high intensity, sustain the effort over long time scales, and over all contiguous areas. ...
... In fact, this one example cost more than one million dollars ($US) over 9 years of TNR implementation (Gunther et al., 2022). Notably, two other large, intensive, and well-funded TNR efforts, conducted in California and Florida over eleven (1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003) and six years (1998)(1999)(2000)(2001)(2002)(2003)(2004), respectively, failed to reduce cat numbers (Foley et al., 2005). Hence, TNR is not considered a viable form of cat management. ...
... In some cases, TNR is preferred by the public since future culling of cats can be prevented under a high intensity TNR program (Boone et al. 2019) and the altered and returned cats are less likely to display nuisance behaviors . While there are some documented successes of TNR programs (Levy et al. 2003;Swarbrick and Rand 2018;Boone et al. 2019;Kreisler et al. 2019;Spehar and Wolf 2020), some studies claim that TNR does not reduce cat populations (Coe et al. 2021;Longcore et al. 2009;Lepczyk et al., 2010;Castillo and Clarke 2003;Foley et al. 2005). The efficacy of TNR and TE programs depends on their goals, which can be quite varied. ...
... Hence, the natural death rate for altered juveniles (d J S ) was (1-0.76 1/6 ) = 0.045. There are no studies reporting the natural death rate for altered adults, so we used the mean cat's life span to estimate the annual survival rate with the equation provided by Foley et al. (2005), annual survival rate = 1-1/ (mean life span). We assumed the mean life span of an altered adult cat is Table 2 Monthly Parameters for the Population Model. ...
Full-text available
Unowned free-roaming cats are a global problem due to predation on wildlife and the spread of infectious diseases. Programs such as Trap, Neuter, and Return (TNR) and Trap-Euthanize (TE) have been used to manage cat populations. We present a bioeconomic model using data from Knox County, Tennessee, U.S., and some parameters from literature that weighs benefits and costs of free-roaming cat management programs. A benefit of a management program is the reduction in wildlife killed by free-roaming cats, illustrated in the model by placing a value on wildlife. The economic management costs include the costs of trapping and neutering, and the procedural and emotional costs of euthanasia. We highlight two key factors that determine which management strategy is most cost effective. The first is whether cat caretakers cooperate with management by decreasing food available for cat groups. The second is the value attributed to saving the lives of wildlife and free-roaming cats. Based on plausible cat and wildlife values, TNR was the most cost effective when caretakers cooperated, but TE was the most cost-effective when caretakers did not cooperate. When the model is adjusted for the local goals, effort, and information, it can be used as a tool for management.
... No TNR programs have been shown to work at the municipal level, as they spay/ neuter fewer than 5% of the feral cats in a community, falling far short of the .70% sterilization rate required for effective population reduction (Jessup 2004, Foley et al. 2005, Longcore et al. 2009). Multi-agency enforcement of existing California State and local city/county codes and regulations regarding unsanitary conditions associated with feral cat colonies (feces accumulation, fleas) by county agencies (e.g., LAC Code 1959& 1964, city ordinances (e.g., Anaheim Municipal Code Section 2015), vector control districts (CA HSC Secs. ...
Conference Paper
Full-text available
Since 2001, flea-borne typhus (FBT) has reemerged as an important vector-borne disease in Los Angeles and Orange counties. The reasons for this resurgence are unclear, but this rise has been accompanied by changes in how public/private animal control groups manage ''nuisance'' animal populations, especially opossums and feral cats, the primary hosts for the cat flea, Ctenocephalides felis, a key vector of FBT in southern California. These changes include no longer recognizing the removal of opossums and feral cats as a flea control strategy and implementation of ''return to field'' programs that place, in lieu of euthanization, impounded cats in neighborhoods without ongoing flea control treatments. Since the first reported FBT case in 2006 in Orange County (the first since 1993), the Orange County Mosquito and Vector Control District (OC Vector) has investigated 237 of the 249 reported FBT cases in the county. Investigations have included patient interviews, ecologic assessments of putative exposure sites, testing of flea and animal specimens collected from these sites for rickettsial bacteria, and public outreach on FBT prevention in affected areas. These investigations have shown that peridomestic animals, such as opossums and feral cats, support populations of fleas infected with Rickettsia typhi and R. felis, the bacteria responsible for human FBT. OC Vector and other local health care agencies have recognized a link between feral cat feeding sites and heightened disease risk in FBT-affected neighborhoods. Despite this observation, local human health care agencies, animal care groups, and city code enforcement agencies have been unwilling, or unable, to implement policies to stop the proliferation of feral cat colonies in areas with high FBT disease risk. The continued rise of FBT has proven difficult to mitigate because of conflicting perspectives among governmental agencies and animal rights advocates, who perceive the zoonotic disease risk associated with cat rescue programs at significantly different thresholds of concern.
... Moreover, the mink feed as well as bedding in empty mink cages also attracted rodents and the juvenile cats may have found more success hunting in these sheds instead of competing with the adults outside of the sheds. Feral and free-roaming cats do exhibit territoriality, but in the presence of an abundant food supply, they tend to either lose this tendency or maintain smaller territories [43]. The latter may have been the case on the mink farms. ...
Full-text available
Zoonotic transmission of SARS-CoV-2 from infected humans to other animals has been documented around the world, most notably in mink farming operations in Europe and the United States. Outbreaks of SARS-CoV-2 on Utah mink farms began in late July 2020 and resulted in high mink mortality. An investigation of these outbreaks revealed active and past SARS-CoV-2 infections in free-roaming and in feral cats living on or near several mink farms. Cats were captured using live traps, were sampled, fitted with GPS collars, and released on the farms. GPS tracking of these cats show they made frequent visits to mink sheds, moved freely around the affected farms, and visited surrounding residential properties and neighborhoods on multiple occasions, making them potential low risk vectors of additional SARS-CoV-2 spread in local communities.
... In the United States, cats are the most common rabies-positive domestic animal [21], and in Pennsylvania, cats pose 2.5 times the relative risk of human rabies exposure compared to bats [51]. Although studies suggest that cats generate a protective antibody titre more effectively than dogs [55], reported vaccination rates of owned cats (54%, [56,57]) and feral cats (1.6%) [52,58]) are wholly insufficient to be the only preventative approach. Limiting free-roaming was an essential intervention in the eradication of enzootic canine rabies in North America [59], and would be a highly impactful approach for cats as well. ...
Full-text available
Domestic animals can serve as consequential conveyors of zoonotic pathogens across wildlife-human interfaces. Still, there has been little study on how different domestic species and their behaviors influence the zoonotic risk to humans. In this study, we examined patterns of bat encounters with domestic animals that resulted in submission for testing at the rabies laboratories of the Canadian Food Inspection Agency (CFIA) during 2014–2020. Our goals were specifically to examine how the number of bats submitted and the number of rabies positive bats varied by the type of domestic animal exposure and whether domestic cats were indoor or free-roaming. The CFIA reported 6258 bat submissions for rabies testing, of which 41.5% and 8.7% had encounter histories with cats and dogs, respectively. A much smaller fraction of bat submissions (0.3%) had exposure to other domestic animals, and 49.5% had no domestic animal exposure. For the bat submissions related to cats, and where lifestyle was noted, 91.1% were associated with free-roaming cats and 8.9% with indoor cats. Model results indicated the probability of a rabies-positive bat was the highest with a history of dog association (20.2%), followed by bats with no animal exposure (16.7%), free-roaming cats (6.9%), cats with unspecified histories (6.0%) and the lowest probability associated with non-free-roaming (indoor) cats (3.8%). Although there was lower rabies prevalence in bats associated with cats compared to dogs, the 4.8 fold higher number of cat-bat interactions cumulatively leads to a greater overall rabies exposure risk to humans from any free-roaming outdoor cats. This study suggests that free-roaming owned cats may have an underappreciated role in cryptic rabies exposures in humans and as a significant predator of bats. Preventing free-roaming in cats is a cost-effective and underutilized public health recommendation for rabies prevention that also synergistically reduces the health burden of other feline-associated zoonotic diseases and promotes feline welfare and wildlife conservation.
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Trap-neuter-return (TNR) is promoted as a "humane" alternative to lethal methods for population control of feral domestic cats (Felis catus). This paper explores feed-backs between feral domestic cats, coyotes (Canis latrans), raccoons (Procyon lotor), and skunks (Mephitis mephitis) at a TNR feral cat colony in Rhode Island, USA. A total of 12,272 photographs from a motion-activated camera were analyzed. Cat population size and visitation frequency of wildlife were estimated during three different feeding regimes. Abundant food on the ground was associated with increased wildlife visits, while elevated or limited food was associated with decreased wildlife visits. During the two-year study period, the population of cats dropped from 17 to 12 individuals and
Significance Although popular companion animals, domestic cats pose numerous problems when free-roaming, including predation of wildlife, hazards to humans, impaired sanitation, and a decrease in their welfare. Thus, managing their populations is essential. The trap–neuter–return method (TNR; capturing, sterilizing, returning/releasing) is widely employed for managing cat populations. However, there is a lack of long-term controlled evidence for its effectiveness. We examined the outcomes of high-intensity TNR by performing a 12-y controlled field experiment. Neutering over 70% of the cats caused population decline when applied over contiguous areas. However, it was limited by a rebound increase in reproduction and survival. These findings provide a robust quantification of the limitations and the long-term effectiveness of TNR.
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Predation by feral Felis catus on burrowing birds of southern temperate and sub-Antarctic islands has resulted in their presence being considered undesirable. Attempts to eradicate or control these cats have included control by trapping and hunting and lately biological control through the introduction of a viral disease. Estimated at 2139+ or -290 adults in 1975, at least 20% of the Marion Island cat population would have to be 'removed' annually to stabilise the otherwise increasing population. In spite of relatively high mortality during the first year of life, the high fecundity and increased survival after 12 months of age, resulted in this population with its stable age distribution increasing at a rate of 17-23% per annum of the 26 yr. In an attempt to reduce the rate of increase, the fully susceptible population had been exposed to the host-specific, contagious disease feline panleucopaenia during March 1977. Data obtained through transect surveys before and after suggest a continued decrease in population size since 1977. Mean cat density in October 1978 was 54% lower than in October 1976, and in June 1980 it was 65% lower than in June 1976.-Author
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This paper reviews the impacts of three species of introduced mammalian predators on native fauna in Australia. The feral cat Felis catus, introduced over 200 years ago, is linked with early continental extinctions of up to seven species of mammals, regional and insular extinctions of many more species of mammals and birds, and the failure of management programs attempting to reintroduce threatened native species to parts of their former ranges. Evidence for cat-impact is largely historical and circumstantial, but supported by observations that afflicted native species are, or were, small (<200 g) occupants of open habitat and hence likely to be especially vulnerable to cat predation. The red fox Vulpes vulpes was released successfully in 1871. Its subsequent spread into all except parts of arid and tropical Australia coincided with local and regional declines of medium-sized (450 - 5,000 g) mammals, birds and chelid tortoises. The fox has also created recent failures of many management attempts to recover threatened native species. Unequivocal demonstration of fox-impact has been obtained in removal experiments, especially on rock-wallables Petrogale lateralis. The dingo Canis lupus dingo, introduced 3,500-4,000 years ago, probably caused the extinction of the thylacine Thylacinus cynocephalus and Tasmanian devil Sarcophilus harrisii on mainland Australia. In effectively suppresses extant populations of large mammals, such as kangaroos, and emus, over large areas. Impacts of all three predators are wrought primarily by direct predation. Negative impacts appear to be increased in spatially fragmented forests where native species are restricted to remnant vegetation, and in arid landscapes when native species become restricted temporarily to scattered oases during drought. Alternative prey especially rabbits Oryctolagus cuniculus, enhance negative impacts on native species by supporting large populations of the predators. It is concluded that feral cats, and especially foxes have major negative impacts on certain small and medium-sized native vertebrates in Australia, whereas dingoes have major negative impacts on large species. Dingoes could have positive effects on smaller native species if they significantly suppress populations of foxes and cats. Further quantification of both the direct and indirect in pacts of the three predators on native fauna is needed and should be obtained from experimental field studies.
Discusses variation in Felis catus social behaviour, distinguishing between: 1) populations in which adult females are generally solitary and those where they are gregarious; 2) variation in group size and social structure, and 3) individual variation within and between age and sex classes in terms of the nature of social relationships. Specifically, note is taken of the significance of spacing, encounters, use of social odours, group size and structure, and social dynamics. Attention is paid to the nature and consequences of the social hierarchy that is established in farm cat societies, which varies considerably between populations. -S.J.Yates
Using gut samples, faecal analysis, records of prey brought home by house cats and uneaten remains in the field, the diet of domestic and feral Felis catus is examined. In descending order of frequency, mammals, birds and (especially below latitude 35o) reptiles predominate. Cat predation on islands, where bird prey is proportionally more significant, often has an adverse impact on native species. Diet is discussed in terms of sex and age differences; seasonal variations; and prey availability. The impacts of cats on farmyard rats; on wild rabbits Oryctolagus cuniculus, voles and other rodents; on game species; on bird populations on continents; and on island wildlife, are all discussed. -S.J.Yates
This is one of two concurrent papers reviewing aspects of the thermal and water physiology and energetics of the genus Peromyscus. In this review paper, initial emphasis is given to developing a thermal life history of young mice during the preweaning and immediate post-weaning periods. Subsequent sections discuss the body temperatures, metabolic rates, body insulation, thermoregulatory mechanisms, and microclimates of adults. A comparative approach is taken throughout, and emphasis is given to acclimation and acclimatization. Major goals are to review the extant literature comprehensively and thereby help to identify directions for future research.
Free-ranging domestic cats on farmsteads were censused annually in August 1977-81 within a 5,182-ha area typical of the cash-grain region in central Illinois. The estimated average number of cats on the area in late summer was 326 (6.3/100 ha). Annual recruitment of immature cats into the late summer population averaged 1.5/adult female. Survival beyond 3-5 years of age was rare; <1% survived 7 or more years. Eleven adult cats were radio-monitored during a 30-day period in summer; four males ranged over larger areas (P < 0.01) than seven females (228 ± 100 ha and 112 ± 21 ha, respectively). When cats were not on farmsteads, approximately 73% of their radiolocation points (N = 1,227) were in edge or linear configurations of cover. Cats made disproportionately high (P < 0.05) use of farmsteads and perimeters, roadsides, and field interfaces and disproportionately low use (P < 0.05) of fields of corn and soybeans. Domestic cats on the area were well fed by humans but routinely deposited prey at their residences.
We report on a preliminary study of the spatial organization, habitat use, and diet of feral cats (Felis catus) in a riparian reserve in central California, to assess potential impacts of this exotic predator on native species. Home ranges of adult cats averaged 31.7 ha and did not differ significantly by sex or season. Home ranges also showed little overlap, suggesting a territorial social system. Cats strongly preferred riparian habitats and foraged primarily on native species of small mammals, especially California voles (Microtus californicus) and Botta's pocket gophers (Thomomys bottae), although birds, insects, and exotic rodents were also eaten. The preference for riparian habitats and native prey suggests that impacts on biodiversity by feral cats may be great, especially in Mediterranean climates where riparian communities already are heavily impacted by urbanization and agriculture.