Free-roaming cats (Felis silvestris catus) in urban environment (Porto, Portugal)

Índice do artigo

Os gatos errantes (Felis silvestres catus) que se encontram em ambiente urbano incluem os animais assilvestrados (sem dono), semi-vadios ou vadios (socializáveis com humanos) e domésticos (sem dono mas não confinados numa propriedade). Em meio urbano estes animais vivem tipicamente em grupos, que podem ser diretamente influenciados pela disponibilidade de recursos alimentares (oferecidos por humanos amigos dos gatos). Para além dos dados recolhidos por organizações não-governamentais de bem-estar animal, pouco é conhecido acerca dos gatos em ambiente urbano em Portugal. Este estudo tem como objetivo descrever e estimar o número e a densidade de gatos errantes e suas colónias na cidade do Porto e compreender a sua distribuição espacial neste habitat urbano. Três áreas diferentes dentro dos limites da cidade foram selecionadas para monitorizar, em três anos diferentes, gatos errantes. Observamos uma ampla distribuição de gatos errantes em cada área de estudo. No entanto, apenas foi identificado um reduzido número de colónias, a maioria ocupando áreas pequenas, com localização associada à existência de alimento e abrigo. Os riscos de saúde pública associados ao aumento do número de gatos errantes em ambiente urbano têm sido reconhecidos em outros países, mas pelo que temos conhecimento, não em Portugal. A consciencialização para as elevadas densidades de gatos nas ruas de Portugal é importante pois acreditamos que esse fato deveria ser considerado um problema, o qual exige a promoção de programas de gestão dos gatos errantes (programas de remoção e/ou esterilização). Por estas razões, os esforços de monitorização e compreensão da ecologia espacial de gatos errantes são fundamentais.



Gomes, Ana Cristina R.1 2*, Cardoso, Alexandra1 3, Carraca, Susana1, Valente, Alexandre1**

1Department of Biology, Faculty of Sciences, University of Porto, Portugal. Rua do Campo Alegre, Edifício FC4, 4169-007 Porto, Portugal.
2CIBIO—Centro de Investigação em Biodiversidade e Recursos Genéticos, Universidade do Porto, Campus Agrário de Vairão, 4485–661 Vairão, Portugal.
3Faculty of Sciences, University of Porto, Portugal. Rua do Campo Alegre, 4169-007 Porto, Portugal.

*Corresponding author: Este endereço de email está protegido contra piratas. Necessita ativar o JavaScript para o visualizar. ; ** Este endereço de email está protegido contra piratas. Necessita ativar o JavaScript para o visualizar.


Free-roaming urban cats, Felis silvestris catus, include feral cats (no owners), semiferal or strays (socialized toward humans) and domestic cats (not confined to owner property). In urban environments, free-roaming cats typically live in groups that might be directly influenced by the availability of food resources (often clumped cat lovers’ hand-outs). Apart from data available at animal welfare Non-Governmental Organizations (NGO), little is known about the free-roaming urban cats in Portugal. This study thus aims to describe and estimate the number and the density of free-roaming cats, and their colonies, within the city of Porto and to understand their spatial distribution in this urban habitat. Three different areas were selected within the city limits and surveyed, during three different years, for the presence of free-roaming cats. We found that free-roaming cat density is widely distributed within each study area. However, only a small number of colonies, most occupying small areas, were identified, with their location associated with food resources and shelter availability. Potential public health problems associated with high numbers of free-roaming cats in urban environments have already been recognized in other countries, but not in Portugal as far as we know. Awareness for the high density of cats in Portugal is important and should be considered a problem, which demands the promotion of free-roaming cat management programs (removal and/or neutering programs). Therefore, efforts for monitoring and understanding cat’s spatial ecology trends are fundamental.

1. Introduction

Pet populations are growing around the world (Clancy and Rowan, 2003; Brown, 2017) and there is an increasing tendency for human preference towards cats, because of their independency as pets and their adaptation to considerable diversified type of environments, especially when considering the different available conditions and life styles at the pet owners’ houses.

The domestic cat, Felis silvestris catus, lives in a commensal relationship with humans (Liberg et al., 2000). Several authors tried to distinguish different categories of domestic cats based on their dependence on humans for their welfare resources, such as food and shelter. This effort seems controversial and, as mentioned by Brickner (2003), these categories may be a continuum where it is difficult to know when one begins and the other ends. In this work, we studied free-roaming cats, in which we include both domestic cats, with owners, and domestic feral ones, that do not have an owner but still can live close to humans.

Free-roaming cats can be found in densely populated areas and can subsist entirely on its own, hunting and scavenging like any wild carnivore, by being fed unintentionally by humans at a refuse depot, or by direct hand-outs from ‘cat lovers’ (Liberg et al., 2000). Therefore, density of free-roaming feral cats shows a wide variation, and the main factors favouring their aggregation and high densities seems to be the availability, accessibility and amount of food (e.g., John, 1977; Izawa et al., 1982; Natoli, 1985; Liberg et al., 2000; Gunther and Terkel, 2002; Flockhart et al., 2016). Furthermore, cats may have the ability to find further resources, without any difficulty, such as shelter in human settlements. According to those factors and also considering that cat density may depend on location and time (Laundré, 1977), the presence of humans might be the main variable determining the growth of free-roaming cat numbers, in urban environments (see Robertson, 2008). Therefore, the large number of cats found in cities is easily understood. Actually, it is in urban areas where cats were reported to live at very high densities (e.g., Liberg et al., 2000; Table 1), forming multimale-multifemale social groups (Izawa et al., 1982; Natoli, 1985; Liberg et al., 2000), or colonies, due to the favourable conditions they find.


Table 1. Cat densities in different urban environments, according to literature.


Density (cats km-2) in
urban environments

Location of the study


0.19 – 0.27

Stewart Island, New Zealand

Harper (2004)

2 - 5

Brooklyn, New York

Calhoon and Haspel (1989)

2.5 – 3.3

Rural area in Southern Sweden

Liberg (1980)

2.6 – 4.1

Harlem and Bedford-Stuyvesant, New York

Kilgour et al. (2017)



Normand (2014)


Herekopare Island, New Zealand

Fitzgerald and Veitch (1985)

132 - 157


Sims et al. (2008)


University of KwaZulu-Natal, South Africa

Jones and Downs (2011)

340 – 580

Onomichi, Japan

Sims et al. (2017)

535 - 743


Finkler et al. (2011)

2300 – 2800


Miramovich (1991)


Ainoshirna Island, Japan

Izawa et al. (1982)


Guelph, Canada

Flockhart et al. (2016)



Cat studies begun to arise in some parts of Europe (and also in the United States), in the 21st century (Slater, 2001). Positive aspects related with the presence of cats in urban environment are reported and include predation of rodents, anti-depressive effect and the possibility to study animal behaviour and nature within an urbanized habitat (reviwed in Gunther and Terkel, 2002). Moreover, the domestic cat is an excellent animal model for the study of ecological factors and spacing organisation in animals, because they occur at high densities throughout the world and are available for study at our doorsteps. On the other hand, negative concerns have also been reported due to cat’s density increase and its relation with public health, namely the spread of zoonotic diseases (Gunther and Terkel, 2002; Robertson, 2008; Duarte et al., 2010) and public nuisance (e.g., hygiene, noise and aesthetics). Additionally, predation of wildlife and extinction of native species (e.g., Fitzgerald and Veitch, 1985; Sims et al., 2008; Woinarski et al., 2017), disruption of ecosystems, hybridization with wild cat populations (Brickner, 2003; Beutel et al., 2017; but see Gil-Sánchez et al., 2015), and the welfare of cats themselves (e.g., transmission of diseases among cats, namely the feline immunodeficiency virus (FIV) and the feline leukemia virus (FeLV); Gunther and Terkel, 2002; Robertson, 2008; Seo and Tanida, 2017) are also negative impacts of high cat densities. Therefore, we are witnessing the growing of a problem with increasingly concern, which should involve human and animal care organizations, as well as public-health governmental officials.

In Portugal, as well as in many other countries around the world, one of the main topic of discussion about free-roaming cats is the control of their numbers through catch-neuter-release programmes (see Sparkes et al., 2013, for discussion on controlling cat numbers). These programs are implemented by municipalities, animal shelters, charity organizations or Non-Governmental Organizations (NGO) that contribute offering neutering clinics with reduced fees (Duarte et al., 2010; d’Avila, 2016). However, the risks of cat disappearance (in the medium-long term), decreasing variability of their populations’ genetic diversity or the decrease of attractiveness to possible partners in natural environment brings controversy to these programmes, especially because they have been being applied without any further management or control. There are yet few scientific studies that supports the arguments given by the pro-cat and anti-cat debate (Gunther and Terkel, 2002; Robertson, 2008), being therefore necessary to increase study efforts in order to improve control methods.

Our aims were to achieve a better knowledge of the free-roaming cats, Felis silvestris catus, in urban environment, and particularly to understand their spatial distribution and the environmental factors involved, as well as to estimate the size of the cats’ population and their colonies. Additionally, this study, as far as we know one of the firsts in Portugal, might be a starting point for more studies and efforts to further understand this problem, in Portugal.

2. Materials and Methods

2.1. Study areas

This study was carried out in three different areas within the city of Porto, Portugal (Figure 1): Contumil, 1.76 km2 (41º10.563’ N / 8º 33.977’ W to 41º 10.058’ N / 8º 35.257’ W); Campo Alegre, 1.10 km2 (41º 9.656’ N / 8º 38.928’ W to 41º 9.045’ N / 8º 37.870’ W); and Porto City Park, 1.03 km2 (41º 10.413 N / 8º 40.251 W to 41º 9.916 N / 8º 41.268 W); study area coordinates were obtained from Google Earth. Cat location coordinates were registered during surveys, in each place, with a GPS device (Garmin, eTrex®).


art gomes fig1

Figure 1. Location of all the study areas: Contumil (blue line), Campo Alegre (yellow line), and Porto City Park (green line) within the Porto city limits (red line; Copyright Google Earth 2017).


The landscape within Porto city is highly variable and includes croplands, green areas (wasteland), scrublands, house areas and others infrastructures, such as roads and subways (classification acording to Monterroso et al., 2009; see Annex Figure A1 for examples). In each area, the proportion of each type of landscape was assessed, according to the previous classification (Figure 2). As a City Park, the Porto City Park study area has higher percentage of green surface than the other two study areas; nevertheless, Contumil green surface is larger than Campo Alegre, which is the more urbanized area (Figure 2).


art gomes fig2


Figure 2. Habitat types available in each study area (habitat classification based on Monterroso et al. (2009); see Annex Figure A1 for examples). (A) Contumil; (B) Campo Alegre; (C) Porto City Park.


2.2. Free-roaming cats sighting surveys

Monthly cat sighting surveys, in all the extent of each study area, were conducted from November/2011 to April/2012, in Contumil study area, from November/2012 to April/2013, in Campo Alegre study area, and from October/2013 to April/2014, in Porto City Park study area. Path coverage was confirmed through a GPS tracking device (Telespial Systems Inc., Trackstick®). Sighting surveys were made monthly during three-day periods, of approximately equal duration, (morning: 09:00 to 13:00; afternoon: 14:00 to 18:00; and night: 20:00 to 24:00). Not all the area was thoroughly monitored because some gardens and wastelands were not accessible or cat sighting was difficult.

For each individual cat, or cat group, sighted, several parameters were registered (Table 2; Annex Figure A1) and each cat, or cat group, was photographed to allow posterior identification. The area occupied by each colony was estimated through Minimum Convex Polygon (MCP; Barratt, 1997), including the observations of all cats of the colony; these areas were then used to estimate cat’s colony densities. The density of free-roaming cat populations, in each of the different study areas, was estimated based on our observations and the respective area of each study area.



Table 2. Parameters registered during cat sighting walks and methodology adopted. Definitions for each classification are found in the respective reference. See Annex Figure A1 for example images from the classification.






Exact coordinates of the sight


Time of the day



Type of food available

Cat lovers’ hand-outs, garbage containers, others

(Liberg et al., 2000)

Food distribution

Dispersed or clumped

Type of shelter

Specific or accidental

(Bradshaw et al., 1999)

Type of cat population

Feral or domestic

(Liberg et al., 2000)

Colony habitat characteristics

Croplands, green areas (wasteland), scrublands, house areas and others

(Monterroso et al., 2009)

Total number of cats sighted

Direct count



2.3. Statistical analyses

All data sets were checked for normality of the distribution (Shapiro-Wilk test; Shapiro and Wilk, 1965) and homogeneity of variances (Levene’s test; Levene, 1961). Whenever data do not follow these assumptions, we used non-parametric tests.

Daily and monthly variability significance, throughout sampling periods, considering all the study areas, were searched with a Krustal-Wallis statistical test (Krustal and Wallis, 1952).

For colony data, we searched for differences among study areas, through a Student’s t-test (Zar, 1999), considering the total number of cats sighted per colony, the colony area, and the colony cats’ density. We also searched for colony differences between food distribution type and shelter availability type, among all the colonies found in the different study areas, with a Student’s t-test. Furthermore, we tested for colony differences between habitat types with a Krustal-Wallis statistical test. Finally, we searched for a relationship between the colony number of cats and the area occupied by each colony with a Spearman’s correlation test.

All the above statistical tests were made in IBM SPSS Statistics 24 (IBM Corp, 2016).

3. Results and Discussion

3.1. Free-roaming cats and their densities

People’s common habit to care for free-roaming cats was witnessed within the city of Porto, as also occurs in many other cities, such as Lisbon metropolitan (Duarte et al., 2010; d’Avila, 2016). In all our study areas, free-roaming cats showed a wide distribution, except in the Porto City Park area where cats were rarely sighted inside the City Park and seems to be concentrated in the outside or limits of the City Park (Figure 3).


art gomes fig3

Figure 3.  Study area with cat, Felis silvestris catus, sightings (location of observations of each cat) represented by dark blue dots. Each map refers to study areas in each year of sampling (Contumil map from 2012, Campo Alegre from 2013, and Porto City Park from 2014). (A) Contumil (limited by blue line); (B) Campo Alegre (limited by yellow line); (C) Porto City Park (limited by green line; City Park limits in orange line); Copyright Google Earth 2017).


Only a small fraction of the cats sighted could be unmistakably identified as domestic cats with owners, by the presence of a collar. Although, in this study, most of the cats sighted are considered feral, these numbers and results should be dealt with careful, because it may include domestic cats with owners, but without collar, which we do not have way to distinguish from the real feral ones. It is also important to note that only in the Porto City Park a considerable number of neutered cats (almost half of the cats sighted; results not shown), recognized by an ear clipping, were seen, contrary to the other two areas.

Our survey revealed high densities and wide distribution of free-roaming cats in Porto and within each study area: 201.14 cats km-2 at Contumil, 130.91 cats km-2 at Campo Alegre, and 33.00 cats km-2 at Porto City Park (Table 3; Figure 3). Porto City Park area having the smallest cat density might be explained by the high number of people walking their pet dogs within the city park limits, thus preventing cat colonization of this area. On the other hand, Contumil is the study area with higher cat density, possibly because it is a residential area located outside the city core and so with less major stressing factors able to affect cat presence. Nevertheless, it is possible that the number of cats (as well as the number of colonies below) in each area is higher and our results are probably still an underestimation of the real cat density, because we are basing our estimates in the cats we saw and counted, on each survey. However, we recognize that we could have not seen some hidden cats due to difficult or impossible access to some areas, or difficult conditions for sightseeing.


Table 3. Cats (number, percentage of cats in groups and cat density) and cat colonies (total number and density) total for each study area and globally considering all total study areas.


Number of cats

Study area (km2)

Cat density

(cats km-2)



% in

Total number










Campo Alegre







Oporto City Park















The number of cats sighted during each sampling period showed some daily variations (Figure 4). However, we did not find any significant differences among different periods of the day (morning vs. afternoon vs. night), neither considering the total number of cats sighted across all the sampling months (Krustal-Wallis test, P = 0.43) nor the cats sighted in each month (Annex Table A1). Therefore, no pattern of cats daily feeding activity was detected, but the periodicity of feeding station refilling (not analysed), might be a factor affecting such activity. Other factors such as temperature and life cycle stage might also be responsible for behavioural changes in cat distribution (not tested). Furthermore, the total number of cats sighted per month showed some variations, as well (Figure 4). February to March are the months with higher cats sighted (Contumil with more cats sighted in February, Campo Alegre in January and February, and Porto City Park in March), which is probably explained by the start of the reproductive period (Mirmovitch, 1995; Natoli and De Vito, 1989; Nowell and Jackson, 1996). We found significant differences among sampling months, either considering each different period of the day (Annex Table A1) or the total number of cats sighted (Krustal-Wallis test, P = 0.004). Thus, this variation in the abundance of cats sighted across different months of the sampling periods are in agreement with previous conclusions that the activity of the domestic cats depends on the season (Gunther and Terkel, 2002).


art gomes fig4

Figure 4. Total number of free-roaming cats sighted during the observation periods along the study. (A) Contumil; (B) Campo Alegre; (C) Porto City Park.


3.2. Free-roaming cats´ colonies distribution

Characteristics of colonies and their habitats, at each study area, are summarized in Table 4 and Annex Table A2. We found that the number of cats in each colony, as well as their occupation range, is highly variable (Table 4). Most of the colonies are small (Campo Alegre: 8 ± 4 cats; and Porto City Park: 4 ± 1 cats), except in Contumil area where most of them are big (16 ± 7 cats). This might be explained, as before, by the location of Contumil area: a suburb area near scrublands and wastelands, where cat numbers have the possibility to increase due to wide areas, while still leaving near people who feed them. With this, it is evident that colony size is significantly smaller in the residential areas located near the city core (Campo Alegre and Porto City Park; Table 5). Therefore, we found that although cats can adapt to a variety of urban environments, they seem to establish their colonies in places near green and open spaces, meaning that besides food and shelter, this is probably an important factor determining colony size.


Table 4. Characteristics of the cat colonies in the study areas (Contumil, Campo Alegre, and Oporto City Park). The location coordinates were obtained from Google Earth and refer to the central point of the colony area (obtained by minimum convex polygon). Cat number is the total number of cats sighted in each colony; Colony area size in ares; colony density within colony area is cats a-1.

Study area  Colony  Location  Colony
Cat number Area (a) Density
Contumil            1 41° 10.228'N / 8° 34.983'W 13 76.83 0.17
2 41° 10.165'N / 8° 34.925'W 18 40.03 0.45
3 41° 9.972'N / 8° 34.717'W 9 94.98 0.09
4 41° 10.475'N / 8° 34.656'W 11 58.37 0.19
5 41° 10.035'N / 8° 35.136'W 23 26.57 0.87
6 41° 10.109'N / 8° 35.162'W 3 7.86 0.38
7 41° 10.739'N / 8° 34.576'W 7 8.20 0.85
8 41° 10.779'N / 8° 34.472'W 15 10.88 1.38
9 41° 10.507'N / 8° 34.229'W 26 13.63 1.91
10 41° 9.971'N / 8° 34.833'W 20 7.78 2.57
11 41° 10.618'N / 8° 34.254'W 26 181.88 0.14
12 41° 10.132'N / 8° 34.975'W 17 22.60 0.75
Campo Alegre 1 41° 9'28.23"N/  8°38'11.98"W 6 23.66 0.25
2 41° 9'7.58"N/  8°38'7.26"W 5 7.69 0.65
3 41° 9'13.54"N/  8°37'58.73"W 7 13.48 0.52
4 41° 9'21.94"N/  8°38'10.74"W 7 72.61 0.10
5 41° 9'24.93"N/  8°38'17.62"W 7 14.06 0.50
6 41° 9'12.73"N/  8°38'2.99"W 4 0.26 15.38
7 41° 9'24.65"N/  8°38'24.14"W 5 3.24 1.54
8 41° 9'32.06"N/  8°38'46.60"W 7 4.64 1.51
9 41° 9'12.93"N/8°37'53.97"W 3 3.93 0.76
Oporto City Park    1 41º 10.234 N / 8º 40.720 W 4 1.90 2.11
2 41º 10.152 N / 8º 41.212 W 5 15.71 0.32
3 41º 10.100 N / 8º 41.171 W 3 3.33 0.90
4 41º 10.073 N / 8º 41.101 W 3 4.26 0.70


The area occupied by colonies at Contumil is 46 ± 52 ares, at Campo Alegre is 14 ± 19 ares, and at Porto City Park is 6 ± 6 ares. It is clear the great variability in these areas estimates (i.e., the high standard deviations of each study area); nevertheless, Contumil (the less urbanized area) and Campo Alegre (the most urbanized area) are again statistically different from each other (Table 5). Colony areas for the remaining study areas comparisons are not significantly different (Table 5). We additionally found that a higher number of cats are significantly found in colonies with higher occupied areas (Spearman’s correlation: β = 0.64; P = <0.001). Finally, cat densities for our study areas are 0.81 ± 0.78 cats a-1 for Contumil, 2.15 ± 4.04 for Campo Alegre, and 1.01 ± 0.77 for Porto City Park. However, these cat densities are not significantly different from each other, among study areas (Table 5).


Table 5. Results of the statistical analyses for differences in the total number of cats sighted in each colony, in the colony area, and in the cats colony density, between the different study areas. Values of the t statistics and p values, from the Students’ t-test are present. See text for mean and standard deviations (SD) information. *Differences statistically significant (p ≤ 0.05)



Differences among study areas (Student’s t-test)


Contumil vs. Campo Alegre areas

Contumil vs. Porto City Park areas

Campo Alegre vs. Porto City Park areas


t (P)

Total number of cats sighted

*5.13 (0.03)

*6.76 (0.02)

2.44 (0.14)

Colony area

*6.76 (0.02)

3.88 (0.07)

0.92 (0.35)

Cat’s density

2.24 (0.15)

0.02 (0.89)

0.76 (0.40)


Furthermore, we showed that cats were mostly seen in groups/colonies in Contumil and Campo Alegre study areas (Table 3). Location of feeding spots, shelter and garbage container, near colonies, is shown in Figure 5. Most feeding spots were clumped (75% in Contumil, 62% in Campo Alegre, and 75% in Oporto City Park; Annex Table A2). Moreover, most of the shelter found for these feral cats is casual rather than specific (92% in Contumil, 85% in Campo Alegre), except for Porto City Park where all the colonies were found with a specific shelter for them (Annex Table A2).


art gomes fig5

Figure 5. Study area with colony locations (central point of the colony area, obtained by minimum convex polygon), known food stations, food type and shelters. Yellow squared marks represent colony location, red circles represent specific shelter, orange circles represent open garbage containers and green circles represent cat lovers’ hand-outs (clumped food distribution). Again, each map refers to study areas in each year of sampling (Contumil map from 2012, Campo Alegre from 2013, and Porto City Park from 2014) (A) Contumil; (B) Campo Alegre; (C) Porto City Park (Copyright Google Earth 2017).


These groups/colonies are clearly associated to the presence of food, either stations regularly filled by cat lovers’ hand-outs (clumped food distribution) or at dumps and refuse depots (dispersed food distribution), because no colony was found far from these food resources (Figure 5; Annex Table A2). However, considering the total number of cats sighted in each colony, its area and densities, we found no differences among different types of food distribution (clumped vs. dispersed; Table 6). Therefore, our study revealed that an important colony factor is the presence of food source, either wittingly or accidentally, which is according to previous studies that also reported that supplemental feeding by people is the main factor for maintain large cat populations in an urban environment (e.g., John, 1977; Izawa et al., 1982; Natoli, 1985; Liberg et al., 2000; Gunther and Terkel, 2002; Flockhart et al., 2016).

Available shelters is not considered a limiting factor, in previous studies, because usually are abundant, although mainly accidental, in urban environments (sensus Bradshaw et al., 1999). However, we found that the number of cats in a colony and the colony area are significantly higher for colonies with accidental shelters than for colonies with specific shelters (Table 6), and one previous study also referred to the availability of shelter as a factor influencing the distribution of cats (Calhoon and Haspel, 1989).

Finally, colony size can also be influenced by urban barriers and traffic kills, as well as by the type of habitat where they stablish. Thus, we further found a significant difference among habitat type, considering the cat colony densities (Table 6).


Table 6. Results of the statistical analyses for differences in the total number of cats sighted in each colony, in the colony area, and in the cats’ colony density, between different categories of food distribution, shelter availability and classes of habitat. For analyses of the differences between food distribution and shelter availability classes, values of the t statistics and p values, from the Students’ t-test, as well as the mean and standard deviations (SD) are presented. For analyses of the differences between habitat type classes, P values from Krustal-Wallis statistical test are presented. † Non-homegeneous variance; * Differences statistically significant (p ≤ 0.05).


Differences between food distribution

Differences between shelter availability

Differences between habitat type


Student’s t-test

Clumped food

Dispersed food

Student’s t-test

Accidental shelter

Specific shelter


t (P)

mean ± SD

t (P)

mean ± SD

Krustal-Wallis Test (P)

Total number of cat sightings

-0.92 (0.37)

9.60 ± 6.95

12.22 ± 7.56

*2.80 (0.01) †

11.91 ± 7.27

5.71 ± 4.19


Colony area

-0.74 (0.48) †

21.44 ± 25.75

36.67 ± 59.27

*2.75 (0.01) †

32.36 ± 42.81

6.72 ± 5.03


Cat’s density

0.53 (0.60)

1.62 ± 3.31

1.02 ± 0.79

0.38 (0.71)

1.55 ± 3.18

1.08 ± 0.62



Considering the total number of colonies in each total area of the study areas, it is clear the difference between the colonies densities of the Porto City Park (3.9 colonies km-2) and the ones at Contumil or Campo Alegre areas (6.8 colonies km-2 and 8.1 colonies km-2, respectively; Table 3). This difference might be explained, as before, by the high number of visitors of the City Park and especially by all the domestic dogs that follow them, preventing the normal spread and establishment of cat populations.


3.3. Urban free-roaming cat estimated densities, in Porto (Portugal), and the "cat problem"

With the surveys of this study, previous number of cats (n=772, 20.42 cats/km2 estimated density) and cat colonies (n=67, 1.8 colonies/km2 estimated number) within the city limits, obtained using local animal welfare NGO’s data (“Animais de Rua” and “Catus Association”), are therefore not surprising (Figure 6). However, such numbers may largely underestimate the real situation. If we were to extrapolate cat densities for the city of Porto, based on the cat densities estimated in this study, we would found far greater values (137.53 cats/km2). However, this estimation may be biased because cats are widely distributed across the city, with different densities, according to food availability, as we shown. Nevertheless, this might be a guideline for future studies, which are needed, especially in different areas or different cities. Considering this extrapolation as a guideline value, our density estimates are thus similar to the ones referred to South Africa, New Zealand and Britain (by Fitzgerald and Veitch, 1985; Jones and Downs, 2011; Sims et al., 2008, respectively; Table 1). Still, some studies on different urban environments found smaller (e.g., Liberg, 1980; Harper, 2004; Kilgour et al., 2017) or much higher (e.g., Izawa et al., 1982; Miramovich, 1991; Flockhart et al., 2016) density estimates, of cat populations (Table 1).


art gomes fig6

Figure 6. Porto city area registering presence of cats and cat colonies, obtained using local animal welfare NGO’s data: Animais de Rua data (2012, unpublished data) in green; Catus association data (2012, unpublished data) in pink (Copyright Google Earth 2017).


Filling feeding spots implies a daily and direct contact between people and free-roaming cats, such contact being potentially harmful to human populations if cats are vectors of diseases. Consequently, cat populations would also be jeopardized due to non-controlled actions against cats, by humans. The “cat problem” is growing and neutering programs are being implemented in the city, both by city municipality and by local animal welfare NGO’s (e.g., “Animais de Rua”; “Catus”), but these might be yet insufficient due to lack of studies revealing the true densities of cat populations. The effort is however higher in the city centre and in places closer to houses, than in the suburbs; the explanation is the higher contact between people and cats, specifically due to the higher number of people feeding free-roaming cats, and to the possibility of cats becoming  pets. This is clear in our results as an higher percentage of neutered cats was found in Porto City Park (41%), which might be due to the proximity to human houses; in fact, although most of the area is green, this study area is surrounded by a dense urban area.

Thus, the implementation of controlled neutering programs, as in other places (e.g., d’Avila, 2016; Kilgour et al., 2017; Tan et al., 2017), would allow to control the spread of diseases to humans and other species (birds, rodents, etc), the impact of feral cats in the predation and extinction of native species, and the probable contact between domestic and wild cats. Rather than just implement these programs in urbanized places near to people, the same arguments also advise the transposition of those measures to other places, such as the city suburbs, forests, and wild areas.

4. Conclusion

Free-roaming cats showed a wide distribution and density in the areas studied within the city of Porto. Actually, if we were to extrapolate densities among the studied areas for the total area of the Porto city, we would found high densities, comparable to other urban environments studied. We conclude that food seems an important factor determining the presence of cat colonies, in urban environments. Other factors such as the existence of open and green areas may also be important to colony size, because is a wide area for cats to live, especially if food is regularly provided. Furthermore, we conclude that the number of cat sighted seems to vary across season, which could be further related to factors such as temperature and life cycle stage. Given the distribution and density of urban cat population there is no doubt that further efforts are necessary to understand cat spatial ecology and densities. In some cases, it might be necessary to consider reducing free-roaming cat densities, eventually implementing controlled neutering programs, as well as funding research on contraception, education for the health of cat and human individuals, and analyses of the disease spreading risk, in both human and animals.


We are grateful to Mrs. Rosa Vieira (Catus Association) for all her support, namely cat data from the city of Porto. Our appreciation also to the “Animais de Rua” association for all information provided. A special thank to Bruna Maciel, Jorge Ferreira, Tiago Azevedo, João Faria and Diana Fernandes for their help during fieldwork.


d’Avila AMED, 2016. Caracterização dos Centros de Atendimento Médico-Veterinários no Concelho de Lisboa que participam no controlo da população de gatos errantes e assilvestrados. Master Thesis, Repositório Científico Lusófuna (ReCiL), 81 pp.

Barratt DG, 1997. Home range size, habitat utilisation and movement patterns of suburban and farm cats Felis catus. Ecography 20, 271-280.

Beutel T, Reineking B, Tiesmeyer A, Nowak C, Heurich M, 2017. Spatial patterns of co-occurrence of the European wildcat Felis silvestris silvestris and domestic cats Felis silvestris catus in the Bavarian Forest National Park. Wildlife Biol wlb.00284.

Bradshaw JWS, Horsfield GF, Allen JA, Robinson IH, 1999. Feral cats: their role in the population dynamics of Felis catus. Appl Anim Behav Sci 65: 273-283.

Brickner I, 2003. The impact of domestic cat (Felis catus) on wildlife welfare and conservation: a literature review. With a situation summary from israel. Department of Zoology, Tel Aviv University 21, 2964 - 2969.

Brown FL, 2017. Dogs and cats: Loving pets in urban homes. In: The city is more than human: na animal history of Seattle. University of Washington Press, Seattle (352 pp).

Cahloon RE, Haspel C, 1989. Urban cat populations compared by season, subhabitat and supplemental feeding. Journal of Animal Ecology 58: 321-328.

Clancy, EA, Rowan, NA, 2003. Companion animal demographics in the United States: A historical perspective. In: DJ Salem and AN Rowan (Eds.), The state of the animals II: 2003 (pp. 9-26). Washington, DC: Humane Society Press.

Duarte A, Castro I, Pereira da Fonseca IM, Almeida V, Madeira de Carvalho LM, Meireles J, Fazendeiro MI, Tavares L, Vaz Y, 2010. Survey of infectious and parasitic diseases in stray cats at the Lisbon Metropolitan Area, Portugal. Journal of Feline Medicine and Surgery 12: 441-446.

Finkler H, Hatna E, Terkel J, 2011. The influence of neighbourhood socio-demographic factors on densities of free-roaming cat populations in an urban ecosystem in Israel. Wildlife Res 38: 235–243.

Fitzgerald BM, Veitch CR, 1985. The cats of Herekopare Island, New Zealand; their history, ecology and effects on birdlife. N Z J Zool 12: 319–330.

Flockhart DTT, Norris DR, Coe JB, 2016. Predicting free-roaming cat population densities in urban areas. Anim Conserv 19: 472–483.

Gil-Sánchez JM, Jaramillo J, Barea-Azcón JM, 2015. Strong spatial segregation between wildcats and domestic cats may explain low hybridization rates on the Iberian Peninsula. Zoology 118: 377-385.

Gunther I, Terkel J, 2002. Regulation of free-roaming cat (Felis silvestis catus) population: a survey of the litereature and its application to Israel. Anim Welfare 11: 171-188.

Harper GA, 2004. Feral cats on Stewart Island/Rakiura: population regulation, home range size and habitat use. DOC Science Internal Series 174. Department of Conservation, Wellington, 35 pp.

IBM Corp, Released 2016. IBM SPSS Statistics for Windows, Version 24.0. Armonk, NY: IBM Corp.

Izawa M, Doi T, Ono Y, 1982. Grouping patterns of feral cats (Felis catus) living on a small island in Japan. Jap J Ecol 32: 373-382.

John L, 1977. The daytime behaviour of domestic cats in a free-roaming population. Anim Behav 25: 990-998.

Jones AL, Downs CT, 2011. Managing feral cats on a university's campuses: how many are there and is sterilization having an effect? J Appl Anim Welf Sci 14: 304-320.

Kilgour RJ, Magle SB, Slater M, Christian A, Weiss E, DiTullio M, 2017. Estimating free-roaming cat populations and the effects of one year Trap-Neuter-Return management effort in a highly urban area. Urban Ecosyst 20: 207-216.

Krustal WH, Wallis WA, 1952. Use of ranks in one-criterion variance analysis. J Am Stat Assoc 47: 583-621.

Laundré J, 1977. The daytime behaviour of domestic cats in a free-roanming population. Anim Behav 25: 990-998.

Levene H, 1961. Robust tests for equality of variances. In: Olkin, I., Ghurye, S.G.,Hoeffding, W., Madow, W.G., Mann, H.B. (Eds.), Contributions to Probabilityand Statistics: Essays in Honor of Harold Hotelling. Stanford University Press,California, pp. 279–292.

Liberg O, 1980. Spacing patterns in a population of rural free roaming domestic cats. Oikos 35: 336-349.

Liberg O, Sandell M, Pontier D, Natoli E, 2000. Density, spatial organisation and reproductive tactics in the domestic cat and other felids. In: Turner DC, Bateson P ed. The Domestic Cat: The biology of its behaviour, 2nd ed. Cambrige, Cambrige University Press.

Miramovich V, 1991. Ecology and social behaviour of free-roaming urban cats (Felis catus). MSc thesis, The Hebrew University of Jerusalem, Israel (In Hebrew).

Mirmovitch, V, 1995. Spatial organization of urban cats (Felis catus) in Jerusalem. Wildl Res 22: 299-310

Monterroso P, Brito JC, Ferreras P, Alves PC, 2009. Spatial ecology of the European wildcat in a Mediterranean ecosystem: dealing with small radio-tracking datasets in species conservation. J Zool 279: 27-35.

Natoli E, 1985. Spacing pattern in a colony of urban stray cats (Felis catus L.) In the historic centre of rome. Appl Anim Behav Sci 14:289-304.

Normand CM, 2014. Feral cat virus infection prevalence, survival, population density, and multi-scale habitat use in an exurban landscape. Arkansas Tech University, ProQuest Dissertations Publishing. 1571366, pp.114.

Natoli E, De Vito E, 1988. The mating system of feral cats living in group.In Truner D.C., Bateson P. (eds), 1988. The domestic cat. The biology of its behaviour. Cmbridge University Press, Cambridge, 99-108.

Nowell K, Jackson P (Eds), 1996. Status survey and conservation action plan. Wild cats. IUCN. Gland, Switzerland. 382 pp.

Robertson SA, 2008. A review of feral cat control. J Feline Med Surg 10: 366-375.

Seo A, Tanida H, 2017. Three-year route census study on welfare status of free-roaming cats in old-town Onomichi, Japan. J Appl Anim Welf Sci, 29:1-8.

Shapiro SS, Wilk MB, 1965. An analysis of variance test for normality (complete samples). Biometrika 52: 591-611.

Sims V, Evans KL, Newson SE, Tratalos JA, Gaston KJ, 2008, Avian assemblage structure and domestic cat densities in urban environments. Divers Distrib 14: 387–399.

Slater MR, 2001. The role of veterinary epidemiology in the study of free-roaming dogs and cats. Prev Vet Med 48: 273-286.

Sparkes AH, Bessant C, Cope K, Ellis SLH, Finka L, Halls V, Hiestand K, Horsford K, Laurence C, MacFarlaine I, Neville PF, Stavisky J, Yeates J, 2013. ISFM Guidelines on Population Management and Welfare of Unowned Domestic Cats (Felis catus). J Feline Med Surg 15: 811-817.

Tan K, Rand J, Morton J, 2017. Trap-Neuter-Return Activities in Urban Stray Cat Colonies in Australia. Animals 7: 46.

Woinarski JCZ, Murphy BP, Legge SM, Garnett ST, Lawes MJ, Comer S, Dickman CR, Doherty TS, Edwards G, Nankivell A, Paton D, Palmer R, Woolley LA, 2017. How many birds are killed by cats in Australia? Biol Conserv 214: 76-87.

Zar JH, 1999. Biostatistical analysis. Pearson Education India, pp. 663.