In their target article, Kalisch et al. propose a theoretical framework for the neuroscientific study of general resilience mechanisms that are applicable to both human and animal research. In this commentary, we argue from a behavioural biologist's point of view that animals could tell us more if (1) this framework were extended and (2) new paradigms were included to assess appraisal styles in animal research, as well.
Concerning (1), more than 50 years ago, in his seminal publication “On Aims and Methods of Ethology,” Nobel prize winner Nikolaas Tinbergen (Reference Tinbergen1963) argued that a comprehensive understanding of behavioural phenomena should give equal attention to the questions of causation, ontogeny, survival value, and evolution, and to their integration. We recommend the Tinbergen framework be applied to the study of resilience in animals as well, because the broader our understanding of resilience, the more successfully we can apply that understanding to humans. Accordingly, the study of resilience in animal research should not be restricted to elucidating the underlying neuronal, cellular, and molecular mechanisms in adult individuals (causation). Rather, it is likely that these mechanisms are shaped by environmental stimuli and genes during sensitive phases of life (ontogeny). Typically, this shaping process is not confined to the early postnatal period, but occurs from the prenatal phase through to adolescence and beyond (Sachser et al. Reference Sachser, Kaiser and Hennessy2013). We therefore recommend that ontogenetic perspectives be integrated into future experimental animal research on resilience mechanisms.
In the target article, a positive appraisal style is considered the key resilience mechanism, and therefore, optimism is regarded as the common final path to mental health. Given that these traits are shaped by natural selection, the question concerning function (survival value) arises. At first sight, it seems plausible that a positive appraisal style would maximize Darwinian fitness. But if a positive style was superior to other appraisal styles, natural selection ought to have selected against non-positive versions. The empirical data available from animals show, however, that as in humans, various appraisal/coping styles exist (e.g., Koolhaas et al. Reference Koolhaas, de Boer, Coppens and Buwalda2010). From this it follows that a positive appraisal style cannot always be the best option in all environmental situations, and hence, cannot be regarded as the only path to mental health. Animal studies addressing the costs and benefits of different appraisal styles could, therefore, make a valuable contribution to the debate on this topic in humans (e.g., Sweeny & Shepperd Reference Sweeny and Shepperd2010).
Current biomedical animal research focuses strongly on so-called model species, and most experimental work on resilience probably is performed using laboratory mice. Addressing the question of evolution by conducting comparative studies with a range of species would broaden this approach and improve our understanding of resilience significantly. Members of any given species are equipped with physiological, motivational, behavioural, and resilience mechanisms to cope with the demands of their specific ecological niche. Because different species inhabit different ecological niches, we also should expect different coping mechanisms to have evolved. This also means that it is very unlikely that a single animal model would reflect human resilience in a comprehensive way. It is, therefore, important to understand resilience mechanisms in a variety of species. This would significantly facilitate translation from animal research to humans.
Concerning (2), Kalisch et al. suggest that a resilience readout in an animal experiment would require two elements: a battery of stressors that adequately models human-life stressors, and a “mental health” testing battery assessing the animal's functioning before and after having experienced the stressors. Such testing may indeed shed light on behavioural and neurobiological pathways to resilience, but it is unlikely to provide a good understanding of the role of stimulus appraisal in the generation of emotional responses. We note that appraisal style can be assessed directly in animals, as well.
In 2004, Harding and colleagues developed methods to determine emotional states by measuring differences in judgement bias in rats. The animals were trained to press a lever to obtain a food reward in response to one tone (“positive tone”), and to refrain from pressing the lever in response to a different tone (“negative tone”) to avoid the presentation of white noise. Having learned to discriminate between these two “reference tones,” the rats' responses to ambiguous, intermediate tones were investigated. Animals that were housed in unpredictable conditions responded less often to the “ambiguous tones” and showed longer response latencies to such tones than did animals that were housed in a predictable environment. Hence, rats in unpredictable – that is, more stressful housing conditions – show behaviour indicating reduced anticipation of a positive event, reflecting a pessimistic judgement bias. They judged “the glass is half empty and not half full.”
Meanwhile, such cognitive bias investigations have been performed in a wide range of species including mice, dogs, sheep, and rhesus macaques (e.g., Mendl et al. Reference Mendl, Brooks, Basse, Burman, Blackwell and Casey2010) and clearly show that positive and negative appraisal styles exist, and can be assessed also in animals. The use of such techniques in studies of resilience would, in this way, help to detect emotional biases in judgements similar to those found in humans.
In summary, the Tinbergen framework provides us with an excellent road map for a thorough investigation of resilience mechanisms in animals. In addition, new behavioural paradigms such as cognitive judgement bias tests allow the assessment of degrees of “optimism” and “pessimism”; that is, different appraisal styles in nonhuman animals. The adoption of a broader range of questions and research tools will significantly advance our understanding of this topic, and thus, contribute to a broader understanding of resilience in humans – because the animals are able to tell us more.
In their target article, Kalisch et al. propose a theoretical framework for the neuroscientific study of general resilience mechanisms that are applicable to both human and animal research. In this commentary, we argue from a behavioural biologist's point of view that animals could tell us more if (1) this framework were extended and (2) new paradigms were included to assess appraisal styles in animal research, as well.
Concerning (1), more than 50 years ago, in his seminal publication “On Aims and Methods of Ethology,” Nobel prize winner Nikolaas Tinbergen (Reference Tinbergen1963) argued that a comprehensive understanding of behavioural phenomena should give equal attention to the questions of causation, ontogeny, survival value, and evolution, and to their integration. We recommend the Tinbergen framework be applied to the study of resilience in animals as well, because the broader our understanding of resilience, the more successfully we can apply that understanding to humans. Accordingly, the study of resilience in animal research should not be restricted to elucidating the underlying neuronal, cellular, and molecular mechanisms in adult individuals (causation). Rather, it is likely that these mechanisms are shaped by environmental stimuli and genes during sensitive phases of life (ontogeny). Typically, this shaping process is not confined to the early postnatal period, but occurs from the prenatal phase through to adolescence and beyond (Sachser et al. Reference Sachser, Kaiser and Hennessy2013). We therefore recommend that ontogenetic perspectives be integrated into future experimental animal research on resilience mechanisms.
In the target article, a positive appraisal style is considered the key resilience mechanism, and therefore, optimism is regarded as the common final path to mental health. Given that these traits are shaped by natural selection, the question concerning function (survival value) arises. At first sight, it seems plausible that a positive appraisal style would maximize Darwinian fitness. But if a positive style was superior to other appraisal styles, natural selection ought to have selected against non-positive versions. The empirical data available from animals show, however, that as in humans, various appraisal/coping styles exist (e.g., Koolhaas et al. Reference Koolhaas, de Boer, Coppens and Buwalda2010). From this it follows that a positive appraisal style cannot always be the best option in all environmental situations, and hence, cannot be regarded as the only path to mental health. Animal studies addressing the costs and benefits of different appraisal styles could, therefore, make a valuable contribution to the debate on this topic in humans (e.g., Sweeny & Shepperd Reference Sweeny and Shepperd2010).
Current biomedical animal research focuses strongly on so-called model species, and most experimental work on resilience probably is performed using laboratory mice. Addressing the question of evolution by conducting comparative studies with a range of species would broaden this approach and improve our understanding of resilience significantly. Members of any given species are equipped with physiological, motivational, behavioural, and resilience mechanisms to cope with the demands of their specific ecological niche. Because different species inhabit different ecological niches, we also should expect different coping mechanisms to have evolved. This also means that it is very unlikely that a single animal model would reflect human resilience in a comprehensive way. It is, therefore, important to understand resilience mechanisms in a variety of species. This would significantly facilitate translation from animal research to humans.
Concerning (2), Kalisch et al. suggest that a resilience readout in an animal experiment would require two elements: a battery of stressors that adequately models human-life stressors, and a “mental health” testing battery assessing the animal's functioning before and after having experienced the stressors. Such testing may indeed shed light on behavioural and neurobiological pathways to resilience, but it is unlikely to provide a good understanding of the role of stimulus appraisal in the generation of emotional responses. We note that appraisal style can be assessed directly in animals, as well.
In 2004, Harding and colleagues developed methods to determine emotional states by measuring differences in judgement bias in rats. The animals were trained to press a lever to obtain a food reward in response to one tone (“positive tone”), and to refrain from pressing the lever in response to a different tone (“negative tone”) to avoid the presentation of white noise. Having learned to discriminate between these two “reference tones,” the rats' responses to ambiguous, intermediate tones were investigated. Animals that were housed in unpredictable conditions responded less often to the “ambiguous tones” and showed longer response latencies to such tones than did animals that were housed in a predictable environment. Hence, rats in unpredictable – that is, more stressful housing conditions – show behaviour indicating reduced anticipation of a positive event, reflecting a pessimistic judgement bias. They judged “the glass is half empty and not half full.”
Meanwhile, such cognitive bias investigations have been performed in a wide range of species including mice, dogs, sheep, and rhesus macaques (e.g., Mendl et al. Reference Mendl, Brooks, Basse, Burman, Blackwell and Casey2010) and clearly show that positive and negative appraisal styles exist, and can be assessed also in animals. The use of such techniques in studies of resilience would, in this way, help to detect emotional biases in judgements similar to those found in humans.
In summary, the Tinbergen framework provides us with an excellent road map for a thorough investigation of resilience mechanisms in animals. In addition, new behavioural paradigms such as cognitive judgement bias tests allow the assessment of degrees of “optimism” and “pessimism”; that is, different appraisal styles in nonhuman animals. The adoption of a broader range of questions and research tools will significantly advance our understanding of this topic, and thus, contribute to a broader understanding of resilience in humans – because the animals are able to tell us more.