How best to introduce and ensure welfare for cohabiting companion cats

The research undertaken in this project will use Denmark as case study country and we will employ a two-stage data collection design, where stage 1 will be an exploratory inquiry aimed at revealing how cat owners perceive and manage multiple cats in the household, including how they have introduced new cats. During stage 2, a questionnaire study will aim to provide representative data on factors promoting or preventing a high level of welfare for cohabiting cats.

Stage 1

Current knowledge about how owners in multi-cat households perceive and manage their cats is limited and it is therefore important to understand and map their actual perceptions and practices. Stage 1 focusses on this mapping. Qualitative methodology is appropriate here (Jamshed, 2014), and qualitative interviews lasting approximately 1 hour will be carried out with 20 participants (i.e., human individuals) from multi-cat households. Participants will be recruited by Norstat, a survey company with a panel of Danish citizens (read more about this panel in the section on Stage 2 below). A sample size of 20 participants is considered sufficient to achieve an appropriate level of variation in accounts. Purposeful, heterogenous sampling (Robinson, 2014) will be used to ensure variation regarding factors such as whether the cats are indoor-only or have outdoor access, the population density of the area of residence, and whether the cats were acquired at the same or at different points in time.

A semi-structured qualitative interview guide (David and Sutton, 2011) will be used to obtain information on the awareness and handling of intra-household cat conflicts, considerations about introduction techniques and actual techniques used (if any), as well as whether techniques were adjusted over time in light of prevailing cat conflicts and/or behavioural problems. Interviews will be audio-taped, transcribed, and coded in the qualitative software program NVIVO by Lumivero. Thematic analysis (Byrne, 2022) will be used to organise and present the main findings. The outcome of this phase will provide an in-depth overview of the cat owners’ awareness of intra-household cat conflicts, perception of the cats’ personalities, knowledge about the cats’ backgrounds, and a typology of management practices. The practices explored will include the introduction techniques currently used, whether the techniques are adjusted over time, and the perceived success of these techniques. This stage will provide key input to stage 2, when measures that reflect the mapped practices will be developed and applied.

Stage 2

This consists of a questionnaire study comprising a representative sample of the Danish cat-owning population. Data will be collected by the survey company Norstat using their panel of Danish citizens, who are invited at random to participate in the panel (the current panel size is 80,000 individuals). Data derived from this kind of panel employ quota sampling, which is of lower quality (but much less expensive) than the probability-based sampling method, yet superior to self-selected sampling and convenience sampling.

A subset of the panel members will be invited to participate and respond to the questionnaire. Approximately 5,000 panel members (randomly recruited within a predefined quota of gender, age, and region of Denmark (NUTS2) so that the sample is representative of the population census) will complete the questionnaire. This pool of respondents will include those who own a cat and those who do not. With an expected 20% prevalence of cat ownership in Denmark, the effective cat-owner sample size available for analysis is expected to be ~1,000 (the remaining ~4,000 non-cat-owning respondents will be screened out).

Based on results from a previous study (Lund and Sandøe, 2021), we expect between 40% and 50% of these respondents to own more than one cat. Questionnaire measures of the cats’ perceived personalities and backgrounds, as well as the different management practices and methods of introduction identified in the qualitative inquiry during stage 1 will be included, alongside measures of cat relationships in multi-cat households, both at the time of the survey and going back in time to gauge the short- and long-term effects of introductions. Furthermore, there will be questions relating to cat management, including the presence, number, and location of basic resources and factors relevant to cat welfare (food and water stations, litterboxes, scratching and climbing opportunities, resting places, indoor/outdoor access, number of cats in the home, etc.).

There will also be questions about the occurrence and type of behavioural problems (conspecific aggression, house soiling, urine marking, destructive behaviour, interspecific aggression, attention-seeking, fearfulness, etc.) and the occurrence of agonistic and affiliative behaviours in multi-cat households (blocking, staring, moving opponent around, nose touching, friendly allogrooming, touching while resting/sleeping, and intraspecific play). The welfare assessment will therefore combine resource-based and animal-based measures.

Finally, the questionnaire will include characteristics of all cats in the household, including breed, gender, neuter status, age, body condition score, neutering status, when they entered the household, and personality type, as well as socio-demographic measures of the owners, such as age, gender, number of people in the household, level of education, income, and area of residence (urban/rural).

The results of the survey will first be presented in a descriptive analysis that will include: a) frequency distribution of the above-mentioned cat characteristics; b) frequency distribution of introduction practices in multi-cat homes; c) distribution and comparison of welfare measures for cats living in single- and multi-cat homes. This will be followed by a multivariate analysis that will include an analysis of the correlation between the prevalence of behavioural problems and physical characteristics of the cats’ environment (e.g., number of cats in the household, indoor/outdoor access, measures of resource availability, health status) for all cats included in the study. The outcome of this will provide an overall indication of factors influencing cat welfare.

In addition, the multivariate analysis will include an analysis of the correlation between the prevalence of short- and long-term agonistic/affiliative conspecific behaviours in multi-cat households and other indicators of cat welfare, and between measures of the method of introduction (identified in the qualitative inquiry during stage 1), measures of resource availability, and cat-related factors, including sex composition. The outcome of this will indicate how detailed aspects of the introduction method, other management factors, and cat-related factors influence both the immediate success of the introduction and the long-term relationships between cats in the household. This will allow us to achieve research objectives (i) and (ii), i.e., mapping factors that affect the level of success in introducing more than one cat to the domestic home, and uncovering factors influencing the quality of long-term relationships between cohabiting cats in the domestic home and behavioural and other welfare problems related to this. With an expected sample size of 500 households with >1 cat, it will be possible to detect statistically significant main effects at the small effect size level when ten predictors are inserted in a multiple linear regression (Cohens f2=0.04, calculated in G*Power, F-test Family (α=0.05, β=0.80)).  

All measures, with the exception of those only relevant to multi-cat households, will also be applied to a comparable sample of single-cat households. The latter sample will serve as a control group that will allow for a genuine comparison of the welfare of cats in multi-cat and single-cat households. This will allow us to reach research objective (iii): to compare the overall welfare of cats in single- and multi-cat households, with an emphasis on the interplay between the quality of the relationship between cats in multi-cat homes and other factors that influence welfare in both single- and multi-cat homes. Effect size calculations will be carried out for the main explanatory variables in all analyses (Cohen, 1992).

Based on findings from the study and the existing literature, a popular guidance document for cat owners and their advisors will be developed in a format that can be presented on a webpage. The guidance document will describe the welfare challenges and potential benefits of having more than one cat in a home, and present best practices for introducing new cats to a home where there is already a resident cat. This will allow us to reach objective (iv) of the project: to make our results available in an online guide.

Members of the group are already involved in a Danish website about cat welfare/cat husbandry (with plans to translate this into English), where the results of this project will be made available to provide information on issues not previously covered, such as how to deal with multiple cats in the household. Also, there are plans to translate the guide into German and Swedish.
It will then be possible to disseminate the results via social media. In parallel, the principal investigator will use his network, including an efficient communication office at the University of Copenhagen, to ensure that the results of the project are disseminated as widely as possible via national and international media. Thereby we will reach objective (v) of the project: a wider dissemination of the results.

The project team consists of four researchers with competences that complement each other well in relation to the requirements of the study:

Associate Professor Irena Czycholl combines an interdisciplinary background as a veterinarian, animal scientist, and animal behaviour and welfare specialist, who will give advice on how to measure cat behaviour. Her research focusses on causal structures leading to certain behaviours across different species and she has hands-on experience of behavioural medicine consultations for dogs, horses, and cats. With an impressive 54 peer-reviewed articles, her recent works have included very successful overview studies based on online questionnaires.

Senior Researcher Ayoe Hoff is an established researcher in bioeconomic modelling, but also a certified cat behaviour consultant (Exam ABC) specialising in cat behaviour. She passed iCatCare’s course ‘Advanced Feline Behaviour for Cat Professionals’ with Distinction. She has first-hand knowledge of the welfare challenges facing companion cats, and more than 10 years’ experience of consulting with cat owners regarding behavioural problems in the home. She has extensive experience in teaching pet professionals (dog trainers, shelter staff, veterinary staff) about cat behaviour, and has written a large number of popular articles about cat behaviour in pet magazines and blogs.

Associate Professor Thomas B. Lund is a sociologist specialising in rural sociology, including research on humans’ use of—and attitudes towards—animals. He has published about human attachment to dogs and cats, as well as the effect of owner-related factors on pet welfare. He specialises in methodology and his research draws on qualitative and quantitative social science data.

Professor Peter Sandøe has a background in philosophy. Since 1990, a major part of his research has been within the field of bioethics, with a particular emphasis on ethical issues related to animals, biotechnology, and food production, and he has worked in a natural science dominated environment since 1997. He has a strong track record of successfully managing interdisciplinary research projects and is committed to interdisciplinary work combining perspectives from natural science, social science, and philosophy, with a well-established reputation for undertaking social science research of the kind envisaged here. Over the past 10 years, he has developed a research interest in companion animals, where the domestic cat has become one of his key interests. He currently has 12 international publications in the field of cat behaviour, cat welfare and cat management. In addition, he has been the driving force behind establishing and ensuring funding for a highly visible and productive Centre for Companion Animal Welfare. He is also strongly committed to ensuring that his research findings are shared with a wider public, both nationally and internationally, and that they are brought into the public debate.

The domestic cat is a popular companion animal in the Western world. According to one estimate, there were more cats (127m) than dogs (104m) kept as companion animals in Europe in 2022 (FEDIAF, 2022). Based on this, it can be estimated that 26% of European families share their home with a cat, with an average of 2.5 cats per household. In Denmark, 14% of families share their homes with cats, amounting to more than 700,000 cats (Lund and Sandøe, 2021). Moreover, between 40% and 50% of cat-owning families in Denmark have more than one cat (unpublished results from a representative survey of Danish families conducted in 2021, cf. Lund and Sandøe, 2021).

However, sharing a home with one or more conspecifics is not always easy for the domestic cat, which is descended from the solitary African wildcat (Felis silvestris lybica), but they are likely to have acquired some social conspecific tolerance during domestication (Bradshaw, 2018, 2016; Finka, 2022). Behaviours indicative of conflict and the resulting signs of reduced welfare (house soiling, marking, conspecific aggression, avoidance behaviour) are often observed (Barcelos et al., 2018; Bradshaw, 2016; Elzerman et al., 2020; Finka and Foreman-Worsley, 2022; Heidenberger, 1997; Pryor et al., 2001; Ramos, 2019). However, cohabiting cats may also display behaviours that indicate positive social relations, thus contributing to positive welfare (Barry and Crowell-Davis, 1999; Elzerman et al., 2020; Finka and Foreman-Worsley, 2022; Lawrence et al., 2019).

A complex relationship therefore seems to exist between social and environmental factors and the welfare of cohabiting cats; a relationship that is far from fully understood (Finka, 2022; Finka and Foreman-Worsley, 2022). One key factor that is likely to contribute to affiliative relationships between cohabiting cats is the method by which new cats are introduced to resident cats in the home (Elzerman et al., 2020; Finka, 2022; Levine et al., 2005). Gradual introduction (including initial separation of cats, scent swapping, short positive meetings, positive reinforcement training, use of pheromones, etc.) is recommended by cat professionals ranging from veterinarians to shelter staff. This approach can allegedly prevent long-term conflicts (Bradshaw and Ellis, 2016; De Souza Dantas, 2020; Ramos, 2019).

However, little research has been carried out on how cat owners introduce new cats to the household in practice. Furthermore, the association between the method of introduction and subsequent quality of the relationship between cohabiting cats is understudied (Bradshaw, 2018; Elzerman et al., 2020; Finka, 2022; Levine et al., 2005) and we do not know how important the method of introduction is relative to other factors that may influence the quality of long-term relationships between cohabiting cats, or the possible long-term welfare consequences of agonistic relationships for cohabiting cats. These factors can include outdoor access, resource abundance, owner experience and age, and the cat’s personality and socialisation history (Elzerman et al., 2020; Finka et al., 2014; Finka and Foreman-Worsley, 2022).

Several studies have investigated factors influencing cat welfare in both the domestic and the shelter environment. Finka (2022), Finka et al. (2014), and Finka and Foreman-Worsley (2022) have provided comprehensive reviews of both, with a special focus on studies that include cohabiting cats. Finka et al. (2014) cover studies of cats in confined environments (typically in shelters and excluding domestic settings), including investigations of how single- vs. multi-cat housing influences the stress levels of confined cats. Finka and Foreman-Worsley (2022) cover studies of cats in the domestic home and focus on studies that indicate the degree to which the number of cohabiting cats affects their welfare in this setting.

Furthermore, Finka and Foreman-Worsley (2022) discuss other factors that have been found to influence the welfare of cats living in a domestic setting, including outdoor access, the number of cat-related resources relative to the number of cats, indoor space per cat in the home, demographics of the owner and household, and cat demographics and personality. Finally, Finka (2022) looks more broadly at studies of factors that influence the social behaviour of cats towards other cats and humans, with a focus on cats that are confined, either in shelters or in the domestic home. Supplementing these reviews, Foreman-Worsley and Farnworth (2019) provide a comprehensive review of studies investigating factors, including conspecific interactions, that influence the welfare of cats confined indoors, including studies of cats in laboratories, shelters and domestic settings.

The mentioned reviews show that the relationship between, and welfare of, cohabiting cats is influenced by a complex interplay between a multitude of factors. These range from the degree of outdoor access, the number of available resources, the size and location of the home (urban/rural), the owners’ experience with cats, owner demographics, cat characteristics such as age, sex, neuter status, time in home, health, weight, and personality, and how successfully new cats are introduced to the home. The relationships and welfare of cohabiting cats are thus not affected by ‘living with other cats’ and ‘group size’ alone. This is emphasised by the literature, in which cohabitation and group size can influence cat welfare both positively and negatively. For example, Heidenberger (1997) shows a positive correlation between anxiety and the number of cats in 550 domestic homes in Germany. In contrast, Adamelli et al. (2005) found that living with conspecifics improved the quality of life (measured in three dimensions: care and environmental aspects, cat behaviour, and physical condition of the cat) in a sample of 62 cats living in domestic homes in and around Padua, Italy.

To the best of our knowledge, there have only been two studies dealing with how the method of introduction affects relationships between cohabiting cats. Levine et al. (2005) investigated the occurrence of fighting (defined as scratching and biting) after introducing a new cat in a domestic setting. The study used a sample of 252 households that had adopted a cat from the Society for the Prevention of Cruelty to Animals (SPCA) of Tompkins County, USA, during the period November 2001 to June 2002. Of these 252 homes, 128 already had resident cats. The method of introduction was one of several factors investigated that could influence the occurrence of fighting between the new and resident cat(s). The introduction methods included ranged from ‘Didn’t change anything, simply put the new cat in the house’ to ‘Separated the new cat from the resident cat(s) for a period of time, then gradually introduced them over time’ (Levine et al., 2005). The study did not find a statistically significant relationship between the method of introduction and long-term fighting (‘long-term’ meaning 2 months to 1 year after the adoption of the new cat). However, the authors argue that this could be because the questions relating to introduction methods may have been too broad, resulting in inaccurate measures.

The study by Levine et al. (2005) found a significant positive correlation between both short-term (meaning the first weeks following introduction) and long-term fighting, and whether the owners perceived the first meeting as ‘unfriendly/aggressive’. It should be noted that Levine et al. (2005) only consider direct fighting as indicative of agonistic relationships. As such, less severe agonistic behaviours that may still be potentially stressful for conspecifics were not recorded (e.g., staring, blocking, or moving opponent away from resources (Heath, 2018)). Moreover, Levine et al. (2005) did not consider the reason why certain methods of introduction were chosen and successful/unsuccessful.

Elzerman et al. (2020) investigated the relationship between various household and cat-related variables and agonistic and affiliative behaviours between resident cats in 2,492 multi-cat households in the United States. Participants were recruited through social media and information material distributed at veterinary clinics and conferences. The study applied the Oakland Feline Social Interaction Scale (OFSIS) developed by DePorter et al. (2019). The questions related to both agonistic and affiliative behaviours. In contrast to Levine et al. (2005), agonistic behaviours included both passive (staring, stalking, twitching tail) and active (chasing, fleeing, hissing, wailing/screaming) displays, while affiliative displays included allogrooming, sleeping in the same room, and touching noses.

Among the included explanatory variables (household variables, cat characteristics) were questions relating to the introduction—both the method (initial separation, scent swapping, utilisation of supplements and medication, utilisation of positive reinforcement training) and how well it went. For the latter, the respondents could answer ‘the introduction went well’ or ‘the introduction did not go well’. Questions relating to cat personality (‘active and curious’ vs. ‘shy and sedentary’) were also included. The study demonstrated a significantly higher rate of affiliative behaviours and a lower rate of agonistic behaviours when the introduction had ‘gone well’, compared to when the introduction ‘had not gone well’. However, the paper does not report how the success of the introduction correlates with the method of introduction.

Despite the scarcity of studies available on the interplay between introduction methods, other relevant factors, and the ensuing long-term relationships between cohabiting cats in the domestic home, it is generally recommended to make gradual introductions, including initial separation and scent swapping, followed by controlled desensitisation and counterconditioning of visual exposure (Bradshaw and Ellis, 2016; De Souza Dantas, 2020; Ramos, 2019). These recommendations are often justified by acclaimed sources who argue that gradual introductions will increase the probability of affiliative relationships between the cats in future (Bradshaw and Ellis, 2016; De Souza Dantas, 2020; Ramos, 2019).

One prominent cat researcher noted that ‘anecdotally, the process of introducing two previously unacquainted cats to one another can be smoothed by allowing each several experiences of odour collected from the other, well before any face-to-face encounter’ (Bradshaw, 2018). However, even though both Levine et al. (2005) and Elzerman et al. (2020) show that an initial friendly/non-aggressive meeting will result in a reduced probability of subsequent fighting, neither of these studies report on how much, or how little, is needed in the way of introduction to facilitate such initial friendly encounters.

A gap therefore exists in the current literature on factors that influence relationships in multi-cat groups in domestic homes, namely which aspects of currently recommended introduction methods have the highest probability of facilitating initial (and thus also long-term according to both Levine et al. (2005) and Elzerman (2020)) affiliative encounters. Furthermore, as Levine et al. (2005) did not find a significant relationship between introduction method and long-term fighting, it is also worth investigating other factors that can influence successful initial encounters and the interplay between these and the introduction methods.

Our study improves on the previous studies of Levine (2005) and Elzerman et al. (2020) in several ways. Firstly, we will use a mixed methods approach, where qualitative interviews will lay the foundation for the questionnaire used in the quantitative survey, in particular with regard to questions relating to introduction methods used by cat owners. Furthermore, we will use a representative sample of Danish cat owners in our quantitative survey, in contrast to the two other studies that both use convenience sampling with the related risk of the results being biased by an overrepresentation of very committed cat owners.

Based on our findings from the two previous studies, we can also ask very specific questions about introduction methods, as we can base our questions on the qualitative interviews and ask about the practical feasibility of these methods, thereby likely improving the practicality of the advice following from our study. In addition, we can test for the influence of different sex compositions on conflict in two-cat households (some experts argue that a neutered male + spayed female combination is more likely to be affiliative than male + male, or female + female combinations, but this has not yet been tested). Finally, by having a control group of single-cat households, we can determine to what extent the cat welfare problems seen in multi-cat households differ from those observed in single-cat households.

Professor James Serpell from the University of Pennsylvania School of Veterinary Medicine has agreed to collaborate with us. James has been the leading force behind developing the Feline Behavioral Assessment & Research Questionnaire (Fe-BARQ) (Duffy et al., 2017). He is currently in the process of developing a new cat behaviour questionnaire that should improve on, and potentially replace, the Fe-BARQ questionnaire. We can play a part in this effort and in return we can make use of items that map feline behaviour problems and other welfare issues from this new questionnaire. The PI of this project has had very fruitful collaborations with James Serpell in the past.

In addition, Daniel Mills, a Professor of Veterinary Behavioural Medicine based at the University of Lincoln has also agreed to collaborate with us. Daniel is a pioneer and international authority on cat behaviour. Among other things, his recent research has investigated the difference and overlap between play and aggression in cats, an insight that is also highly relevant for our study. He has also worked with Fe-BARQ and has been involved in developing other Feline Welfare Assessment tools (De Assis and Mills, 2021). As such his input—in terms of both the questionnaire outline and questions related to agonistic and affiliative cat behaviour—will be of great value to the project. Daniel also has contacts in international cat welfare organisations (e.g., he is a member of the Feline Wellbeing Panel of International Cat Care), and he can use these contacts to help boost the dissemination of findings from the project.

We expect at least two papers to be published in relevant international journals with peer review, one on factors affecting the successful introduction of additional cats and the quality of their long-term relationships and one comparing the welfare of cohabiting and single-housed cats. These papers will be available open access, which will allow a very wide and efficient dissemination to other scholars in the field and to a wide range of professionals. We also have a very ambitious plan to disseminate the results of the project to cat owners – by developing a guide aimed at cat owners and by circulating the results via different media in a number of countries, including Denmark, Sweden, Germany, Austria, UK, USA, and Canada.

In terms of time planning, all data collection, the primary analysis of data, submission of two papers to relevant international journals, and drafting of the guide will be complete by the end of 2024. However, the papers must be published before any dissemination activities are carried out. We expect the papers to be published in the first half of 2025 and will initiate dissemination activities immediately after that. All activities relating to the project will be concluded by the end of 2025.

Activities in 2024 will be based on the funding from SKK Agria and on co-funding from the Danish Centre for Companion Animal Welfare, whereas activities in 2025 and publication fees for the two papers will rely solely on co-funding from the Centre.

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