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

Índice do artigo

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.