We illustrate the application of the ten-stage protocol by using wild free-roaming horses as an example. This species was chosen since the study is part of a broader project regarding the practical assessment of the welfare of wild free-roaming horses and implications for their management.

In light of the above observations, we describe a ten-stage protocol designed to guide wildlife biologists and others who wish to apply a systematic, scientifically based approach for assessing the welfare of individual free-roaming wild animals. We use the term ‘free-roaming’ to distinguish between wild species that roam freely in a wild habitat from those that are in captivity (e.g., in a zoo or sanctuary). We use the term ‘wild’ to mean animals that are of a wild species or those that are non-domesticated (feral). We have avoided the term ‘feral’ since this is associated with negative public perceptions, and whether an animal is truly wild or feral has no influence on the principles of how its welfare is assessed. This protocol, based on the ‘Five Domains Model’ [ 48 51 ], will ensure that such assessments are as scientifically objective, systematic, structured, transparent and comprehensive as possible. Applying the protocol will also enable researchers to present a clear understanding of the limitations imposed on their particular assessment by the circumstances in which data collection and interpretation necessarily occur when studying wild free-roaming animals. We also suggest methods that may be employed to capture robust data to support such welfare assessments.

Methods for assessing welfare have been well developed for a range of captive animals [ 15 23 ], including for wild species [ 24 28 ]. Although a need to develop methodologies for assessing the welfare of free-roaming wildlife has been highlighted [ 1 ], to date, such assessments have been largely restricted to impacts of non-lethal or lethal control of unwanted species, such as rodents, possums, rabbits, kangaroos, camels, badgers, and horses [ 29 47 ]. Whilst a recent study explored some aspects of welfare in the daily lives of free-roaming wild dogs [ 8 ], protocols for purposefully, systematically and scientifically assessing the welfare of free-roaming wild animals undertaking their normal daily activities, remain elusive. Therefore, little is known about what positive and negative welfare impacts they might be experiencing and why. Moreover, robust scientific methods for capturing reliable and informative data to enable assessment of free-roaming wild animal welfare have not been well described.

There is a growing awareness of how human activities, including wildlife population management and rehabilitation, land management and other conservation activities, may influence the welfare of free-roaming animals in the wild [ 1 8 ]. Conservation and wildlife management practices have traditionally focused on assessing animal populations, using metrics like abundance, density and diversity; demographic parameters like sex ratios and age classes; and fitness metrics like survivorship and reproductive success. While valuable for some conservation purposes, such metrics provide little information about the welfare of individual animals within populations. However, survival does not necessarily imply good welfare since animals can survive despite persistently experiencing chronically unpleasant states [ 9 12 ]. Furthermore, the welfare of individual animals can influence the success of some conservation activities. For example, poor welfare may reduce fitness and reproductive success, and thus alter population trajectories. In addition, the public are increasingly aware of, and concerned about wild animal welfare [ 3 13 ]. Therefore, having knowledge of the welfare status of individual wild animals may contribute information directly relevant to ethical, legal and political debates about the ways in which we interact with wild animals and their associated habitats [ 14 ].

2. The Ten-Stage Protocol

Acquire an understanding of the principles of Conservation Welfare

Acquire an understanding of how the Five Domains Model is used to assess welfare status

Acquire species-specific knowledge relevant to each Domain of the Model

Develop a comprehensive list of potential measurable/observable indicators in each physical domain, distinguishing between welfare status and welfare alerting indices

Select a method or methods to reliably identify individual animals

Select methods for measuring/observing the potential welfare indices and evaluate which indices can be practically measured/observed in the specific context of the study

Apply the process of scientific validation for those indices that are able to be measured/observed, and insert validated welfare status indices into the Five Domains Model

Using the adjusted version of the Model that includes only the validated and practically measurable/observable welfare status indices, apply the Five Domains grading system for grading welfare compromise and enhancement within each Domain

Assign a confidence score to reflect the degree of certainty about the data on which welfare status has been graded

Including only the practically measurable/observable welfare alerting indices, apply the suggested system for grading future welfare risk within each Domain.

2.1. Stage 1: Acquire an Understanding of the Principles of Conservation Welfare A new discipline of Conservation Welfare has recently been proposed to align traditional conservation approaches that historically focused on measures of ‘fitness’ (physical states), with more contemporary animal welfare science concepts which emphasise ‘feelings’ (mental experiences or affective states), that result from physical states. This enables a more holistic understanding of animals’ welfare states [ 52 ]. A common language and understanding relating to wild animal welfare are important starting points, since the way in which welfare is conceived influences the way it is evaluated and the emphases put on its different features [ 52 ]. The reader is referred to Beausoleil et al. 2018 [ 52 ] for a more detailed consideration of the value of seeking a shared welfare-related understanding between conservation scientists and animal welfare scientists under the heading of Conservation Welfare. 53,54,55,57,58,59,60,61,49,50,51,52, Animal welfare is characterised mainly in terms of an animal’s mental experiences, in other words, how the animal may be experiencing its own life [ 52 56 ]. In animal welfare science, welfare is conceptualised as a property of individuals, belonging to species considered have the capacity for both pleasant (positive) and unpleasant (negative) mental experiences, a capacity known as sentience [ 52 62 ]. Contemporary animal welfare science aims to interpret indicators of biological function and behaviour in terms of the mental experiences that those indicators are likely to reflect [ 52 ]. Mental experiences, or affective states, are subjective and cannot be measured directly, but indirect indices can be used to cautiously infer affective experiences [ 48 63 ]. 2.1.1. Negative Affective States 58,62,63,64,65,66,67,68,69,70,73,74, There is a growing body of neurophysiological and behavioural evidence in non-human animals regarding the basis of negative affective states such as breathlessness, thirst, hunger, pain, fear, nausea/sickness, dizziness and weakness, and there are also validated links between measurable indicators of physical/functional states and some of these mental experiences [ 36 71 ]. For example, body condition is a measurable physical state that can be used as an indicator of hunger in some situations [ 72 75 ]. Likewise, certain behaviours can be used as indices of pain. For example, in horses, the combination of rolling, gazing and/or kicking at the abdomen along with inappetence may be interpreted as reflecting abdominal pain [ 76 ]. Some affective experiences are generated by the animal’s brain processing sensory inputs that register specific features of their internal physical/functional state. For example, water deprivation causes dehydration which leads to osmoreceptor-stimulated neural impulses passing to the brain generating the affective experience of thirst [ 67 ]. Thirst elicits the behaviours of seeking water and drinking, in order to correct dehydration, after which the mental experience of thirst ceases. 64,66,67,69, Other affective experiences may arise from externally stimulated sensory inputs that contribute to the animal’s perception of its external circumstances. For example, threatening situations such as the presence of predators or humans, separation from conspecifics, or environmental hazards such as fire, are registered via cognitive processing of sensory inputs from visual, auditory and/or olfactory receptors giving rise to anxiety and fear [ 52 71 ]. 49, Whilst some negative experiences such as thirst and hunger motivate the animal to be behaviourally active in order to achieve resolution of the experience, others motivate the animal to reduce its activity. For example, weakness, sickness and pain often induce inactivity and seeking to be isolated from other animals [ 50 ]. These and other types of behaviour are referred to as ‘sickness’ behaviours and may facilitate recovery from disease and injury thereby enhancing survival [ 49 77 ]. Experiencing negative emotions to some degree is therefore essential in order to motivate life-sustaining behaviours, but it is the incidence, intensity and duration of these experiences that are important in determining the overall impacts on an animal’s welfare state. It is when negative experiences become extreme, prolonged or unavoidable, that an animal experiences the most severe compromises to its welfare [ 3 50 ]. 2.1.2. Positive Affective States 66,78,79,80,77,82,51,55,78,79,55,64,69,83, Animals can also experience a range of positive affective states, and when experienced, these may enhance the animal’s welfare state [ 50 81 ]. Some positive mental experiences may occur as a result of behaviours that are directed at minimising negative affects [ 50 ]. For example, the smell, taste, textural and masticatory pleasures of eating a range of foods and the comfort of post-prandial satiety may occur with eating that is directed at relieving hunger [ 50 83 ]. Alternatively, other positive experiences may replace negative experiences when an animal is able to express more of its behavioural repertoire [ 50 80 ]. For example, foraging, affiliative social interactions, adolescent play behaviour, maternal behaviour and sexual activity are behaviours that infer positive mental experiences [ 50 84 ]. Despite living in stimulus-rich environments, expression of rewarding behaviours can be hindered in wild free-roaming animals. For example, in malnourished horses, more time and energy is spent searching for food. Hunger is also likely to dominate awareness and this, in turn, may reduce motivation to undertake rewarding behaviours [ 50 51 ]. Conversely, when food is plentiful, relief from the negative experience of intense hunger may re-motivate animals to utilise existing opportunities to engage in a range of rewarding behaviours [ 51 ]. Therefore, it is important to consider indicators of positive, as well as negative welfare states in wild free-roaming animals and to understand particular features of their ‘natural’ circumstances may compromise or enhance their welfare [ 85 ].

2.2. Stage 2: Acquire an Understanding of How the Five Domains Model Is Used to Assess Welfare Status 49,50,49,50,36,49,50,51,55,86,87,88,89, The Five Domains Model [ 48 51 ] is consistent with, and structurally represents, the understanding that physical and mental states are linked ( Figure 1 ). It is a device that facilitates systematic and structured welfare assessment of individual sentient animals, based on current understanding of the functional bases of negative and positive subjective experiences that animals may have [ 48 51 ]. Originally developed to assess welfare compromise in animals used in research, teaching and testing [ 48 ], it has since been broadened for use in companion animals, livestock, captive wild animals and animals designated as ‘pests’ [ 27 90 ]. 50,50, The Five Domains Model comprises four interacting physical/functional domains of welfare; ‘nutrition’, ‘environment’, ‘health’ and ‘behaviour’, and a fifth domain of mental state (affective/mental experience) ( Figure 1 ). The physical/functional domains focus on internal physiological and pathophysiological states (Domains 1–3) and external physical, biotic and social conditions that may alter the animals’ behavioural expressions (Domain 4) [ 49 51 ]. Following measurement of animal-based indices within each physical domain, the anticipated negative or positive affective consequences are cautiously assigned to Domain 5. It is these experiences that contribute to descriptions of the animal’s welfare state [ 49 51 ]. It is imperative that a sound understanding of the principles of Conservation Welfare (Stage 1) and the Five Domains Model (Stage 2) is gained prior to progressing to the next stages of the protocol.

2.3. Stage 3: Acquire Species-Specific Knowledge Relevant to Each Domain of the Model 50, In order to appropriately apply the Five Domains Model to assess animal welfare, detailed species-specific knowledge is required. Table 1 illustrates the species-specific information within each of the four physical/functional domains, that is required to enable assessment of the welfare of free-roaming horses. Without a thorough understanding of what is normal for a species under optimal conditions, it is not possible to identify or interpret abnormalities. Acquiring species-specific knowledge will likely require extensive reading and advice from others having species-relevant practical experience, in addition to species-relevant nutritional, environmental, health and behavioural expertise. Accordingly, such holistic welfare assessments require multidisciplinary input [ 49 51 ]. All of the information required to make an informed assessment of the animal’s welfare status may not be available for the wild species of interest. However, systematically undertaking Stage 3 will help to identify knowledge gaps and related limitations in welfare assessments, thus guiding further research.

2.5. Stage 5: Select a Method or Methods to Reliably Identify Individual Animals 123,126,127,128,129, In order to assess animal welfare at an individual level, individuals need to be identifiable. Non-interventional identification methods may be suitable for some species. For example, in horses a combination of coat colour and natural markings may be used [ 122 124 ]. Where such approaches are not possible, alternative methods may be required, such as marking with paints or dyes, or applying tags [ 125 ]. Factors such as distance from the animal during observations and visibility are important considerations in choice of identification method. Animal welfare impacts associated with capture/handling/restraint, application of any marks/tags, wearing of the mark, and impacts of observations should be assessed. The welfare impacts of different methods of marking have been previously reviewed, and should be considered along with other advantages and disadvantages of the marking method, before deciding upon those most appropriate for identification [ 125 130 ].

2.6. Stage 6: Select Methods for Measuring/Observing the Potential Welfare Indices and Evaluate Which Indices Can Be Practically Measured/Observed in the Specific Context of the Study Having decided, based on species-specific knowledge (Stage 3), what resource-based and animal-based indices are important for assessing welfare in the species of concern (Stage 4), and how individual animals are going to be identified (Stage 5), the methods of practically measuring/observing the required indices then need to be considered. Collecting information on the welfare of wild free-ranging individual animals is logistically challenging: their habitats may be difficult to access; the animals may be difficult to observe because of natural features such as vegetation and topography, in addition to fear of humans, and they may be unobservable for significant periods or at repeated intervals. In some situations it may also be challenging to locate the individuals that may be experiencing the worst welfare impacts, as they may hide, be less mobile, more distant from conspecifics and in habitats/terrain that make visualising them difficult. 100,101,102,122,123,124,125,126,127,128,129,130,126,127,128,129,133,134,135,136, Historically, data on free-roaming animals have been obtained using methods such as direct observations (e.g., herd size, behaviour, body condition score), trapping (e.g., sex, weight, size) and GPS collaring (e.g., home range, distance travelled) [ 99 131 ]. Although these methods can yield useful information, they themselves often have significant welfare implications [ 125 130 ], provide a very narrow range of data, and there may be bias of the individuals sampled (e.g., direct observation is likely biased to those individuals within habitats where direct visualisation is possible). With more recent advances in technologies, it is now possible to obtain a wider range of information about free-roaming animals, and for longer periods of time, using techniques such as camera traps and drones [ 132 137 ] ( Table 4 ). Advantages and limitations of each potential method need to be considered for the species and context of the research, and the highest yielding methods may vary. For example, for free-roaming horses residing on open grassland or desert habitat, direct observations or drones may be the most effective way to obtain animal-based data. In contrast, in a woodland habitat, where trees may interfere with direct visualisation of animals, camera traps may be more appropriate. Combined, these methods can provide complementary information ( Figure 7 ). In some situations, direct animal-based indices may be impractical, but there could be alternative indices that indirectly provide relevant information. For example, it is not practical to assess the dentition of free-roaming wild horses, but some indices can be observed that are indirectly suggestive of clinically significant dental disease ( Figure 5 ). Methods should be evaluated by undertaking pilot studies to identify which of the potential indices are practically feasible to measure/observe in the context of the study. Indices that are not practically able to be measured/observed with currently available methods should be archived. This enables them to be considered at a later stage when evaluating the limitations of the welfare assessments (Stage 9), and to be revisited when future technological advances may make them more feasible to measure or observe.

2.7. Stage 7: Apply the Process of Scientific Validation for Those Indices that Are Able To Be Measured/Observed, and Insert Validated Welfare Status Indices into the Five Domains Model Once it has been established which indices can be practically measured/observed in the species and context of interest (Stage 6), these indices then need to be scientifically validated. Ideally, validation of welfare indices requires prior demonstration of the relationship between an observed indicator and the physical/functional impact (Domains 1–4), and of the relationship between the physical/functional impact (Domains 1–4) and the inferred mental experience (Domain 5). These steps of scientific validation have been described in detail elsewhere [ 63 ]. For example, detection of raised plasma osmolarity by osmoreceptors increases water-seeking and drinking behaviour, and drinking eliminates water-seeking behaviour [ 67 ], validating the link between the externally observable indicator of water-seeking behaviour/drinking, and the internally measurable indicator of dehydration, plasma osmolarity. Affective neuroscience provides evidence of the link between the physical state of dehydration (increased plasma osmolarity) and the mental experience of thirst, via neurohormonal pathways transmitting afferent inputs from osmoreceptors to higher brain centres associated with emotions [ 67 ]. Ideally, evidence of these relationships should relate to the species and context of interest, but where this is not available, evidence from the same species in a different context (e.g., in captivity), or a similar species, can be cautiously extrapolated. In many situations, the complete body of evidence to achieve such validation is not available and the level of confidence in the validation of indices should be indicated [ 63 ]. Thus, this process will also highlight further knowledge gaps, and what further evidence may be required to strengthen the confidence between the suggested animal-based indices and inferred mental experiences. In some cases, a direct animal-based indicator may not be practical to measure/observe in free-roaming animals, but there may be scientific evidence to support the use of an indirect indicator, which may be resource-based. For example, in free-roaming animals, water seeking or drinking behaviours can be difficult to observe. Therefore, thirst may be indirectly judged based on the resource-based indices of how available water sources are in relation to required frequency of drinking, based on the best available data for the species of interest. In the absence of direct measures, strength of motivation to drink could also be assessed by the distance the animal is willing to travel to reach a water source. Factors other than location of water sources would also need to be considered since impaired water access may occur for other reasons, such as illness or injury. Indices that cannot be scientifically validated as indicators of the animals’ mental experience (e.g., poor hoof condition in the absence of an abnormal gait), should be archived for consideration in future validation studies. Some of these archived indices may still provide valuable alerting information. All welfare alerting indices ( Table 3 ) should be evaluated and graded separately from welfare status indices, as described in Stage 10.

2.8. Stage 8: Using the Adjusted Version of the Model that Includes Only the Validated and Practically Measurable/Observable Welfare Status Indices, Apply the Five Domains Grading System for Grading Welfare Compromise and Enhancement Within Each Domain Once the indices that can be practically measured/observed (Stage 5), which are deemed to be sufficiently validated (Stage 7), have been inserted into the Five Domains Model, the next stage is to apply the grading system. 51,87, In order to standardise the assessment of animal welfare across different individuals and/or different assessors, and to monitor animal welfare over time, a reliable, repeatable and practical method of grading is required. Grading welfare compromise and welfare enhancement, and the operational details of the Five Domains Model have been previously described [ 50 88 ]. It should be noted that such grading does not necessarily provide a comprehensive assessment of welfare status; rather it provides an assessment of those indices of welfare that can be assessed and interpreted in terms of the mental experience they are associated with, in the particular species and context of interest. In the case of free-roaming animals the range of welfare-relevant indices that can be assessed will usually be more limited than that for animals in captivity. 51, Grading the impact of mental experiences on welfare status involves a different approach depending on whether the experiences are negative (welfare compromise) or positive (welfare enhancement) [ 50 87 ] ( Table 5 ). 2.8.1. Grading Welfare Compromise (Negative Mental Experiences) 51, The grading system applies a five-tier scale (A–E) to each of the Five Domains, representing increasingly severe impacts, ranging from none to very severe ( Table 6 ) [ 50 87 ]. Information from the scientifically validated measurable/observable indices decided upon in Stage 7 is used to assign the grade of physical impact (A–E) in the first 4 domains. Knowledge of the association between those physical impacts and the associated mental experiences is used to infer the type of unpleasant experiences in Domain 5. The grades assigned in Domains 1–4 are used to infer the severity and duration of those experiences in Domain 5. The grade assigned in Domain 5 is usually the same as the highest of the grades in Domains 1–4, to reflect the most severe negative mental experience. This grade is the overall welfare compromise grade ( Table 6 ). There may, however, be insufficient information to define impacts with the degree of precision implied by a five-tier scale, and in this case the grading matrix can also be adapted to a simpler three-tier scale to represent ‘no to low’, ‘moderate’, and ‘severe’ compromise [ 51 ] ( Table 7 ). 2.8.2. Grading Welfare Enhancement (Positive Mental Experiences) 51, opportunities for the animal to engage in self-motivated rewarding behaviours, (ii) the animals actual utilisation of those opportunities, (iii) making a cautious judgement of the degree of ‘positive affective engagement ’. For example, in free-roaming horses, when grading positive mental experiences (Domain 5) associated with impacts in Domain 4 (behaviour), opportunities for horses to engage in free movement, exploration, foraging a range of vegetation of varying tastes and textures, to have affectionate social interactions with bonded conspecifics and engage in maternal, sexual or play behaviour, would be expected. However, for a variety of reasons, a horse may not be able to utilise these opportunities, and consequently will not exhibit behaviours that would provide evidence of positive mental experiences. This may occur where there is welfare compromise. For example, malnutrition, dehydration, hypothermia, injury and illness may all impair an animal’s ability to engage in activities that may otherwise be pleasurable [50,70, The described grading system applies a four-tier scale (0, +, ++, +++), representing ‘no’, ‘low-level’, ‘medium-level’ and ‘high-level’ enhancement [ 50 87 ], but as above could also be simplified to a two- or three-tier scale when information relating to positive mental experiences is sparse. Grading of welfare enhancement has three elements; (i) the availability offor the animal to engage in self-motivated rewarding behaviours, (ii) the animals actualof those opportunities, (iii) making a cautious judgement of the degree of’. For example, in free-roaming horses, when grading positive mental experiences (Domain 5) associated with impacts in Domain 4 (behaviour), opportunities for horses to engage in free movement, exploration, foraging a range of vegetation of varying tastes and textures, to have affectionate social interactions with bonded conspecifics and engage in maternal, sexual or play behaviour, would be expected. However, for a variety of reasons, a horse may not be able to utilise these opportunities, and consequently will not exhibit behaviours that would provide evidence of positive mental experiences. This may occur where there is welfare compromise. For example, malnutrition, dehydration, hypothermia, injury and illness may all impair an animal’s ability to engage in activities that may otherwise be pleasurable [ 36 71 ]. The ability to engage in positive social interactions may also be impacted by aspects of social organisation and group composition [ 139 ] ( Figure 8 ). Table 5 illustrates one way in which the interaction between compromise and enhancement has been conceptualised, i.e., severe compromise hinders enhancement. 49,50,51,87, The use of numerical scores in the grading system is explicitly rejected in order to avoid scientifically unjustified aggregation of scores and to avoid implying a degree of precision that is not achievable when qualitatively assessing subjective affective states [ 48 51 ]. Scientifically informed best judgement is an important aspect of grading with the Five Domains Model, and so the grading scheme should act as a guide only, but be utilised alongside informed interpretation [ 50 51 ]. Detailed examples of species and situational specific grading matrixes and application of this grading system can be found elsewhere [ 50 88 ].

2.9. Stage 9: Assign a Confidence Score to Reflect the Degree of Certainty about the Data on Which Welfare Status Has Been Graded When the grading system is applied to assess individual animal welfare (Stage 8), a confidence score should then be assigned to the overall welfare status grade, to reflect the degree of certainty about the data upon which the grade was based [ 88 ]. We recommend a three-tier scoring system where L = low confidence, M = moderate confidence and H = high confidence. The confidence score should reflect the knowledge gaps and limitations of the assessment, including gaps in species-specific knowledge (Stage 3), any challenges with individual animal identification (Stage 5) and the archived indices that could either not be practically measured/observed with currently available methods (Stages 6), or which could not be sufficiently validated (Stage 7). These are critical actions both for directing further research to improve future welfare assessments, and for informing the level of confidence with which individual welfare can currently be assessed in the species and context of interest. In addition, a range of other factors should be considered including: whether all indices in the grading scheme could be measured/observed in the individual being assessed; the number of and/or duration of observations of the animal; whether indices were measured/observed from several methods combined or a single method; the implications if all methods could not be applied (e.g., still images only vs. video recordings vs. direct observations); and the distance of the assessor from the animal/image/video recordings when measurements/observations were made. The importance of some of these factors may also vary depending on the degree of welfare compromise. For example, if a welfare compromise status grade of E is assigned to a horse with a body condition score of 1/9, or a horse with a broken leg, the confidence in that score may be high despite the possibility that the grade was based on data from a single still image of the horse. In contrast, if a welfare status grade of A was assigned to a horse based on a single still image, the confidence in that score would likely be low.