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Both humans and nonhuman animals need to show self-control and wait for a larger or better reward instead of a smaller or less preferred but instant reward on an everyday basis. We investigated whether this ability undergoes ontogenetic development in domestic pigs (similar to what is known in human infants) by testing if and for how long nine- and 16-week-old pigs wait for a larger amount of their preferred reward. In a delay-of-gratification task, animals first learned that a small reward was hidden under a white cup and a large reward under a black cup, and then the delay to deliver the large reward was gradually increased. The results show that older pigs could wait longer for a larger reward than younger pigs (10.6 ± 1.3 s vs. 5.2 ± 1.5 s), thereby confirming our hypothesis of ontogenetic development of self-control in pigs. This self-control is likely to be regulated by the behavioural inhibition system and associated systems. Self-control or, more specifically the lack of it may be involved in the development of abnormal behaviours, not only in humans but also in animals. Therefore, research on self-control in decision-making might provide a new perspective on abnormal behaviours in captive animals.

Understanding animal emotions is an important scientific and ethical question but assessing emotional valence is still considered challenging. As the observation of lateralization (hemispheric asymmetries in structure and/or function) can provide insight into the underlying processes of the cognitive, physiological and behavioural components of emotions, it is a promising approach for studying them. The emotional valence hypothesis states that positive emotions are mostly processed by the left hemisphere, while negative emotions are mostly processed by the right hemisphere. Support for this hypothesis is still not conclusive; therefore, our study tested it in the context of visual laterality for viewing positive or negative emotionally conditioned stimuli. Ninety male piglets were either positively (food-reward) or negatively (mild punishment) conditioned to an object. Afterwards, the object was presented without the reinforcer under three different treatments: patch on the left or right eye (reducing input to the contralateral hemisphere) or patch between the eyes (the control). Monocular viewing had no clear effects on the negatively conditioned subjects. In contrast, in the positively conditioned group, covering the right eye caused a longer interruption of vocalization, a longer latency to touch the object, a shorter duration of exploring the arena and an increased vagal activity compared to the control. This suggests that reduced processing in the left hemisphere leads to heightened attention that is accompanied by a general orienting response, possibly resulting from a reduced positive appraisal. These findings therefore partially support the emotional valence hypothesis and suggest an important role of the left hemisphere in the quick recognition of a positive stimulus. This study demonstrated that investigating the lateralized processing of emotions can provide insight into the mechanisms of positive appraisal in animals.

Vocal expression of emotions has been observed across species and could provide a non-invasive and reliable means to assess animal emotions. We investigated if pig vocal indicators of emotions revealed in previous studies are valid across call types and contexts, and could potentially be used to develop an automated emotion monitoring tool. We performed an analysis of an extensive and unique dataset of low (LF) and high frequency (HF) calls emitted by pigs across numerous commercial contexts from birth to slaughter (7414 calls from 411 pigs). Our results revealed that the valence attributed to the contexts of production (positive versus negative) affected all investigated parameters in both LF and HF. Similarly, the context category affected all parameters. We then tested two different automated methods for call classification; a neural network revealed much higher classification accuracy compared to a permuted discriminant function analysis (pDFA), both for the valence (neural network: 91.5%; pDFA analysis weighted average across LF and HF (cross-classified): 61.7% with a chance level at 50.5%) and context (neural network: 81.5%; pDFA analysis weighted average across LF and HF (cross-classified): 19.4% with a chance level at 14.3%). These results suggest that an automated recognition system can be developed to monitor pig welfare on-farm.

In humans, speech perception is lateralized, with the left hemisphere of the brain dominant in processing the communicative content and the right hemisphere dominant in processing the emotional content. However, still little is known about such a division of tasks in other species. We therefore investigated lateralized processing of communicative and emotionally relevant calls in a social mammal, the pig (Sus scrofa). Based on the contralateral connection between ears and hemispheres, we compared the behavioural and cardiac responses of 36 young male pigs during binaural and monaural (left or right) playback to the same sounds. The playback stimuli were calls of social isolation and physical restraint, whose communicative and emotional relevance, respectively, were validated prior to the test by acoustic analyses and during binaural playbacks. There were indications of lateralized processing mainly in the initial detection (left head-turn bias, indicating right hemispheric dominance) of the more emotionally relevant restraint calls. Conversely, there were indications of lateralized processing only in the appraisal (increased attention during playback to the right ear) of the more communicative relevant isolation calls. This implies differential involvement of the hemispheres in the auditory processing of vocalizations in pigs and thereby hints at similarities in the auditory processing of vocal communication in non-human animals and speech in humans. Therefore, these findings provide interesting new insight in the evolution of human language and auditory lateralization.

The reliable detection of estrus is an important scientific and practical challenge in dairy cattle farming. Female vocalization may indicate reproductive status, and preliminary evidence suggests that this information can be used to detect estrus in dairy cattle. The aim of this study was to associate the changes in the vocalization rate of dairy heifers with behavioral estrus indicators as well as test the influence of the type of estrus (natural estrus vs. superovulation-induced estrus). We analyzed 6 predefined estrus-related behavior patterns (standing to be mounted, head-side mounting, active mounting, chin resting, being mounted while not standing, and active sniffing in the anogenital region) and vocalization rates in the peri-estrus period (day of estrus ± 1 d) of 12 German Holstein heifers using audio-visual recordings. Each heifer was observed under natural estrus and a consecutive superovulation induced by FSH and cloprostenol. Estrus was determined by behavioral patterns and confirmed by clinical examination (vaginoscopy and ultrasound imaging of the ovaries) as well as by the concentration of peripheral progesterone. Estrus behavior and vocalization rates were analyzed in 3-h intervals (an average of 19 intervals for each heifer), and an estrus score was calculated based on the 6 behaviors. The interval with the highest estrus score (I0) was considered the estrus climax. We demonstrated similar time courses for the estrus score and vocalization rate independent of estrus type. However, in natural estrus, the maximum vocalization rate (±SE) occurred in the interval before estrus climax (I−1; 42.58 ± 21.89) and was significantly higher than that in any other interval except estrus climax (I0; 27.58 ± 9.76). During natural estrus, the vocalization rate was significantly higher within the interval before estrus climax (I−1; 42.58 ± 21.89 vs. 11.58 ± 5.51) than under superovulation. The results underscore the potential use of vocalization rate as a suitable indicator of estrus climax in automated estrus detection devices. Further studies and technical development are required to record and process individual vocalization rates.

Motor lateralization is hypothesized to depend on the complexity of the motor function, but it might at the same time reflect hemispheric dominance within an individual across motor functions. We investigated possible motor lateralization patterns in four motor functions of different complexity (snout use in a manipulative task, foot use in two-stepping tasks and tail curling) in the domestic pig, a tetrapod species relevant as farm animal but also as a model in human neuroscience. A significant majority of our sample showed individual biases for manipulation with their snout and for curling their tail. Interestingly, the tail curling was lateralized towards the right at the population level and showed stronger lateralization patterns than the snout. Using a cluster analysis with combined tail and snout laterality, we identified groups of individuals with different lateralization patterns across motor functions that potentially reflect the individuals' hemispheric dominance. To conclude, our results suggest that pigs show lateralization patterns that depend on the motor function and on the individual. Such individual lateralization patterns might have broader implications for animal personality and welfare. Our study lays the methodological groundwork for future research on laterality in pigs.

The assessment and provision of welfare in farm animals has become a major issue in animal science. A key element for providing good welfare is the enabling of positive affective states in the animals. As the serotonergic system plays a central role in regulating affective behavior, an increase in centrally available serotonin (5-HT) via dietary supplementation of its precursor, tryptophan (TRP), might be an approach to induce positive affective states. Therefore, the aim of our study was to investigate the effects of dietary TRP supplementation on brain TRP metabolism and 5-HT levels but also on affective state and behavioral reactivity in pigs. All subjects were fed a standard diet until 8 weeks of age, then feed was changed for all animals, with half the animals (control) receiving a diet with the recommended TRP content (2.5 g/kg), whereas the other half (TRP+) received a TRP-enriched diet (10.2 g/kg). In part 1 of our study, we investigated the effects of the dietary TRP supplementation on TRP metabolism in brain areas related to affective and cognitive processing. We found significantly increased concentrations of TRP and its metabolites in nearly all analyzed brain tissues. In part 2 of our study, we analyzed the effects of these alterations on the affective state as measured in a cognitive bias test, namely the spatial judgment task, but also on behavioral reactivity as measured in a combined open field/novel object test. The TRP enrichment revealed no significant behavioral changes in the open field/novel object tests. In the spatial judgment task, the TRP+ group showed more pessimistic behavior after dietary change than before. Thus, our results do not support the suggestion that TRP supplementation induces positive affective states and thus improves animal welfare in pigs.

Public concern for farm animal welfare calls for reliable scientific tools to measure it. Measuring cognitive bias, i.e., the influence of affective states on cognitive processing, has gained importance during recent years. The one most often adapted experimental design to test cognitive bias in non-human animals is the spatial judgement task, where animals must judge a goal box/pot based on its location. First, they learn to discriminate between a positively reinforced and a negatively reinforced location, and then they are confronted with ambiguous, intermediate locations. The responses to these so-called probe cues are assumed to be influenced by the animal’s affective state. Current methods for the assessment of cognitive bias in domestic pigs, one of the most intensively housed farm animal species in Europe, still need to be refined and validated. Hence, we developed an improved, reliable design to test pigs repeatedly and induce graded responses to graded probe cues. For this, we used a spatial judgement task in a go/no-go paradigm with partial reinforcement and a mildly aversive negative reinforcer. A total of 16 female juvenile German Landrace pigs underwent discrimination learning of the rewarded vs. punished goal box location; this was then repeatedly tested on three probe locations in between (eight times in four weeks). The subjects learned to discriminate between the two reference locations and showed graded responses (latency to open the goal box) to the probe locations. Analyses of the subjects’ general behaviour implied that additional information on the animals’ state can be derived. The subjects showed no signs of learning with respect to the outcome of probe cues but exhibited stable response levels during test weeks two to four. In conclusion, the design presented in this paper is suitable for spatial judgement tasks in domestic pigs. After successful validation it might be used for testing affective valence based on judgement bias and hence can contribute to our understanding of the welfare of this intensively farmed species.

Altering one’s emotional state in response to the emotional expressions of others, called emotional contagion, is a well-studied phenomenon in humans and many nonhuman animals. Here we describe the methods that are typically used to assess changes in the emotional state in demonstrators and the transmission of emotions to naïve observers. We then review the evidence for the transmission of positive and negative emotions in farm animals. We conclude by highlighting examples of how a better understanding of emotional contagion in farm animals can lead to novel and innovative interventions to improve their welfare.

Emotions, unlike mood, are short-lived reactions associated with specific events. They can be characterized by two main dimensions, their arousal (bodily activation) and valence (negative versus positive). Knowledge of the valence of emotions experienced by domestic and captive animals is crucial for assessing and improving their welfare, as it enables us to minimize the negative emotions that they might experience and to promote positive ones. Emotions can affect vocalizations directly or indirectly through the brain, lungs, larynx or vocal tract. As a result, vocal expression of emotions has been observed across species, and could serve as a non-invasive and potentially very reliable tool to assess animal emotions. In pigs (Sus scrofa), vocal expression of emotions has been relatively well studied. However, it is not known if the vocal indicators revealed in previous studies are valid across call types and contexts. To find this out, we conducted a meta-analysis of the effects of emotional valence on pig vocalizations, including calls recorded in the most common emotional situations encountered by pigs throughout their lives, from birth to slaughter. Our analyses revealed that pigs produced calls characterized by a higher center of gravity, a shorter duration, less noise (lower Wiener entropy), lower formants (measured using the formant dispersion) and LPC coefficients in positive compared to negative contexts. Overall, these vocal parameters could be very useful for developing automated methods to monitor pig welfare on-farm.

The growing recognition of animals as individuals has broader implications for farm animal welfare research. Even under highly standardized on-farm conditions, farm animals show heterogeneous but individually consistent behavioural patterns towards various stimuli, based on how they appraise these stimuli. As a result, animal welfare is likely to be highly individual as well, and studying the proximate mechanisms underlying distinct individual behaviour patterns and appraisal will improve animal welfare research. We propose to extend the framework of affective styles to bridge the gap between existing research fields on animal personality and affective states. Affective styles refer to consistent individual differences in emotional reactivity and regulation and can be predicted by baseline cerebral lateralization. Likewise, animals with consistent left or right motor biases—a proxy measure of individual patterns in cerebral lateralization—have been shown to differ in their personality, emotional reactivity, motivational tendencies or coping styles. In this paper, we present the current knowledge of the links between laterality and stable individual traits in behaviour and affect in light of hypotheses on emotional lateralization. Within our suggested framework, we make recommendations on how to investigate affective styles in non-human animals and give practical examples. This approach has the potential to promote a science of affective styles in nonhuman animals and significantly advance research on animal welfare.

Vocalizations have long been recognized to encode information about an individual's emotional state and, as such, have contributed to the study of emotions in animals. However, the potential of vocalizations to also encode information about an individual's emotional reactivity has received much less attention. In this study, we aimed to test whether the vocalizations of domestic pigs contain correlates of emotional reactivity that are consistent between different contexts. We recorded vocalizations of 120 young female pigs in an experimental arena in two consecutive recording contexts, social isolation and an encounter with a familiar human. Simultaneously, we measured their heart rate and behaviour to determine their emotional reactivity in the same context (within-context). In addition, we aimed to determine the subjects' emotional reactivity in other contexts (across-context) by measuring their behaviour in four common tests of emotional reactivity, the human approach test, the open door test, the open field test and the novel object test. Using a cluster analysis, we identified four different call types. Significant inter-context correlations were found for all call types, suggesting that pig vocalizations are consistent within an individual across contexts. The call rate and the proportions of the individual call types were found to correlate significantly with indices of emotional reactivity both within and across contexts. Thereby, we found more significant correlations to indices of emotional reactivity within context (behavioural and physiological response during recording) compared to across context (behavioural response in the four emotional reactivity tests). The consistency of the vocal correlates to emotional reactivity between the different contexts depended on the call type. While we found moderate evidence that the high grunt is indicative of more active, more explorative and less fearful individuals both within as well as across contexts, the other call types provided less consistent results. Thus, it seems that some call types are better suited to provide information on a caller's emotional reactivity than others, and further research is needed to clarify the underlying influential factors.

Studies on consequences of affect on cognitive processing (cognitive bias) seem to provide proxy measures of the valence (positive/negative) of affective states. This article presents a pilot study testing a design for studies of judgment bias we adapted to the specific needs and abilities of domestic pigs. We used a spatial judgment paradigm, in which subjects learned to discriminate a food rewarded from an unrewarded position of a goal box during a training period, and then were tested for their responses to 3 equidistant intermediate probe positions during a testing period. After the training period, half of the subjects were repeatedly isolated from their social group to manipulate their affective state. Subjects learned the required discrimination task (positive vs. negative stimulus) and showed consistent differences in approach latencies toward and exploration of stimuli of different valence. Hence, the subjects' expectations could be inferred from their behavior. However, repeated social isolation had no effect on judgment of ambiguous stimuli and on both basal and test-related cortisol levels. In conclusion, the spatial judgment approach seems to provide a useful tool to detect and discriminate diverse affective states in domestic pigs based on their responses to graded ambiguous stimuli.

Abnormal behaviour is a potential indicator of pain, suffering and injury in captive animals. Especially stereotypies, i. e. repetitive invariant behavioural patterns without obvious function or goal, can be observed as a consequence of inadequate housing conditions. Hence, they are often considered indicators of impaired welfare. In context of the ongoing scientific debate on captive animal welfare, the number of publications on stereotypies has increased, most notably in veterinary and farm animal research. Based on biological principles and definitions, we present several examples of stereotypic behaviour in (mainly) farm animals, and discuss approaches of preventing or reducing them. The occurrence of abnormal behaviour is often, but not necessarily, associated with the fact that modern housing and management precludes various evolutionary emerged highly motivated behaviours, or poses challenges the animals are unable to cope with adequately. Numerous studies show that stereotypies can be indicative of (current or past) suffering and impaired welfare. They can be avoided or at least reduced by increasing the biological relevance of the housing environments through environmental enrichment which stimulates species-specific behaviour.

In the study of animal emotions, emotional valence has been found to be difficult to measure. Many studies of farm animals' emotions have therefore focussed on the identification of indicators of strong, mainly negative, emotions. However, subtle variations in emotional valence, such as those caused by rather moderate differences in husbandry conditions, may also affect animals' mood and welfare when such variations occur consistently. In this study, we investigated whether repeated moderate aversive or rewarding events could lead to measurable differences in emotional valence in young, weaned pigs. We conditioned 105 female pigs in a test arena to either a repeated startling procedure (sudden noises or appearances of objects) or a repeated rewarding procedure (applesauce, toy and straw) over 11 sessions. Control pigs were also regularly exposed to the same test arena but without conditioning. Before and after conditioning, we measured heart rate and its variability as well as the behavioural reactions of the subjects in the test arena, with a special focus on detailed acoustic analyses of their vocalisations. The behavioural and heart rate measures were analysed as changes compared to the baseline values before conditioning. A limited number of the putative indicators of emotional valence were affected by the conditioning. We found that the negatively conditioned pigs showed changes that were significantly different from those in control pigs, namely a decrease in locomotion and an increase in standing. The positively conditioned pigs, however, showed a stronger increase in heart rate and a smaller decrease in SDNN (a heart rate variability parameter indicating changes in autonomic regulation) compared to the controls. Compared to the negatively conditioned pigs, the positively conditioned pigs produced fewer vocalisations overall as well as fewer low-frequency grunts but more high-frequency grunts. The low-frequency grunts of the negatively conditioned pigs also showed lower frequency parameters (bandwidth, maximum frequency, 25% and 50% quartiles) compared to those of the positively conditioned pigs. In any of the statistically significant results, the conditioning accounted for 1.5-11.9% of variability in the outcome variable. Hence, we conclude that repeated moderate aversive and rewarding events have weak but measurable effects on some aspects of behaviour and physiology in young pigs, possibly indicating changes in emotional valence, which could ultimately affect their welfare. The combination of ethophysiological indicators, i.e., the concurrent examination of heart rate measures, behavioural responses and especially vocalisation patterns, as used in the current study, might be a useful way of examining subtle effects on emotional valence in further studies.

In domestic pigs, vocalisation can be an indicator of distress and negative emotional states. It might play a role in the transfer of emotion between individuals (‘emotional contagion’ or ‘empathy’), which could result in impaired animal welfare on a group level based on the distress in an individual member of the group. The aim of this study was to characterise the responses of pigs to conspecific distress calls. We performed a playback experiment in an open arena with 24 juvenile German Landrace pigs, during which individual subjects were exposed to both conspecific distress calls and an artificial sine tone (control) on consecutive experimental days. Both behavioural (locomotion, vocalisation, elimination and distance to the speakers) and physiological responses (heart rate and heart rate variability) were measured for 2 min before, during and after the playback of the stimuli (distress calls/control). Subjects showed decreased locomotion and vocalisation rates during both stimuli, suggesting that the animals responded to both stimuli. Heart rates decreased at the onset of both stimuli due to an activation of the parasympathetic system, indicating an orientation response to sudden stimuli. However, heart rates decreased after the end of the distress calls but not after control stimuli, illustrating that conspecific calls and other sounds are evaluated differently. We conclude that pigs exposed to isolation are attentive to conspecific distress vocalisation and hence the information about threat possibly conveyed in it, but they do not share the distress of the caller. Therefore, we could not find direct effects of distress calls of unfamiliar conspecifics on the welfare of isolated juvenile pigs. However, the state of heightened attention elicited by conspecific distress calls may affect a pig's subsequent evaluation of its environment.

In a pilot study we have injected the amygdala of five female pigs (age 8 weeks) with acetylcholine (ACh, 5.5 μM/20 μl) and recorded short latency utterances. The evoked vocalizations displayed the characteristics of natural screams in sonagram appearance and hearing impression. Quantitative analyses, too, revealed the similarity of the ACh-evoked vocalizations with natural screaming. The results demonstrate for the first time that aversive vocalizations can be triggered by cholinergic amygdala stimulation.