Background

Clinical scenario

Many cats are kept in captive environments such as rehoming centres (also referred to as rescues/shelters), often for indefinite periods of time. In trying to accommodate these individuals as optimally as possible, it is important that they are provided with suitable housing conditions, which aim to minimise exposure to stress in order to maximise welfare.

Whilst recommendations for the housing of the domestic cat in laboratories, rehoming centres and other facilities have been put forward [1–3], the strength of evidence in support of these recommendations is rarely considered critically, and can be contradictory. This study was conceived as part of the development of evidence-based guidelines on the housing of cats in such contained environments, specifically cat rehoming centres. The aim was to assess the evidence on whether housing cats singly as compared to groups of two or more in these types of environments results in changes to physiological and/or behavioural measures of stress, and therefore which system should be recommended as preferable in order to minimise stress.

Introduction

A recent survey of cat rehoming organisations within the UK estimated their total intake of cats over a 12 month period to be 156,826, and 70% of these organisations were usually or always operating at full capacity [4]. Unfortunately, the provision of optimal housing for such large quantities of cats within these environments is inherently difficult, and under such conditions there is the potential for individuals to develop signs of physical and psychological ill health.

As a species, Felis catus is thought to have originated from primarily solitary dwelling felids [5–7], and whilst populations of free living F. catus may reside in groups, they may also live independently [8–11]. The feline social system is therefore one of variability and flexibility. In cat colonies, social structuring, relationships and potential conflicts may be the result of complex interactions between age, gender, sex ratio, relatedness and individuality [12]. It is thought that the occurrence of group living and the subsequent population densities of free ranging cats are ultimately influenced by the abundance of food resources rather than an inherent need for protection or regular social contact/interaction per se[13–17]. In contrast to free ranging populations, group living in domestic companion cats may often take the form of temporary or transitory housing during a stay in a rehoming centre, or when living in a domestic home environment. In both contexts, individuals may have limited choice or control over the nature of their ‘group living’, especially when their environment prevents them from making the choice to live independently (for example, multiple cats kept in a single enclosure at a rehoming facility, or multiple cats kept strictly indoors in the home).

It is likely that most rehoming centres will contain diverse populations of cats of varied ages and temperaments. Some cats may be related or familiar with each other (which may facilitate more amicable relationships in certain instances [18]), but the majority are potentially unrelated and also unfamiliar. For many individuals, being forced to reside in close proximity to other cats under these types of conditions may result in stress, conflict and potentially compromised health and welfare [19, 20]. Organisations caring for such animals often operate under conditions of limited resources of space, staffing, time and finances. Currently, there is conflict in which housing practices are recommended to maximise use of resources but simultaneously preserve a basic standard of welfare for the cats.

The aim of this study was therefore to review the evidence on the impact of single housing compared to multi-cat housing on stress in cats, as measured by physiological or behavioural effects.

Focussed clinical question

In [cats kept in confined environments] does [single housing compared to multi-cat housing] result in [changes in physiological and/or behavioural measures of stress]?

Methods

Search strategy

The search strategy included the use of three separate electronic databases; CAB Abstracts (1910 – present, via the Ovid interface), Zoological Records (1998 – 2007) and Medline (In-process & other non-indexed citations, 1946- present, via the Ovid interface). The search was conducted in October 2012.

After accounting for specific syntax associated with each database, each search had similar components (search terms are listed in Additional file 1) and all were searched as both keywords and subject heading terms, joined using Boolean operators. All references obtained were imported into Endnote, combined into a master database, and all duplicates (identified based on title, date published and authors) were removed.

Inclusion criteria

Studies were not excluded on any grounds of quality, only on relevance to the study aim. For inclusion, papers had to include:

  • Domestic cats kept in an enclosed area from which they were unable to exit (omitting the domestic home), for example, rehoming centres, boarding catteries and laboratories.

  • Comparison of both single and multi-cat (i.e. two or more cats) housing conditions within a single study, with outcome measures that were either behavioural, physiological or both, and were classed as indicators of stress. Our working definition of stress was:

  • “an inferred internal state which denotes a real or perceived perturbation to an organism’s physiological homeostasis or psychological well-being”, as used by Ward et al.[21], and similar to that used by McEwen [22], as we felt it was appropriate to this context. However many other definitions exist [23–25], and in the present study papers were not included or excluded on the basis of this definition.

  • Original observed or experimental data.

Studies were also required to be peer-reviewed, with the full text available in English.

Screening process

Two stages of eligibility screening were carried out. The first stage was completed independently by two of the authors (LF and JS), and any references that clearly did not fit the eligibility criteria were excluded. After this, in stage two, the remaining references were screened again by all three authors. For this stage, full text was retrieved for any papers where the information contained within the abstract was deemed insufficient to make a decision upon eligibility. Where there was initial disagreement over eligibility, the papers were read and discussed until consensus was reached among the reviewers [26].

Critical appraisal

All remaining papers were independently appraised by all three authors, using critical appraisal tools developed by the Department for Emergency Medicine at Manchester Royal Infirmary (http://www.bestbets.org/) and used extensively in the literature [27–29]. These appraisals were then collated by the lead author (LF) into a summary table. All three authors re-checked this summary of evidence for consistency of interpretation.

Results

959 papers were initially identified. Following screening as in Figure 1, six papers fulfilled all of the inclusion criteria. The results of the appraisal can be seen in Table 1.

Figure 1
figure 1

Results of searches and screening processes used to identify relevant papers.

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Table 1 Summary of appraisal of the six papers meeting the inclusion criteria of assessing single versus multi-cat environments on physiological and behavioural measures of stress in confined domestic cats
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Summary of the evidence

The findings of the appraisals are summarised in Table 1. There was a lack of agreement overall as to whether single or multi-cat housing was associated with higher levels of stress. The majority of the studies (four out of six) showed no difference in stress levels between single and multi-cat housing [30–33]. However, one of these studies only compared single cats with those housed with one or two other familiar conspecifics and not with larger multi cat groups [33]. One study suggested that stress levels were higher in cats housed singly in barren environments as compared to singly and group-housed cats provided with varying levels of enrichment [34].The final study included showed no difference in stress levels between single and group housing in socialised cats, but found that cats previously unsocialised to conspecifics showed fewer signs of stress when single housed [35].

There were significant limitations to all of the identified studies. These included differential treatment of the groups within the study. For example cats in the single housing conditions either had inconsistent handing [34], were exposed to their housing condition for a much shorter period of time [30, 34], were deliberately given barren, non-enriched housing [34], or experienced a non-stable environment over the course of the study period [30], when compared with group-housed cats. Sample size calculation was performed in only one study [33], and some of the studies involved very small numbers of cats, which in one case amounted to six cats each exposed to three different interventions [30]. In none of the studies was the assessor of the outcome blinded to the intervention.

Additionally, the diverse populations under study and variations in methodology complicate comparison. Group sizes in the multi-cat environment were variable, from 2 to eight [33, 34]. The effect of population density was not assessed, as this information was not available for all studies; however this may clearly be a potential confounding factor. The previous social experience of the cats varied, with some cats living in established social groups [30], some having been assessed as non-socialised to other cats by shelter staff [35] and others with no known or stated history of socialisation. A cats prior social experience was identified by one study as a factor in its stress levels in group housing, and the same study showed that the introduction of an “unsocialised” cat to a stable group caused an increase in the stress levels of all of the cats under observation [35].

There were also substantial differences in duration of the data collection periods across all studies, ranging from a single instance [31] to fifteen days [34], which could have affected the extent to which the cats had the opportunity to habituate to their respective study environments, or resulted in some cats exhibiting acute and others chronic signs of stress.

Thus these studies may not be truly comparing single and multi-cat environments, so much as suggesting the presence of several other factors that may be equally important in determining stress levels. These include: how consistent handling and husbandry routines are [34], as well as the amount of environmental manipulation, such as changes in housing location and type, that the individual is exposed to [30]. In one study, stress levels in their stable, long-term and group housed control population were lower than in any other experimental condition (i.e. individual, pair and group) [32], suggesting that group stability (and presumably familiarity) were also important mitigators of stress levels.

Discussion

The majority of the studies did not find significant differences between single and group housed cats in regards to their stress levels. Whilst this may suggest that group size does not in fact impact upon the stress of confined cats in rehoming and similar environments, it is arguable whether this can be assumed unequivocally. This is due to the lack of overall agreement between studies, as indicated by the conflicting evidence found in two of four such studies [34, 35], as well as the various confounding elements of study designs found throughout the reviewed papers. These included factors such as differential provision of enrichment or human contact between groups, differences in the cats’ socialisation and housing experience prior to the studies, and potential differences in sizes of groups in the group housing conditions. These results also suggest that a stable environment (both social and physical) may be an important factor in managing stress, and that some cats (such as those previously successfully socialised to conspecifics) may cope better in a multi-cat environment than those with little, or aversive previous experience of conspecifics. Therefore, when providing housing for cats, it is important to consider their likely prior social experience. When housing cats communally, keeping cats in large group sizes may also be more stressful than keeping them in smaller groups [33] although there is only a small amount of relevant data to support this, and it is possible that population density may also be a confounding factor.

Measuring stress in non-human animals is inherently difficult, and it is unlikely that any one measure can accurately capture how stressed an animal is [36, 37]. However, the more separate (suitable) measures considered within a single study, the greater the potential for robustness. As there is no consistent definition used within the scientific literature for this term nor specific aetiology or prognosis for stress [23], it is important that where studies attempt to measure stress, a clear definition of this concept is given. This will facilitate in the ease of assessing the suitability of study methodology, as well as determining whether the main aims and objectives of a study have been achieved. All of the studies aimed to measure stress, but only one of them attempted to provide a clear definition of it [33]. Of the six papers that were critically appraised, only one study used both behavioural and physiological measures to assess stress [30] and only one used more than one set of behavioural outcome measures [33]. Only one study assessed whether the study cats were previously socialised with conspecifics [35], which again makes direct comparison between group housing conditions across the different studies difficult, because this appeared to influence the stress levels experienced by cats when housed in groups.

Comparison between the studies is further complicated by the variety of methods used to assess stress, all of which have their limitations (further details of these methods used are provided in Table 2). The duration of time over which individuals were exposed to specific housing conditions also varied considerably (both within and between studies). This affects the comparison of stress levels between cats under different housing conditions due to potential confounds of comparing cats which are acutely stressed (e.g. from being taken from stable enriched group housing to barren single housing) to cats which are chronically stressed, or to those that have actually begun to habituate to their environment. The physiological and behavioural signs of acute as compared to chronic stress may vary [38, 39] making it difficult to isolate the specific effects of the environment, from the effects of period of exposure, upon the stress levels experienced by cats. However, by implication, the acutely raised stress levels in some of the single housed cats may have been as attributable to the acute change in environment rather than to the actual housing condition itself.

Table 2 Further detail of behavioural outcome measures used in the studies reviewed
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Whether individuals have previous experience of the housing environment may also be another important mitigator of stress. Previous research indicates that cats that have been housed in rehoming centre environments previously may cope better under these conditions than those that have not [47]. It is unknown if any of the study populations within the appraised papers had been housed under such conditions before, but this may have been an important factor to consider.

Conclusion

On the basis of the evidence available, the below recommendations for practice have been provided. It is however important to consider the complex nature of stress, and the methodological limitations of the above studies, in relation to their ability to help us isolate and assess the effects of multiple and single housing alone on stress in cats (Table 1). There are also numerous other factors which have not been considered here, particularly disease control, which is also of great importance in rehoming centres [48–50]. These results should draw attention to the importance of other potential mitigating factors which may influence how stressful single or multiple housing can be for individuals, and suggest ways these may be utilised practically to improve the welfare of confined cats in these types of environments.

Recommendations for practice

  • Especially where the previous social history of cats towards conspecifics is unknown, individuals should be housed singly, but with the appropriate environmental enrichment in place (e.g. places to hide and perch, toys, consistent positive human handling where appropriate).

  • Cats should be exposed to as few environmental changes/manipulations as possible during their stay and husbandry routines should be as consistent as possible.

  • If cats are to be housed in groups, they should ideally be housed together with other cats considered socialised to conspecifics.

  • If cats are to be housed in groups, or with those that are initially unfamiliar, wherever possible, groups should have a stable composition (i.e. group members are not constantly changed).