What is animal happiness?

Today, we see a growing concern for the quality of life of nonhuman animals and an accompanying call for viable means of assessing how well animals thrive. Past research focused on minimizing negatives such as stress, while more recent endeavors strive to promote positives such as happiness. But what is animal happiness? Although often mentioned, the term lacks a clear definition. With recent advances in the study of animal emotion, current interest into positive rather than negative experiences, and the call for captive and domesticated animals to have good lives, the time is ripe to examine the concept of animal happiness. We draw from the human and animal literature to delineate a concept of animal happiness and propose how to assess it. We argue that animal happiness depends on how an individual feels generally-that is, a typical level of affect.

Introduction

There is increasing public concern for the treatment of captive and domesticated animals, including laboratory, farm, work, zoo, companion, and managed wild animals. Western societies now call for “a good life” for these animals.1, 2, 3 A concern for animal welfare is based on the acceptance of animal sentience, which is defined as the capacity to feel pain and pleasure. The traditional approach to animal welfare was that pain, suffering, distress, and other negative physical or mental experiences should be minimized.4 Consequently, there is a bias in the science of animal welfare toward the study of negative experiences at the expense of positive ones.5 Advances in our understanding of animals, in particular mammals, and the associated evolution in societal views have led to the gradual inclusion of positive experiences into definitions and assessments of animal welfare.4, 6, 7 It is now evident that although the study of negative experiences may have more moral urgency, simply aiming at an absence of negative experiences cannot translate into a good life.3, 6

The increased focus on the positive has been paralleled by an increased interest in the emotional lives, or affective states, of animals, from a conceptual and practical point of view.8, 9, 10 As with definitions of animal welfare, definitions of affect, whether in relation to animals or humans, are diverse. It is generally agreed that affect is a subjective experience that varies in pleasantness or unpleasantness (valence) as well as activation (arousal).11 In line with an increased interest in animal affect, a growing body of methodologies to assess affect in animals are being proposed.12 These methodologies involve the measurement of physiological, behavioral, or cognitive variables thought to vary with, or be an inherent part of, affective experiences.9

Questions

With a growing interest in promoting positive experiences and the call for a good life, the concept of animal happiness requires exploration. Although a number of articles addressing animal welfare mention the term animal happiness, this term is either not defined3, 13, 14, 15, 16, 17 or defined inconsistently, with authors sometimes referring to a personality trait,6 a short‐term emotion or a longer term mood,6, 8, 12 or providing a vague definition.6 Yet others equate happiness with quality of life and apply a definition similar to that found in the human literature.18 Given the inconsistent use of the term happiness in the context of animal welfare, the time is ripe to consider the concept of animal happiness and answer the following key questions: What exactly is animal happiness? How does animal happiness relate to animal welfare? and How can we assess animal happiness?

Approach

The study of human welfare, or quality of life, has benefited from many more years of thought and study, and from the human capacity to report subjective feelings verbally. Human psychology research provides animal researchers with new insights into potential definitions and methods.19 The aim of our review is to propose a framework for the concept and assessment of animal happiness. To this end, we first study the literature on human quality of life, in particular, human happiness, and identify concepts that may also apply to animals and compare these with notions of animal welfare. Following this, possible methodologies to assess the proposed concept of animal happiness are examined.

Human happiness

The concept of human happiness has been examined for millennia by philosophers (Box 1), and for just over a century by psychologists. In humans, happiness has been related to quality of life, including satisfaction with life and well‐being, and the meanings of these terms must hence be briefly presented before we examine the concept of human happiness.

Throughout the ages, philosophers have contemplated the definition of a good life. Aristotle (384–322 BC) developed a theory of happiness (eudaimonia) that focused on fulfilling an ideal human life and living life according to the virtues.20 Epicurus (341–270 BC) reasoned that happiness was to be in a state of ataraxia, which means to be untroubled by worries or to be content.21 Most premodern thinkers do not attribute the concept of happiness to animals. This was not necessarily because they did not attribute positive affective experiences to animals but because they were concerned with the concept of happiness as a phenomenon connected to higher cognitive abilities such as abstract reasoning, seeing the meaning in one's life and assessing one's situation across past and future; abilities that we do not readily attribute to animals. By the 18th century, such diverse thinkers as Jean‐Jacques Rousseau and Jeremy Bentham had started including animals into their normative theories due to animals’ presumed ability to experience pain and pleasure.22, 23

Four different notions of human quality of life

There are many notions, or views, of human quality of life and these can be sorted into a fourfold matrix24 (Fig. 1): (1) livability of the environment, (2) life‐ability of the individual, (3) usefulness of life, and (4) satisfaction with life. This matrix draws on two distinctions. The first distinction is between the chances for a good life and the actual outcomes of a life, with chances and outcomes being related but not the same; individuals may fail to realize chances, but they may also make much out of poor chances. The second difference is between external (in the environment) and internal (in the individual) qualities. External and internal qualities refer to conditions of which an individual need not be aware of subjectively to have a high quality of life.

Figure 1

The four different views of human quality of life, based on Veenhoven.24 The matrix draws a distinction between chances for a good life and the outcomes of a life, and between external (environmental) and internal (individual) qualities. A distinction is also made between cognitive and affective appraisals linked to the satisfaction with life quadrant.

Liveability of the environment represents the view that human quality of life has to do with the quality of living conditions. This view refers explicitly to a characteristic of the environment and does not have the limited connotation of material conditions. One could also speak of the “habitability” of an environment. Contemporary economists often refer to this as “welfare” or “standard of living.”25, 26

Life‐ability of the individual represents the view that human quality of life has to do with how well individuals are equipped to cope with challenges. Doctors and psychologists use the terms quality of life and well‐being to denote this specific meaning.27 In biology, the phenomenon is referred to as adaptive potential28 and in health care as health.29 Psychological terms include efficacy and potency.30 Life‐ability will typically add to a subjective appreciation of life (bottom‐right quadrant), but should not be equated with that. Even the best life‐abilities can fail to overcome severe environmental challenges, and the benefits of specific life‐abilities depend on which environment one lives in.

Usefulness of life represents the view that human quality of life has to do with higher values. In other words, a good life is one that is good for something other than itself, it should have a meaning and purpose, such as a commitment to socially shared values. It is sometimes referred to as “the meaning of life,” which then is intended to denote true (objective) significance, instead of a mere subjective sense of meaning.31 Note that this external benefit does not require inner awareness. A human's life may be useful without them knowing, especially if the effects manifest after their death. A useful life is not necessarily a happy life, for instance, not when one sacrifices one's personal happiness for a greater good.32 Only aspects of usefulness for which an individual is aware may impact on the subjective appreciation of life (bottom‐right quadrant).

Satisfaction with life represents the view that human quality of life is in the eye of the beholder and designates “subjective appreciation of one's life as a whole.” This is commonly referred to using terms such as subjective well‐being,33 life satisfaction,34 and happiness35 in a limited sense of this word. Much of the present day happiness research focuses on this human quality of life, and human happiness can hence be defined as the enjoyment of one's life as a whole. A life will have more of this quality, the more and the longer a life is enjoyed. The four views of the human quality of life described above are causally interrelated. Chances for a good life affect outcomes of life, but inversely outcomes can also affect changes; satisfaction with life (outcome) can foster life‐abilities such as resilience (chances).

Assumed and apparent quality of life

The term quality of life includes four separate notions. The first three of these notions are the assumed quality of life, while the last one, happiness (also referred to as satisfaction with life), is the apparent quality of life. Most research on human quality of life aims at identifying optimal life‐chances, that is, environmental conditions that policies should provide, and inner capabilities that education should cultivate (upper two quadrants in Fig. 1). Yet, it is easier to count the presence of such conditions than to ascertain that they are really required for a good life, and if relevant, to what degree, in what combinations, and for what kinds of people. Hence, notions of livability and life‐ability depend heavily on values and, for that reason, common sum‐scores of life‐chances reflect assumed quality of life.36 Whether such combinations actually result in a good life is determined by how long and happy people live, which Veenhoven calls apparent quality of life.36 In his view, we can identify good life‐chances (top two quadrants in Fig. 1) by studying happiness levels in different cases (right bottom quadrant).

Two components of human happiness

Happiness was defined above as the enjoyment of one's life as a whole. When appraising how happy they are, humans draw upon two sources of information: (1) how well their life‐as‐it‐is compares to standards of how they believe life should be (conscious demands) and (2) how well they feel in general. These subappraisals are seen as components of happiness; the cognitive component and the affective component, respectively.37, 38, 39, 40 Although the cognitive and affective components of happiness represent different mechanisms, they are found to strongly correlate.34

Cognitive component

Bentham22 thought of happiness as the end product of a mental calculus. Many scholars in the field also see it as the result of a cognitive process—a weighted average of earlier life‐aspect evaluations41 or a series of comparisons of life‐as‐it‐is with various standards of how‐life‐should‐be.42 The cognitive component of happiness requires conscious awareness: Do I get what I want from life? Veenhoven refers to the cognitive component of happiness as “contentment,” in essence, the “degree to which an individual perceives that his or her aspirations are met.”43 Note that this component of happiness has no connection to the cognitive component of affect, which has to do with the bidirectional link between affect, on the one hand, and judgment, attention, and memory, on the other hand.44

Affective component

Individuals who experience positive affect frequently and negative affect infrequently report high levels of happiness.40 This affective component of happiness includes both positive and negative affect, which are thought to be regulated by separate dedicated systems.32, 45, 46 Positive affect is thought to be regulated by the behavioral activation system, which promotes approach, while negative affect is thought to be regulated by the behavioral inhibition system, which promotes avoidance.45 Evolutionary biologists propose that individuals receive a positive affective signal for events that help them thrive and adapt to the environment and a negative affective signal for events that compromise survival or reproductive success.47 As Spruijt et al.48 state, “Under normal conditions those things that are pleasurable, i.e. causing and reinforcing behavior at this moment, are those things that are good in the long run, i.e. have high fitness value.” This implies that under nonnatural conditions, such as those linked to modern living, pleasurable things may, in fact, lead to low fitness in the long run. This low fitness may, in turn, be linked to a high frequency of negative experiences, for example, those linked to being overweight, tired or ill from eating high‐fat and high‐sugar foods, which will subsequently lead to low levels of affective happiness.

It is the frequency, not the intensity, of affect that seems to have the highest weight in overall reports of happiness:40 humans reporting high levels of happiness do not experience more intense positive emotions, but rather more frequent positive emotions of average intensity.49 Therefore, affective happiness, although sometimes referred to as the average level of affect, is based on the frequency of positive and negative affect (separately or the ratio thereof) and not per se on an average, which would imply that the intensity/value of each transient emotion or mood is of importance. Moreover, the affective component of happiness does not require conscious awareness.50 One can feel well most of the time without being aware of one's typical level of affect. In essence, affective happiness is a background typical level of affect that one may only become aware of when one needs to report it.

Affective happiness cannot be equated with emotions and moods. Moods are generally defined as affective states that are derived from the cumulative experience of shorter term (acute) emotions, which occur in response to specific external or internal stimuli.8 Moods are transient states and are generally said to last hours to weeks.51 Affective happiness, that is, hedonic or typical level of affect, draws on affective experiences, such as emotions and moods, but is not the same. Affective happiness is not an emotion or a mood but the frequency of pleasantness in all affective experiences. All affective states are transitory, but the frequency of positive and negative affect, that is affective happiness, can be quite stable (Fig. 2). Since affective happiness is defined as how well one feels most of the time, it has some stability by definition. This is not to say that affective happiness is a fixed trait; how well one feels on the balance is basically a state though typically reproduced in stable conditions. In livable conditions, we tend to feel well, that is, experience more positive than negative affect, studies in contemporary affluent societies showing ratios of around 3 to 1.52

Figure 2

Schematic representation of the distinction between emotion, mood, and affective happiness. Although emotions and moods are transient affective experiences, affective happiness is stable under more or less stable conditions.

There are good reasons to believe that overall happiness is mainly extrapolated from affective rather than cognitive experience.34 One reason for this is that “life‐as‐a‐whole is not a suitable object for calculative evaluation.”53 Life has many facets and there is generally no straightforward ideal to compare it with. Another reason seems to be that cognitive appraisals are often instigated by affective cues.54 This corresponds with the theory that affective systems are evolutionarily older than cognition and that cognition works as an addition to the navigation system rather than as a replacement.55

Notions of animal welfare and links to human quality of life and happiness

Notions of animal welfare

In modern animal welfare research, three main views of animal welfare have been identified by Fraser: (1) basic health and functioning, (2) natural living, and (3) affective states.13, 14

The basic health and functioning view places emphasis on freedom from disease, injury, and stress, and meeting basic requirements for life, including appropriate nutrition, water, and so on. Criticism of this view by adherents of other views includes the concern that a perfectly healthy and well‐functioning animal may still be housed in an environment providing little stimulation, hence little opportunity for positive experiences, possibly leading to negative affective states of boredom, frustration, or depression.13

The natural living view places emphasis on the level of “naturalness” in the lives of animals—on the importance of natural species‐specific behaviors and on an environment containing natural elements.13 Some would argue that the welfare of animals is improved the closer they are maintained to their natural, wild state.56 Adherents of other views criticized the natural living view based on the fact that wild ancestors of domesticated species may have faced difficult challenges, such as poor nutritional or climatic conditions. There is also the difficulty of deciding what exactly constitutes natural behaviors or environments for highly selected, domesticated animals.

The affective states view places emphasis on the feelings of animals.14 This view focuses on minimizing negative affect and maximizing positive affect. This view is based on the assumption that animals can subjectively experience their feelings, necessitating some basic form of consciousness, which is often referred to as a sentience—the ability to experience pain and pleasure.3, 57, 58

These three views of animal welfare, as with the four notions of human quality of life, interrelate and show some overlap.13 Many animal welfare authors suggest that all three of these notions should be combined to obtain the most accurate and complete definition of animal welfare.13, 59 If these three views are combined into a unified definition of animal welfare, it follows logically that they must be considered as equally important components of animal welfare. Many researchers and stakeholders, however, will favor one of these views. Below, we compare these three views to the notions of human quality of life and to the different components of human happiness.

How do the notions of animal welfare compare to those of human quality of life?

The basic health and functioning view of animal welfare can be related to internal chances, that is, how well individuals are equipped to cope with challenges (e.g., immunity and resilience). Natural living in animals has to do with the livability of the environment and the extent to which this is linked to the adaptive repertoire of individuals (life‐ability). Natural living can thus be related to both external and internal qualities of the chances axes in the human quality of life framework (Fig. 1). The affective states view of animal welfare is an internal outcome, and can hence be linked to human satisfaction with life or happiness. The affective states view of animal welfare might hence be equated with, or part of, animal happiness. To determine whether the affective states view is animal happiness or only part of this concept, we must first compare the three views of animal welfare to the components of human happiness.

The human quality of life usefulness is not represented in our selected animal welfare concepts. However, given that animal welfare becomes a point of concern in animals used for human benefit, and hence with some usefulness, it seems that usefulness is an inherent part of all animal welfare discussions.

How do the components of animal welfare compare to components of human happiness?

We will now consider the three views of animal welfare described above, as integrated components of a unified animal welfare concept and compare them to the components of human happiness, namely the satisfaction with life view of the human quality of life. As mentioned above, human happiness draws from two separate components: affective and cognitive. In human happiness research, health or (natural) living conditions are not included as components of happiness, but rather as factors that impact on human happiness60 or possible outcomes/consequences of happiness.61, 62 Only affect is included as a component of both animal welfare and human happiness.

Vertebrate animals are sometimes accepted as sentient beings based on evidence that they can feel both pain and pleasure (see Ref. 5). If animals can feel good or bad, the concept of a typical level of affect, or the affective component of human happiness, applies; even if animals are not aware of their frequency.50 In this respect, animals might be comparable to human infants.63 The importance of affect to animal welfare, and in particular the importance of the balance between positive and negative affective experiences, is reflected in previously proposed definitions of animal welfare (see Ref. 64 for a review of frameworks of positive animal welfare). Simonsen65 defined animal welfare as “the animal's positive and negative experiences.” McMillan66 proposed that animal quality of life, which is now roughly accepted as synonymous with animal welfare,17 “may be viewed as a set of scales, with pleasant feelings on one side and unpleasant feelings on the other.”67 Yeates and Main68 proposed that animal welfare is based on everyday sensational pleasures, among other things. Finally, Green and Mellor3 argued that a good animal life could be defined as a life where “the balance of salient positive and negative experiences is strongly positive.” Many other applied ethologists have also emphasized the important role of affect or affect balance in the study of animal welfare or quality of life.6, 8, 48, 69, 70

With respect to a possible cognitive component of animal happiness, many definitions of animal welfare propose that some level of cognitive activity is involved in the level of welfare an animal experiences. Yeates and Main68 emphasize the importance of allowing individuals to realize their own goals. Franks and Higgins19 suggest that animal welfare is a function of needs satisfaction and that it is based on the ability to realize own goals, gather information, and have some level of control over the environment. Finally, McMillan67 writes, “Quality of life is the affective and cognitive (to the degree that the animal can form such a cognitive construct) assessment that an animal makes of its life overall,” which very closely resembles current definitions of human happiness. Animals have goals that they are motivated to reach, in that they are willing to work hard to achieve them; when increasing cost is placed on fulfilling these goals, animals will increase their rate of work to achieve them.71 This is not only the case for physiological necessities such as food. Animals will go a long way to defend access to aspects, such as social contact, novelty, and occupation.72, 73, 74 Animals, moreover, display individual preferences that are linked to liking (see Ref. 75). Animals show indications of increased welfare when their goals are met and preferences catered for. For instance, play behavior is observed in juveniles of many species in the absence of welfare threats.76 But see Ref. 77 for a recent critical review of the link between welfare and play.

It is, however, unclear to what extent animals are able to conceptualize the degree to which their goals are met and it is thus unknown whether the cognitive component seen in human happiness is also present in (certain) species of animals. We take the tentative stance that the happiness level of (most) vertebrate animals probably depends much more, if not only, on the affective component of happiness. This is supported by the fact that even in the most cognitively complex species on Earth, that is humans, the cognitive component takes a secondary position relative to the affective component.34 Further research will have to determine whether certain animals can conceptualize to which degree their life meets their standards.

Given the above reflection on how different concepts of animal welfare can be related to the human qualities of life views and human happiness components, we define animal welfare comparably to Fraser,13 but also similarly to human quality of life, in terms of various views that are interrelated but separate. The affective states view corresponds to an internal outcome and can hence be linked to animal happiness as far as it can be conceptualized in a manner similar to that in humans. As with humans, we can speculate about the environmental conditions and the individual capabilities that make for a good life for particular kinds of animals and individuals within that species, and the apparent importance of these will depend greatly on the individual animal, on people's values, and on our current understanding of the species at that given moment. On that basis, environmental conditions and individual capabilities estimate assumed quality of life. For example, we can ask animals to indicate preferences for various resources, but these preferences will depend on the options presented, which are themselves dependent on human choices. Moreover, we may assume that sick animals have low levels of happiness, but this may not be the case if the disease does not impact on the subjective experiences of the animals. The only way to establish how a particular resource or disease impacts animal happiness is to study apparent quality of life and hence attempt to assess how happy animals feel (affective states view), and on that basis, infer in what conditions they do best, rather than assume what is good for them (Fig. 3).

Figure 3

Linking the views of animal welfare as identified by Fraser et al.14 with the concept of animal happiness, defined in terms of affect balance. Natural living and biological functioning are linked to assumed welfare, because various environmental or psychological aspects are assumed to be better for welfare (e.g., more natural environment or good health). Affective states instead are linked to apparent welfare, because it is based on assessing the subjective experience of an animal. When affective states are investigated in the context of an individual's life as a whole, this view of animal welfare can be translated as animal happiness.

Measures of affective happiness in humans

Above, we defined affective happiness as “how well one feels most of the time.” In humans, this can be measured in several ways. The most common way is to use self‐reports of how well one feels generally (trait approach) or repeated self‐reports of how well one feels now (summed state approach). How one feels generally, hence the trait approach to affective happiness, is not the same as measuring mood, as stated above. Self‐reports are the gold standard in human happiness research. Not all humans, however, are able to report how they feel, for example, human infants; for these cases, several nonverbal measures have been developed, which focus either on typical affect level or on affect at one given moment with the ultimate aim of computing the ratio of pleasant and unpleasant affect, that is, affect balance. Nonverbal indications of affect are seen in expressive behaviors and in physiological attendants of pleasant and unpleasant experiences. These different approaches are introduced below with some examples. There has been much more research into markers of cumulative negative experiences but the focus here is on measures that can go into the positive as well as negative ranges of affective happiness. We will hence not describe in detail measures of chronic stress or depression.

Trait approaches—estimates of typical affect

Typical affect can be assessed nonverbally using proxy reports, which are estimates made by humans who know an individual of interest well, such as a parent or a friend. Such ratings draw upon both verbal and nonverbal communications from the individual of interest. Studies that compare sick children or adolescent self‐reports of happiness to parent proxy reports find a gap between the two, indicating that proxy reports, at least those of parents, are not always accurate reports of happiness.78, 79 Behavioral measures include systematic observations of nonverbal behaviors deemed indicative of a human's typical level of affect and are commonly made by teachers and therapists, usually using an observation schedule such as the German “Allensbacher Ausdrücktest,” which involves facial expression and body posture. A study that compared interviewer rating on the Allensbacher with respondent's self‐reports found modest correlations of around +0.40.80

Typical affect has been shown to influence human physiology on three levels: the neuroendocrine, immune, and cardiovascular81, 82, 83, 84 level, thus physiological response may be a valid nonverbal indicator of the typical level of affect. For example, repeated positive affect has been linked to lower plasma fibrinogen during a single stress test.82 Fibrinogen is a positive acute phase protein, the plasma levels of which rise in response to inflammation. Single physiological markers, however, vary in response to many different factors, including disease, which makes them not entirely reliable when it comes to assessing affective happiness. A better marker could be a composite indicator. One example of a composite indicator of cumulative biological risk reflecting complex multisystemic dysregulation is the allostatic load (AL) model.85, 86, 87 AL increases with accumulated stress and can be measured at a single time point by recording the levels of a number of biomarkers.87 Exact biomarkers used and formulations and statistical tests applied vary per study and there is hence not one accepted method to assess AL.87 Next to an accumulation of stress, AL was recently also linked to an accumulation of positive affect, and could hence be used as a physiological indicator of typical affect.88

Another method to assess typical affect is to investigate brain structure and function. There is no single pleasure center in the brain; instead, hedonic valence seems to be generated by a set of limbic and paralimbic brain structures.89, 90 Major depressive disorder, for example, is linked to changes in the size of certain brain structures91 and Urry et al.92 found greater left than right superior frontal activation associated with higher levels of both affective and cognitive components of happiness. Prefrontal activation asymmetries linked to emotions and affective happiness are reviewed by Davidson93 and Davidson et al.94 Affective happiness was, moreover, linked to fractional amplitude of low‐frequency fluctuations in the right amygdala.95 For a review on brain changes linked to chronic stress, see Ref. 96.

At the genetic level, there is evidence that individuals with the transcriptionally more efficient version of the serotonin transporter gene report significantly higher levels of typical affect.97 Typical affect is also linked to telomere length, with shorter telomere length being associated with repeated stress,98 and vice versa.99 Finally, gut microbiota was linked to depression and positive mood,100 and may hence in the future prove useful in assessing affective happiness in humans.

Summed states approach—estimates of affect balance

Another way to assess typical hedonic level is to use multiple‐moment observations to compute affect balance: the ratio of pleasant to unpleasant affect. First, one can repeatedly request self‐reports of momentary affect from individuals. This technique is referred to as the experience sampling method (ESM) and typically involves sending signals to individuals via their smartphones at random times of the day, on average seven times per day for 1 week, to ask how they are feeling in that precise moment.101 The day reconstruction method is a variant of ESM in which respondents first list their activities of the previous day and then rate how well they have felt during each of these activities.102 Second, one can sample expressive behaviors, such as laughing or weeping, at regular time intervals; the method is referred to as time sampling. This method has been used to measure affect balance in human infants using the frequency of smiling and laughing versus crying.63 Finally, some physiological measure can be repeatedly sampled in humans to compute affect balance, for example, salivary cortisol82, 103 or heart rate (in men),82 with a higher frequency of positive affect across the day being linked to lower average cortisol and heart rate over the day.

Measures of affective happiness in animals

The study of animal affect has grown over the past decades and we now know of several possible, more or less validated methodologies that can be used to assess animal affect.6, 12, 104, 105 With animals, we lack the gold standard of self‐reports and must make use of indirect indicators of affect instead. These include behavioral indicators of momentary affect (e.g., spontaneous postures and behaviors, facial expressions, vocalizations, approach or avoidance responses to novel stimuli), cognitive biases linked to particular affective states (judgment, attention, and memory), or physiological changes linked to acute or chronic affect (e.g., oxytocin).106, 107, 108, 109, 110 Physiological indicators of both momentary and long‐term affect include neuroendocrine,106 immune,6 and cardiovascular changes,111 as in humans. We do not describe these behavioral, cognitive, or physiological methods in detail here as these are reviewed elsewhere.6, 9, 12, 104, 105 We do, however, point to how these methods can be applied to our concept of animal happiness. Moreover, as in humans, there has been much research into markers of chronic stress112, 113, 114 and depression115, 116 in animals, usually in laboratory animals used to study human pathologies, but we once again will focus on measures that have been found to tap into the positive range of affective happiness.

Trait approach—estimates of typical affect

The judgment bias test (JBT) is used to assess changes in judgment of ambiguity (optimism) in animals.108 The theory is that cumulative experience of positive and negative affect leads to a more or less optimistic judgment of ambiguous cues.8 If the judgment of ambiguity is based on transient mood as some suggest,117, 118 then JBT does not measure the typical level of affect and is not a valid method for assessing affective happiness, unless it can be repeated in time, which has been questioned.119 However, if JBT is measuring a stable, constant affective state instead, which is suggested by studies linking it to depression,120 then it would be a valid measure of typical affect. To decide which of these scenarios is valid, one will have to compare the results of the JBT with another method that measures affect balance (see below). JBT is associated with some practical and theoretical limitations, as it is time‐consuming, often requires testing animals outside of their home environment, and possibly acts as cognitive enrichment, thereby impacting affect in itself.121, 122, 123 These limitations require future research attention. Animal measures of depression, such as sensitivity to reward loss, can be used to assess the typical level of affect, although it is unclear how far into the positive range these measures might tap.124, 125, 126, 127

Proxy reports of happiness have also been adapted to certain animal species, including great apes and felids.128, 129, 130, 131 For example, the happiness level of chimpanzees, including a component reflecting affect, was rated by familiar keepers and was moderately associated with objective observations of behavior.128 Similarly, in a method called qualitative behavior assessment, a subjective assessment is used to assess the welfare state of captive and domesticated animals by rating them using terms such as positively occupied.132 Results of this approach have been reported to show variable inter‐ and intra‐rater reliability.133, 134, 135 These types of proxy tools may be criticized as being subjective and unreliable. Furthermore, it is unclear whether these methods assess transient emotions or moods, or affective happiness.

Physiological correlates of affective happiness that can be assessed at one moment in time in humans might be applicable to animals, in particular, mammalian species—which share many brain structures and physiological systems with humans. Physiological markers of depression115 and chronic stress112 in animals could provide some measure of cumulative stress, however, as in humans, single markers are unlikely to be reliable measures of affective happiness or even affect for that matter. Reviews of physiological correlates of affect in animals have previously been published.6, 12 Markers of cumulative stress, moreover, may fail to capture the positive experiences and hence the positive range of affective happiness. The concept of AL seems promising for assessing happiness in animals (personal communication by Louise Kremer, Wageningen University & Research, the Netherlands), and has been previously applied to defining the concept of animal welfare.136, 137, 138 Telomere length as a measure of affective happiness in animals is very promising as it presents a measure of cumulative stress as well as cumulative positive experiences.139 Gut microbiota may also offer interesting possibilities to assess typical level of affect in animals if it were to prove useful in human happiness research.

If current methods to assess momentary affect in animals are valid and repeatable over a period of time to enable a computation of the frequency of positive and negative affect, as, for example, with time sampling in human infants, they could potentially be used to compute affect balance, hence affective happiness, in animals. As mentioned above, methods to assess transient affect are described in detail elsewhere and will not be covered here. One promising behavioral indicators of acute affect in animals is vocalization. Vocalizations in animals have been found to reflect both affect valence (e.g., frequency and arousal) and arousal (e.g., loudness and duration).140, 141 In rats, for example, two categories of ultrasonic vocalizations (USVs) have been linked to affect.142 Minimally frequency‐modulated 22‐kHz USVs emitted in putatively aversive situations have been labeled “alarm calls” and are assumed to reflect negative affect.143 High‐frequency–modulated 50‐kHz USVs emitted during putatively positive or rewarding situations are assumed to reflect positive affect.143, 144, 145 Another example is the snorting sound produced by horses that was recently linked to positive affective experiences.146 Many more studies have linked vocalizations to emotions in various species. The advantage of vocalizations is that they can be recorded and analyzed in an automated fashion, which may allow for long‐term computations of affect balance.

Other possible indicators of acute affect that could be repeatedly sampled over time include play behavior, thought to reflect positive emotion6 (although the heterogeneity of play behavior and differences between juveniles and adults complicate the use of this indicator in this context, see Ref. 77), certain body, ear, and tail postures that can be linked to positive or negative affect,107, 111, 124 facial expressions,147, 148 and potentially also physiological markers.106 Recording these indicators of affect repeatedly over a period of time would be time‐consuming, making those indicators that can be sampled automatically, using, for example, sensors attached to an animal's body, very valuable in this context.

To conclude, there has been very little work to date done on assessing affective happiness in animals, which consequently makes our discussion of how to measure animal happiness rather short in comparison with the rest of this article. Instead, we present possible avenues for future research. Work on assessing momentary animal affect is still in its infancy but has shown promising results, pointing to some more or less practical and reliable behavioral, cognitive, and, to a lesser extent, physiological markers of affect. The repeated recording of these over a set period of time presents a promising avenue for assessing the typical level of affect in animals. Such affect balance methods will require some level of validation, which will be heavily dependent on human happiness research which benefits from the gold standard of verbal self‐reports. One possibility would be to use physiological markers of affective happiness validated in humans to validate behavioral and cognitive measures in animals though this will require similar brain structures and physiological systems between the animal species of interest and humans. Once validated in some way by physiology, affect balance methods can subsequently be used as standards to identify trait approach methods, such as possibly the JBT.

One advantage of the affect balance measurements is that it enables us to assess absolute positive and negative states rather than simply relative positive and negative states.77 A negative ratio—a higher frequency of negative over positive affect—reflects an absolute negative state, with the number of the ratio indicating how negative this is. A positive ratio—a higher frequency of positive over negative affect—reflects an absolute positive state.

Conclusions and implications

The aim of our review was to delineate a concept of animal happiness, drawing from literature on human quality of life and happiness and on definitions of animal welfare, and to propose possible assessment methods. The growing public concern for the present‐day welfare of captive and domesticated animals and the increasing importance of positive experiences in these concerns make this review particularly topical.

We suggest that animal happiness is most likely only based on an affective component of happiness, contrary to human happiness, which draws both on affective experience and cognitive comparison. Animal happiness, we suggest, can hence be defined as how an animal feels most of the time. Animal happiness defined in this way is about the balance of positive and negative affect, hence reflects the view of animal welfare commonly referred to as affective states. However, the typical level of affective happiness cannot be equated with emotions and moods, which represent, in most definitions, short‐term and highly variable affective states.8 Happiness is a long‐term, typically stable state, which reflects how one feels most of the time, that is, the typical level of affect. Our review suggests that certain notions of human happiness can be transferred to animals, and other notions, such as the cognitive component of human happiness, cannot, at least on the basis of existing knowledge. We may yet find in future research that certain animal species can conceptualize to what extent their goals are met, and hence form a cognitive appraisal of their happiness level.

We provide here an attempt at a conceptual framework for the understanding and study of animal happiness. Since objective measures of happiness in animals have to date not received much research attention, we advocate further research into assessing affect balance using existing markers of acute affect in animals. We encourage further research on affective vocalizations and the physiological correlates of affect in both humans and other animals. These could potentially provide objective and practical (e.g., automated) assessments of animal happiness in the future. Tools to compute affect balance in individual animals with the aim of assessing animal happiness will help us understand what animals require for a good life, in terms of both environmental and internal qualities.

Author contribution

All authors wrote the manuscript.

Competing interests

The authors declare no competing interests.