How Many Animals Do Cats Kill In A Year

Abstruse

Anthropogenic threats, such as collisions with homo-made structures, vehicles, poisoning and predation by domestic pets, combine to kill billions of wildlife annually. Free-ranging domestic cats take been introduced globally and have contributed to multiple wildlife extinctions on islands. The magnitude of mortality they cause in mainland areas remains speculative, with large-scale estimates based on not-systematic analyses and little consideration of scientific data. Here we bear a systematic review and quantitatively estimate mortality caused by cats in the United states. We gauge that complimentary-ranging domestic cats kill 1.3–4.0 billion birds and vi.3–22.3 billion mammals annually. Un-owned cats, as opposed to endemic pets, cause the majority of this bloodshed. Our findings suggest that complimentary-ranging cats crusade substantially greater wild animals bloodshed than previously thought and are likely the single greatest source of anthropogenic mortality for United states birds and mammals. Scientifically sound conservation and policy intervention is needed to reduce this impact.

Introduction

Domestic cats (Felis catus) are predators that humans take introduced globally^1,2 and that accept been listed among the 100 worst not-native invasive species in the world^three. Free-ranging cats on islands take caused or contributed to 33 (14%) of the mod bird, mammal and reptile extinctions recorded by the International Matrimony for Conservation of Nature (IUCN) Red Listing^iv. Mounting evidence from three continents indicates that cats can also locally reduce mainland bird and mammal populations^5,6,vii and cause a substantial proportion of full wild animals mortality^8,9,10. Despite these harmful effects, policies for management of free-ranging cat populations and regulation of pet buying behaviours are dictated by animal welfare issues rather than ecological impacts¹¹. Projects to manage free-ranging cats, such as Trap-Neuter-Return (TNR) colonies, are potentially harmful to wild animals populations, simply are implemented beyond the United States without widespread public knowledge, consideration of scientific bear witness or the ecology review processes typically required for deportment with harmful environmental consequences^11,12.

A major reason for the current not-scientific approach to management of free-ranging cats is that total mortality from true cat predation is often argued to be negligible compared with other anthropogenic threats, such every bit collisions with man-made structures and habitat destruction. Nonetheless, assessing the conservation importance of a bloodshed source requires identification of which species are being killed (for example, native versus non-native invasive species and rare versus common species) in improver to interpretation of total numbers of fatalities. Estimates of annual US bird mortality from predation past all cats, including both owned and united nations-owned cats, are in the hundreds of millions^13,fourteen (we ascertain un-owned cats to include farm/barn cats, strays that are fed by humans merely non granted access to habitations, cats in subsidized colonies and cats that are completely feral). This magnitude would place cats among the tiptop sources of anthropogenic bird mortality; withal, window and building collisions have been suggested to cause even greater mortality^{15,sixteen,17}. Existing estimates of mortality from cat predation are speculative and non based on scientific data^{13,fourteen,15,16} or, at best, are based on extrapolation of results from a single study¹⁸. In improver, no large-scale mortality estimates exist for mammals, which form a substantial component of cat diets.

Nosotros conducted a data-driven systematic review of studies that estimate predation rates of owned and un-endemic cats, and estimated the magnitude of bird and mammal mortality acquired past all cats across the contiguous United States (all states excluding Alaska and Hawaii). Nosotros guess that gratis-ranging domestic cats kill ane.iii–4.0 billion birds and 6.3–22.three billion mammals annually, and that un-endemic cats cause the majority of this bloodshed. This magnitude of mortality is far greater than previous estimates of cat predation on wildlife and may exceed all other sources of anthropogenic mortality of U.s. birds and mammals.

Results

The magnitude of bird bloodshed acquired by cat predation

Afterwards excluding studies that did non meet a priori inclusion criteria designed to increment the accuracy of our analysis, we developed probability distributions of predation rates on birds and mammals. We combined predation rate distributions with literature-derived probability distributions for US cat population sizes, and nosotros besides accounted for the proportion of owned cats allowed outdoors, the proportion of owned and un-owned cats that hunt, and imperfect detection of owned cats' prey items.

We generated an estimated range of bird and mammal mortality caused by cat predation by incorporating the above distributions—including separate predation rate distributions for owned and un-endemic cats—and running 10,000 calculation iterations. We augmented U.s.a. predation information by incorporating predation rate estimates from other temperate regions (Supplementary Tabular array S1). For birds, we generated three Usa mortality estimates based on predation data from studies in: (ane) the United States, (2) the United States and Europe and (3) the United States, Europe, and other temperate regions (primarily Australia and New Zealand). Owing to a lack of United states studies of un-owned true cat predation on mammals, we estimated mammal bloodshed using information groupings ii and 3. We based all other probability distributions on Usa studies (distribution details in Tabular array 1; data in Supplementary Tabular array S2).

Tabular array 1 Probability distributions used for parameters in cat predation model.

Full size table

The three estimates of bird mortality varied moderately, with a 19% difference among median estimates (Tabular array two). We focus interpretation on the estimate generated using United states of america and European predation information because it is the lowest value. Furthermore, this estimate is more likely to exist representative of the US than the approximate based on incorporation of information from Australia and New Zealand, where the wildlife brute and climate are less similar to the United states. Nosotros estimate that cats in the contiguous United States annually kill between 1.3 and four.0 billion birds (median=2.4 billion) (Fig. 1a), with ∼69% of this mortality caused past un-owned cats. The predation approximate for un-owned cats was higher primarily due to predation rates past this group averaging three times greater than rates for owned cats.

Table 2 Median estimates of almanac wild fauna bloodshed acquired by cat predation in the contiguous United states of america.

Full size table

Figure 1: Estimates of cat predation on Usa birds and mammals.

(a) Probability distribution of estimated bird bloodshed caused past all gratuitous-ranging cats in mainland areas of the contiguous United States. (b) Probability distribution of estimated mammal mortality caused by all free-ranging cats in mainland areas of the face-to-face United States.

Full size image

The magnitude of mammal mortality caused by cat predation

Our guess of mammal mortality was robust to the choice of predation data as evidenced by a 1.6% difference between the 2 median estimates (Table 2). We focus estimation on the lower estimate, which was based on Us and European predation data and United states of america values of other parameters. We estimate annual mammal mortality in the contiguous Us at between 6.3 and 22.3 billion (median=12.3 billion) (Fig. 1b) with 89% of this mortality caused by un-owned cats. The estimate that incorporated European data (simply non data from Australia and New Zealand) may exist slightly lower considering wild animals across much of Europe were historically exposed to predation past a similarly-sized wild cat (Felis sylvestris) and, therefore, may exist less naive to predation by domestic cats. Notwithstanding, information technology is unlikely that European wildlife accept fully adapted to the unusually loftier densities of domestic cats in much of this continent⁹.

Factors explaining estimate dubiousness

For both birds and mammals, sensitivity analyses indicated that un-endemic cat parameters explained the greatest variation in full bloodshed estimates (Fig. 2). Un-owned cat population size explained the greatest variation in mortality estimates (42% for birds and 51% for mammals), and the un-owned cat predation rate explained the second greatest variation (24% for birds and 40% for mammals). The just other parameters that explained >five% of variation in mortality estimates were the owned cat predation rate on birds (16%) and the correction factor for imperfect detection of owned cats' prey items (8%).

Figure two: Factors explaining uncertainty in estimates of wildlife mortality from cat predation.

Corporeality of variation in estimates of wildlife mortality in the contiguous Us contributed past each parameter in the cat predation model (percentages represent adjusted R ⁱⁱ values from multiple regression models).

Total size paradigm

Discussion

Our approximate of bird mortality far exceeds whatever previously estimated U.s.a. figure for cats^13,14,16, as well as estimates for whatever other direct source of anthropogenic mortality, including collisions with windows, buildings, communication towers, vehicles and pesticide poisoning^{13,fifteen,sixteen,17,18,19,20,21}. Systematic reviews like ours, which includes protocol conception, a data search strategy, data inclusion criteria, data extraction and formal quantitative analyses²², are scarce for other anthropogenic bloodshed sources.²¹ Increased rigour of mortality estimates should be a loftier priority and will allow increased comparability of mortality sources²³. Nonetheless, no estimates of whatever other anthropogenic bloodshed source approach the value we calculated for cat predation, and our estimate is the first for cats to be based on rigorous data-driven methods. Notably, we excluded high local predation rates and used assumptions that led to minimum predation rate estimates for united nations-owned cats; therefore, actual numbers of birds killed may be even greater than our estimates.

Complimentary-roaming cats in the Usa may also have a substantial impact on reptiles and amphibians. However, U.s.a. studies of cat predation on these taxa are scarce. To generate a first approximation of US predation rates on reptiles and amphibians, we used the same model of true cat predation along with estimates of cat predation rates on these taxa from studies in Europe, Australia and New Zealand. We gauge that between 228 and 871 million reptiles (median=478 one thousand thousand) and between 86 and 320 million amphibians (median=173 million) could be killed past cats in the contiguous United States each year. Reptile and amphibian populations, and, therefore, true cat predation rates, may differ between the regions where we gathered predation information for these taxa and the Us. Furthermore, reptiles and amphibians are unavailable as prey during winter across much of the The states. Additional inquiry is needed to analyze impacts of cats on US herpetofauna, especially given numerous anthropogenic stressors that threaten their populations (for instance, climate change, habitat loss and infectious diseases) and documented extinctions of reptiles and amphibians due to true cat predation in other regions^4,24.

The uncommonly high approximate of mammal mortality from cat predation is supported past individual US studies that illustrate high annual predation rates by individual un-endemic cats in backlog of 200 mammals per twelvemonth^{6,25,26,27,28} and the consistent finding that cats preferentially depredate mammals over other taxa (Supplementary Table S1). Even with a lower yearly predation rate of 100 mammals per cat, annual mortality would range from 3–eight billion mammals merely for un-owned cats, based on a population estimate of between 30 and eighty million united nations-endemic cats. This estimated level of bloodshed could exceed any other directly source of anthropogenic mortality for small mammals; notwithstanding, nosotros are unaware of studies that take systematically quantified straight anthropogenic mortality of small terrestrial mammals across large scales.

Native species make upward the majority of the birds preyed upon by cats. On average, merely 33% of bird prey items identified to species were non-native species in 10 studies with 438 specimens of 58 species (Supplementary Table S3). For mammals, patterns of predation on native and non-native species are less clear and announced to vary by landscape type. In densely populated urban areas where native pocket-size mammals are less common, non-native species of rats and mice tin make upwards a substantial component of mammalian casualty²⁹. However, studies of mammals in suburban and rural areas found that 75–100% of mammalian prey were native mice, shrews, voles, squirrels and rabbits^26,thirty,31. Further enquiry of mammals is needed to clarify patterns of predation by both owned and un-owned cats on native and not-native mammals, and across different landscape types.

Sensitivity analyses signal that additional research of un-owned cats volition continue to amend precision of mortality estimates. Our finding that un-owned cat population size and predation rate explained the greatest variation in bloodshed estimates reflects the current lack of knowledge almost un-endemic cats. No precise guess of the un-endemic true cat population exists for the U.s. considering obtaining such an estimate is price prohibitive, and feral united nations-owned cats are wary of humans and tend to be lone outside of urban areas. In add-on, human subsidized colonies of un-owned cats are maintained without widespread public knowledge. For example, in Washington DC lonely there are >300 managed colonies of united nations-owned cats and an unknown number of unmanaged colonies. Population size estimates can exist improved by incorporating observations of gratis-ranging cats into a wildlife mortality reporting database²³.

Context for the population impact of a mortality source depends on comparing mortality estimates to estimates of population affluence of private species. Nonetheless, continental-scale estimates of wildlife population affluence are uncertain due to spatio-temporal variation in numbers. For mammals, clarification of the population impacts of true cat predation is hindered by the absence of nationwide population estimates. For all Northward American land birds, the grouping of species near susceptible to mainland cat predation (Supplementary Table S3), existing estimates range from 10–20 billion individuals in North America³². A lack of detail about relative proportions of different bird species killed by cats and spatio-temporal variation of these proportions makes it hard to identify the species and populations that are most vulnerable. The magnitude of our bloodshed estimates advise that cats are likely causing population declines for some species and in some regions. Threatened and endangered wildlife species on islands are about susceptible to the effects of true cat predation, and this may as well exist true for vulnerable species in localized mainland areas⁵ because pocket-size numbers of fatalities could crusade significant population declines. Threatened species in close proximity to cat colonies—including managed TNR colonies^xi,12—face an specially high level of risk; therefore, true cat colonies in such locations comprise a wildlife direction priority. Claims that TNR colonies are constructive in reducing cat populations, and, therefore, wildlife mortality, are non supported by peer-reviewed scientific studies¹¹.

Our estimates should alert policy makers and the general public most the large magnitude of wildlife bloodshed caused past free-ranging cats. Structured decisions about actions to reduce wild animals bloodshed require a quantitative evidence base. We provide evidence of large-scale true cat predation impacts based on systematic analysis of multiple data sources. Futurity specific management decisions, both in the United States and globally, must be further informed by fine scale enquiry that allows assay of population responses to cats and cess of the success of detail management actions. We are not suggesting that other anthropogenic threats that kill fewer individuals are biologically unimportant. Near nothing is known about the cumulative population impacts of multiple mortality sources. Furthermore, comparing of full mortality numbers has express apply for prioritization of risks and development of conservation objectives. Combining per species estimates of bloodshed with population size estimates will provide the greatest information nearly the risk of population-level impacts of cat predation. Although our results suggest that owned cats take relatively less impact than un-endemic cats, owned cats yet crusade substantial wildlife mortality (Tabular array ii); elementary solutions to reduce mortality caused past pets, such as limiting or preventing outdoor access, should be pursued. Efforts to better quantify and minimize bloodshed from all anthropogenic threats are needed to increase sustainability of wild fauna populations.

The magnitude of wild animals mortality caused past cats that we written report here far exceeds all prior estimates. Available evidence suggests that mortality from cat predation is probable to exist substantial in all parts of the world where free-ranging cats occur. This bloodshed is of particular concern within the context of steadily increasing populations of endemic cats, the potential for increasing populations of un-owned cats¹², and an increasing abundance of direct and indirect mortality sources that threaten wild animals in the United States and globally.

Methods

Literature search

We searched JSTOR, Google Scholar, and the Web of Scientific discipline database (formerly ISI Web of Science) within the Web of Knowledge search engine published by Thomson Reuters to identify studies that document true cat predation on birds and mammals. Nosotros initially focused this search on U.s.a. studies, but due to a limited sample of these studies, we expanded the search to include predation enquiry from other temperate regions. We as well searched for studies providing estimates of true cat population sizes at the scale of the contiguous United States and for US studies that estimate the proportion of endemic cats with outdoor access and the proportion of cats that chase wild animals. The search terms we used included: 'domestic cat' in combination with 'predation,' 'prey,' 'nutrition,' 'food item' and 'mortality'; all previous terms with 'domestic cat' replaced by 'Felis catus,' 'feral,' 'stray,' 'farm,' 'free-ranging,' and 'pet'; 'trap-neuter-return colony'; 'TNR colony'; and 'true cat predation' in combination with 'wildlife,' 'bird,' 'mammal,' and 'rodent'. We checked reference lists of manufactures to identify additional relevant studies. Atomic number 82 authors of 3 studies were too contacted to ask whether they knew of ongoing or completed unpublished studies of cat predation in the United States.

Nomenclature of cat ranging behaviour

Nosotros grouped studies based on the ranging behaviour of cats investigated. We defined owned cats to include owned cats in both rural and urban areas that spend at least some time indoors and are also granted outdoor access. We defined un-owned cats to include all un-endemic cats that spend all of their time outdoors. The un-owned cat group includes semi-feral cats that are sometimes considered pets (for case, farm/befouled cats and strays that are fed past humans only non granted admission to habitations), cats in subsidized (including TNR) colonies, and cats that are completely feral (that is, completely contained and rarely interacting with humans). We did non classify cats by landscape type or whether they receive food from humans because the amount of time cats spend outdoors is a major determinant of predation rates^33,34 and because predation is independent of whether cats are fed by humans^6,34,35.

Written report inclusion criteria

Studies were only included if: (1) they conspicuously reported cat ranging behaviour (that is, a clarification of whether cats were owned or un-owned and whether they were outdoor cats or indoor-outdoor cats), and (2) the grouping of cats investigated fit exclusively into one of the two groups we defined above (that is, nosotros excluded studies that lumped owned and un-owned cats in a single predation rate estimate). For some studies, we extracted a portion of information that met these criteria merely excluded other information from cats with unknown ranging behaviour. We just included mainland and big island (New Zealand and United kingdom of great britain and northern ireland) predation studies, because true cat predation on pocket-sized islands is often exceptionally high^36,37 and focused on colony nesting seabirds³⁸. We excluded studies from outside temperate regions and those with predation charge per unit estimates based on fewer than 10 cats, <1 month of sampling, or on cats that were experimentally manipulated (for instance, by plumbing equipment them with bells or behaviour altering bibs). We included studies that used true cat owners' records of casualty returns, but nosotros excluded those that asked owners to estimate by prey returns considering such questionnaires may lead to bias in estimation of predation rates³⁹. (For a list of all included and excluded studies, see Supplementary Tabular array S1).

Data extraction and standardization of predation rates

Most studies written report an estimate of true cat predation charge per unit (that is, daily, monthly or annual prey killed per cat) or nowadays data that allowed us to calculate this rate. When studies only reported predation rate estimates for all wildlife combined, we calculated divide predation rates by extracting taxa-specific prey counts from tables or figures and multiplying the total predation rate by the proportion of prey items in each taxon. If taxa-specific counts were non provided, nosotros direct contacted authors to obtain this information. For studies that presented low, medium and high estimates or low and high estimates, nosotros used the medium and average values, respectively. For studies that presented more i predation estimate for cats with similar ranging behaviour (for example, owned cats in rural and urban areas), nosotros calculated the boilerplate predation charge per unit.

Nearly all studies of un-endemic cats report numbers or frequencies of occurrence of different taxa in stomachs and/or scats. For studies reporting numbers of prey items, we estimated annual predation rates by assuming ane stomach or scat sample represented a cat's average daily prey intake (for instance, an average of one prey item per breadbasket or scat=365 prey per true cat per year). This assumption probable resulted in conservative estimates because cats mostly assimilate prey within 12 h (ref.²⁸28) and can produce two or more scats each solar day²⁹. For studies reporting occurrence frequencies of casualty items, we assumed this proportion represented a true cat'due south average daily casualty intake (for example, a 10% bird occurrence charge per unit=0.1 bird per stomach or scat=36.5 birds per cat per twelvemonth). This supposition results in coarse predation rate estimates, just estimates from this arroyo are even more conservative than those from the get-go assumption considering many stomachs and scats undoubtedly included more than ane bird or mammal.

Predation charge per unit estimates from many studies were based on continuous yr-round sampling or multiple sampling occasions covering all seasons. Withal, seasonal coverage of some studies was incomplete. To generate full-year predation rate estimates in these cases, we adjusted partial-yr predation estimates co-ordinate to the boilerplate proportion of prey taken in each month every bit determined from twelvemonth-round studies reporting monthly data (birds and mammals^viii,33, birds simply^vii,40). For partial-year estimates from the northern hemisphere, we outset monthly estimates from southern hemisphere studies by 6 months. The final annual predation rate estimates for all studies are presented in Supplementary Tabular array S1. The twelvemonth-circular studies we used stand for different geographical regions (for birds—England, Kansas (United states), Australia and New Zealand; for mammals—England and Australia) with varying climates and slightly varying seasonal patterns of predation. For both birds and mammals, averaging across full-twelvemonth studies resulted in higher proportions of predation in the jump and summer compared with fall and winter, an expected design for much of the United States. The reference studies we used, therefore, provide a reasonable baseline for correcting to full-yr bloodshed estimates. This approach greatly improves upon the assumption that bloodshed is negligible during the period of the year not covered past sampling.

Quantification of annual mortality from cat predation

Nosotros estimated wild fauna bloodshed in the face-to-face United States by multiplying data-derived probability distributions of predation rates by distributions of estimated cat affluence, following⁴¹. Quantification was conducted separately for owned and united nations-endemic cats and for birds and mammals. Every bit there was a relatively small sample of U.s.a. studies that estimated predation rates (due north=14 and 10 for birds and mammals, respectively), we repeated calculations using predation rate distributions that were augmented with predation rates from Europe and all temperate zones. However, we only used studies from the face-to-face Us to construct all other probability distributions (listed below).

We estimated mortality using the following model of cat predation:

where npc is the number of owned cats in the contiguous The states, pod is the proportion of owned cats granted outdoor access, pph is the proportion of outdoor owned cats that chase wildlife, ppr is the annual predation charge per unit by endemic cats, cor is a correction factor to account for owned cats not returning all prey to owners, nfc is the number of united nations-endemic cats in the face-to-face The states, pfh is the proportion of un-endemic cats that hunt wild animals, and fpr is the annual predation charge per unit past un-endemic cats. From the probability distribution of each parameter (encounter Table ane and Supplementary Methods for details virtually the specific probability distributions used), we randomly drew i value and used the above formulas to calculate mortality. Random draws were made using distribution functions in Programme R (rnorm and runif commands for normal and uniform distributions, respectively). We conducted 10,000 random draws to estimate a potential range of annual predation on each wildlife taxa. For all analyses, we written report median mortality estimates and lower and upper estimates bracketing the key 95% of values.

Sensitivity analyses

We used multiple linear regression analysis to assess how much variance in mortality estimates was explained past the probability distribution for each parameter. We treated total mortality estimates as the dependent variable (due north=10,000) and we divers a predictor variable for each parameter that consisted of the 10,000 randomly drawn values. We used adjusted R ² values to translate the percentage of variance explained by each parameter.

Boosted data

How to cite this commodity: Loss South.R. et al. The touch of gratis-ranging domestic cats on wild animals of the U.s.a.. Nat. Commun. 4:1396 doi: 10.1038/ncomms2380 (2012).

Change history

• 12 December 2013

  The original version of this Article independent incorrect estimates of the number of animals killed by gratuitous-ranging domestic cats, which were based on 90% confidence levels rather than the stated 95% confidence levels. Reanalysis of the original data after publication using 95% confidence levels resulted in larger ranges for the estimated number of animals killed. The estimates have now been corrected throughout the PDF and HTML versions of the Commodity. Unrelated to the changes above, four estimates of cat predation rates on wildlife from temperate zone studies in Supplementary Table S1 were based on partial year values that had non been adjusted to year-circular estimates. The values have now been revised in Supplementary Table S1.

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Acknowledgements

S.R.L. was supported by a postdoctoral fellowship funded by the US Fish and Wild animals Service through the Smithsonian Conservation Biology Institute's Postdoctoral Fellowship plan. P. Blancher provided insight for development of the model of true cat predation magnitude, and R. Kays, C. Lepczyk and Y. van Heezik provided raw data from their publications. C. Machtans facilitated data sharing, and participants in the 2011 Order of Canadian Ornithologists' anthropogenic mortality of birds symposium provided context and perspectives. C. Lepczyk and P. Blancher provided comments on the manuscript. The findings and conclusions in this article are those of the authors and do not necessarily correspond the views of the Smithsonian or The states Fish and Wildlife Service. All data used for this analysis is available in the Supplementary Materials.

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Authors and Affiliations

1. Migratory Bird Center, Smithsonian Conservation Biological science Plant, National Zoological Park, P.O. Box 37012 MRC 5503, Washington, District of Columbia 20013, USA

   Scott R. Loss & Peter P. Marra

2. Sectionalization of Migratory Birds, U.South. Fish and Wildlife Service, Midwest Regional Part, 3815 American Boulevard East, Bloomington, Minnesota 20013, USA

   Tom Will

Contributions

Due south.R.L. designed the study, collected and analysed information, and wrote the newspaper. T.W. and P.P.M. designed the written report and contributed to paper revisions. All authors discussed the results and commented on the manuscript.

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Correspondence to Scott R. Loss.

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The authors declare no competing financial interests.

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Loss, South., Will, T. & Marra, P. The impact of gratuitous-ranging domestic cats on wild animals of the United States. Nat Commun 4, 1396 (2013). https://doi.org/ten.1038/ncomms2380

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• Received: 06 September 2012

• Accustomed: 12 December 2012

• Published: 29 Jan 2013

• DOI : https://doi.org/10.1038/ncomms2380

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