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Animal welfare scientists have accumulated knowledge and developed interventions to improve livestock welfare, but these are poorly adopted in commercial practice. Animal welfare interventions are rarely tested for economic viability and this limits their uptake. This study employs Stochastic Partial Budgeting (SPB) to determine the viability of animal welfare improvements. Aggression between pigs is used as an example because there is a large literature base from which to draw interventions, and the problem has persisted for decades without resolution. Costs and benefits of three interventions to control aggression (pre-weaning socialisation, synthetic maternal pheromones and large social groups) were estimated by reviewing the academic and industry literature and by conducting a survey of sixteen pig farmers. The net effects were compared to farmers' willingness to pay (WTP) for interventions to reduce aggression as identified by recent research. Results are consistent with prior research which indicates that improving animal welfare generally comes at a cost to producers. Nevertheless, pre-weaning socialisation resulted in a neutral or positive net effect 38% of the time and should be central to campaigns promoting the control of aggression in the industry. Exposing pigs to synthetic maternal pheromones did not improve profitability but the net costs were small and within the realms of WTP for a sub-group of farmers with animal welfare goals. The net costs of converting existing buildings in order to house pigs in large social groups were beyond the realms of farmers' WTP. The approach adopted in this study, of combining SPB with WTP from the sector, should be extended to other animal welfare issues., Competing Interests: The authors have declared that no competing interests exist.

Early play fighting mimics later aggression in many species, and may, therefore, be expected to reduce costs from later aggressive interactions. Using social network analysis (SNA) the effect of a central play fighting network position on later skin lesions from aggression was assessed in domestic pigs. Piglets (n = 263) were kept in litter groups or socialised pre-weaning with another litter to enhance play fighting experience. Play fighting was recorded for 1.5 h per day over 6 days pre-weaning. Play fighting network centrality was quantified using measures of individual network position and entire network structure (degree, eigenvector, betweenness, clustering coefficient). Skin lesions from aggression were counted after a dyadic contest and at 24 h and 3 weeks following group mixing. Pigs with play fighting interactions with many partners experienced fewer lesions from the dyadic contest (in-degree, p = 0.01) and tended to received fewer lesions 3 weeks after group mixing (degree, p = 0.088) but no other play fighting centrality measures affected the number of lesions at any point. The benefits of play fighting were therefore limited to specific aggressive social contexts. The tendency of socialised piglets to play fight with non-littermates did not affect subsequent lesions. We advocate the use of SNA over approaches that only consider dyadic interactions to further our understanding of the influence of early social group interactions on later life experience.

Contest behaviour, and in particular the propensity to attack an unfamiliar conspecific, is influenced by an individual's aggressiveness, as well as by experience of winning and losing (so called 'winner-loser effects'). Individuals vary in aggressiveness and susceptibility to winner-loser effects but the relationship between these drivers of contest behaviour has been poorly investigated. Here we hypothesise that the winner-loser effect on initiation of agonistic behaviour (display, non-damaging aggression, biting and mutual fighting) is influenced by aggressiveness. Pigs (n = 255) were assayed for aggressiveness (tendency to attack in resident-intruder tests) and then experienced two dyadic contests (age 10 and 13 weeks). Agonistic behaviour, up to reciprocal fighting, in contest 2 was compared between individuals of different aggressiveness in the RI test and experiences of victory or defeat in contest 1. Winner-loser effects were more influential than aggressiveness in determining initiation of agonistic behaviour. After accruing more skin lesions in contest 1, individuals were less likely to engage in escalated aggression in contest 2. The interaction between aggressiveness and winner-loser experience did not influence contest behaviour. The results suggest that aggressiveness does not compromise learning from recent contest experience and that reducing aggressiveness is unlikely to affect how animals experience winning and losing.

When deciding whether to invest in an improvement to animal welfare, farmers must trade-off the relative costs and benefits. Despite the existence of effective solutions to many animal welfare issues, farmers' willingness to pay for them is largely unknown. This study modelled pig farmers' decisions to improve animal welfare using a discrete choice experiment focused on alleviating aggression between growing/finishing pigs at regrouping. Eighty-two UK and Irish pig farm owners and managers were asked to choose between hypothetical aggression control strategies described in terms of four attributes; installation cost, on-going cost, impact on skin lesions from aggression and impact on growth rate. If they did not like any of the strategies they could opt to keep their current farm practice. Systematic variations in product attributes allowed farmers' preferences and willingness to pay to be estimated and latent class modelling accounted for heterogeneity in responses. The overall willingness to pay to reduce lesions was low at £0.06 per pig place (installation cost) and £0.01 per pig produced (running cost) for each 1% reduction in lesions. Results revealed three independent classes of farmers. Farmers in Class 1 were unlikely to regroup unfamiliar growing/finishing pigs, and thus were unwilling to adopt measures to reduce aggression at regrouping. Farmers in Classes 2 and 3 were willing to adopt measures providing certain pre-conditions were met. Farmers in Class 2 were motivated mainly by business goals, whilst farmers in Class 3 were motivated by both business and animal welfare goals, and were willing to pay the most to reduce aggression; £0.11 per pig place and £0.03 per pig produced for each 1% reduction in lesions. Farmers should not be considered a homogeneous group regarding the adoption of animal welfare innovations. Instead, campaigns should be targeted at subgroups according to their independent preferences and willingness to pay., Competing Interests: The authors have declared that no competing interests exist.

There is considerable interest in how early life experiences shape behavioural development. For example, the socialisation of unfamiliar pigs pre-weaning has been suggested to decrease aggression during later life. However, the behavioural mechanisms behind this socialisation effect remain unexplored. We allowed 12 litters of domestic pigs (Sus scrofa) to move freely between their home pen and a neighbouring pen (socialisation) during the lactation period, while keeping 12 litters isolated in their home pen (control). Contrary to predictions, socialisation did not result in higher levels of social play. However, control individuals engaged in more sow directed play than those that underwent socialisation. Consistent with predictions, males performed more piglet directed play than females. Social play behaviour pre-weaning was found to be highly concordant within individuals from both treatments. Post-weaning, 148 pigs were selected to perform two resident-intruder tests to assay aggressiveness. As predicted, socialised individuals were quicker to attack than controls, although females were more aggressive than males. Additionally, play fighting experience was found to negatively correlate with attack latency in females, supporting the hypothesis that early-life play experience is likely to be sexually dimorphic when males and females show pronounced differences in their later-life social behaviour.

Social conflict is mostly studied in relation to aggression. A more integral approach, including aggressive and affiliative behaviour as well as physiology, may however give a better understanding of the animals' experience during social conflict. The experience of social conflict may also be reflected in the spatial distribution between conspecifics. The objective was to assess the relationship between behaviour, physiology, and spatial integration in pigs (Sus scrofa) during social conflict. Hereto, 64 groups of pigs (9 wk of age) were studied in a 24 h regrouping test whereby pairs of familiar pigs were grouped with 2 unfamiliar pairs, in either barren or straw-enriched housing. Data on aggressive and affiliative behaviour, skin lesions, body weight, and haptoglobin could be summarized into three principal component analysis factors. These three factors were analysed in relation to spatial integration, i.e. inter-individual distances and lying in body contact. Pigs stayed up to 24 h after encounter in closer proximity to the familiar pig than to unfamiliar pigs. Pigs with a high factor 1 score were more inactive, gave little social nosing, had many skin lesions and a high body weight. They tended to space further away from the familiar pig (b = 1.9 cm; P = 0.08) and unfamiliar ones (b = 0.7 cm; P = 0.05). Pigs that were involved in much aggression (factor 2), and that had a strong increase in haptoglobin (factor 3), tended to be relatively most far away from unfamiliar pigs (b = 0.03 times further; P = 0.08). Results on lying in body contact were coherent with results on distances. Pigs in enriched housing spaced further apart than pigs in barren housing (P<0.001). The combined analysis of measures revealed animals that may either promote or slow down group cohesion, which may not have become clear from single parameters. This emphasizes the importance of an integral approach to social conflict.

Indirect Genetic Effects (IGEs), also known as associative effects, are the heritable effects that an individual has on the phenotype of its social partners. Selection for IGEs has been proposed as a method to reduce harmful behaviours, in particular aggression, in livestock and aquaculture. The mechanisms behind IGEs, however, have rarely been studied. The objective was therefore to assess aggression in pigs which were divergently selected for IGEs on growth (IGEg). In a one generation selection experiment, we studied 480 offspring of pigs (Sus scrofa) that were selected for relatively high or low IGEg and housed in homogeneous IGEg groups in either barren or enriched environments. Skin lesion scores, a proxy measure of aggression, and aggressive behaviours were recorded. The two distinct IGEg groups did not differ in number of skin lesions, or in amount of reciprocal fighting, both under stable social conditions and in confrontation with unfamiliar pigs in a 24 h regrouping test. Pigs selected for a positive effect on the growth of their group members, however, performed less non-reciprocal biting and showed considerably less aggression at reunion with familiar group members after they had been separated during a 24 h regrouping test. The enriched environment was associated with more skin lesions but less non-reciprocal biting under stable social conditions. Changes in aggression between pigs selected for IGEg were not influenced by G×E interactions with regard to the level of environmental enrichment. It is likely that selection on IGEg targets a behavioural strategy, rather than a single behavioural trait such as aggressiveness.

Background: Stress, elicited for example by aggressive interactions, has negative effects on various biological functions including immune defence, reproduction, growth, and, in livestock, on product quality. Stress response and aggressiveness are mutually interrelated and show large interindividual variation, partly attributable to genetic factors. In the pig little is known about the molecular-genetic background of the variation in stress responsiveness and aggressiveness. To identify candidate genes we analyzed association of DNA markers in each of ten genes (CRH g.233C>T, CRHR1 c.*866&#95;867insA, CRHBP c.51G>A, POMC c.293&#95;298del, MC2R c.306T>G, NR3C1 c.*2122A>G, AVP c.207A>G, AVPR1B c.1084A>G, UCN g.1329T>C, CRHR2 c.*13T>C) related to the hypothalamic-pituitary-adrenocortical (HPA) axis, one of the main stress-response systems, with various stress- and aggression-related parameters at slaughter. These parameters were: physiological measures of the stress response (plasma concentrations of cortisol, creatine kinase, glucose, and lactate), adrenal weight (which is a parameter reflecting activity of the central branch of the HPA axis over time) and aggressive behaviour (measured by means of lesion scoring) in the context of psychosocial stress of mixing individuals with different aggressive temperament., Results: The SNP NR3C1 c.*2122A>G showed association with cortisol concentration (p = 0.024), adrenal weight (p = 0.003) and aggressive behaviour (front lesion score, p = 0.012; total lesion score p = 0.045). The SNP AVPR1B c.1084A>G showed a highly significant association with aggressive behaviour (middle lesion score, p = 0.007; total lesion score p = 0.003). The SNP UCN g.1329T>C showed association with adrenal weight (p = 0.019) and aggressive behaviour (front lesion score, p = 0.029). The SNP CRH g.233C>T showed a significant association with glucose concentration (p = 0.002), and the polymorphisms POMC c.293&#95;298del and MC2R c.306T>G with adrenal weight (p = 0.027 and p < 0.0001 respectively)., Conclusions: The multiple and consistent associations shown by SNP in NR3C1 and AVPR1B provide convincing evidence for genuine effects of their DNA sequence variation on stress responsiveness and aggressive behaviour. Identification of the causal functional molecular polymorphisms would not only provide markers useful for pig breeding but also insight into the molecular bases of the stress response and aggressive behaviour in general.

Innovations in the breeding and management of pigs are needed to improve the performance and welfare of animals raised in social groups, and in particular to minimise biting and damage to group mates. Depending on the context, social interactions between pigs can be frequent or infrequent, aggressive, or non-aggressive. Injuries or emotional distress may follow. The behaviours leading to damage to conspecifics include progeny savaging, tail, ear or vulva biting, and excessive aggression. In combination with changes in husbandry practices designed to improve living conditions, refined methods of genetic selection may be a solution reducing these behaviours. Knowledge gaps relating to lack of data and limits in statistical analyses have been identified. The originality of this paper lies in its proposal of several statistical methods for common use in analysing and predicting unwanted behaviours, and for genetic use in the breeding context. We focus on models of interaction reflecting the identity and behaviour of group mates which can be applied directly to damaging traits, social network analysis to define new and more integrative traits, and capture-recapture analysis to replace missing data by estimating the probability of behaviours. We provide the rationale for each method and suggest they should be combined for a more accurate estimation of the variation underlying damaging behaviours. [ABSTRACT FROM AUTHOR]

Aggression between unfamiliar pigs at mixing is a major animal welfare problem in commercial farming. It has been studied since the 1970s and remains an important topic in animal welfare research. Methods to reduce pig aggression at mixing have been reviewed previously, but there has been little translation of the advocated techniques and building designs into practice. As a result, the problem persists on many commercial units. A similar situation exists for many other animal welfare issues. This article takes a new approach in not only reviewing the recent scientific literature, but also reviewing the evidence of uptake in industry. Firstly, the current state of aggression mitigation research is reviewed; including the most successful recent developments in breeding against aggression, early life socialisation, the use of pheromones and nutrition. Secondly, information is extracted from both peer reviewed and industry literature to establish the extent to which these strategies have been transferred from research to practice. Finally, we discuss why in spite of the amount of research on reducing aggression at mixing the problem has not reduced in intensive farming systems. The limited uptake in practice appears to be due to low prioritisation of the problem, the practicalities of implementation, lack of information on cost-effectiveness and ineffective communication of research to the farming community. To bridge this gap, industry must be involved in the design of practical solutions and the cost-effectiveness of these must be quantified. This approach should also be considered for other animal welfare issues under investigation. We recommend a better alignment between research questions and industry interests to increase the success of research efforts to improve animal welfare in practice. [ABSTRACT FROM AUTHOR]

Sustainable animal production should take animal welfare into account. How animal welfare is incorporated into farming practices will, however, largely depend on the farmer’s choices. These choices may depend on the farmer’s perception of animal welfare and the willingness to change the current situation. Aggression between group housed pigs is a longstanding welfare issue and research efforts have resulted in little to no change in practice. Our objective was to gain insight into farmers’ perception of aggression between growing pigs and their opinion about a method shown by research to reduce the expression of this behaviour. Pig farmers in the UK were asked about their management and perception regarding aggression through a postal survey. Respondents (n = 167) had a breeder-to-finisher farm (n = 114; 585 ± 123 sows; range 0–7000), breeder-weaner farm (n = 10; 718 ± 433 sows; range 15–45000), or grower/finisher farm (n = 32; 1291 ± 187 grower/finishers; range 24–5000). The majority of the respondents (73%) did not consider aggression at weaning as a problem which needed to be addressed. For mixing at the finisher stage, 43% did consider aggression a problem and indicated they would consider a solution if available. Farmers who considered aggression at the grower/finisher facilities a problem were on average younger (55 ± 12 years) than farmers who did not consider it a problem (61 ± 12 years; P = 0.02). When respondents ranked welfare issues on what they found most important to reduce at the grower/finisher phase, they ranked tail/ear/flank biting as more important than mounting and lameness, but not different from aggression. Currently, 27% of the pig breeders (respondents keeping sows) applied co-mingling of piglets (i.e. ‘socialization’), in which piglets are pre-weaning introduced to piglets of other litters to reduce aggression later on, and 22% had worked with such a strategy in the past. Farmers applying co-mingling did not differ in their perception of aggression compared to farmers who did not co-mingle. Respondents expressed concerns about co-mingling with regard to practical management (48%), aggression of the sow towards piglets (33%), reduced piglet growth rate (24%), fights between piglets (22%), cross-suckling (20%), missed suckling bouts (16%), and stress for the animals (16%). Half of the breeders were in favour of co-mingling (51%) whereas the other half was against (49%). As part of a sustainable approach, integrating all stakeholders, knowledge of farmers’ perception of aggression may help align research questions in this area with the concerns of the stakeholders. [ABSTRACT FROM AUTHOR]

Commercial pigs globally are routinely mixed into new social groups. This results in regrouping aggression predominantly during the first 24 h which compromises welfare and productivity. Chronic aggression persists thereafter and is also undesirable. Management strategies are needed that reduce the costs of aggression in both of these contexts. Pigs vary greatly in aggressive behaviour and numbers of skin lesions. This study examined how regrouping behaviour affects immediate and long-term lesion counts with a specific focus on understanding the behaviour of pigs with few lesions in both social contexts. Aggressive behaviour from 1163 growing pigs was observed for 24 h post-regrouping and fresh lesions were counted 24 h and 3 weeks post-regrouping. Similarity between pigs was calculated using all behavioural traits recorded during the 24 h post-regrouping. Clusters of pigs were formed using furthest neighbour clustering with a stopping rule of 80% similarity. Five clusters of pigs representing 90% of the population (1047 pigs) were identified. For each regrouping aggressive behaviour trait and for fresh lesion counts 24 h post-regrouping the means differed significantly (P < 0.0001) between clusters. The most extreme clusters were characterised by extremely high or low levels of aggression with the other three clusters characterised by pigs that were unaggressive losers, selectively aggressive or with long fights. Statistically significant (P < 0.05–P < 0.001) but numerically small differences between clusters were found in lesion count 3 weeks post-regrouping. Pigs were separately categorised based upon their combination of lesion counts recorded 24 h and 3 weeks post-regrouping. Pigs showing similar behaviour at regrouping displayed wide ranging combinations of acute and chronic lesion outcomes. Pigs with particularly low lesion counts at both regrouping and 3 weeks post-regrouping were found in all 5 clusters. Avoidance of aggressive behaviour at regrouping resulted in few lesions at 24 h but more lesions at 3 weeks. Increasing the proportion of pigs in the population that receive few lesions from both regrouping and chronic aggression may require management strategies that manipulate behaviour in both contexts. Long-term costs of avoiding regrouping aggression, represented by lesion counts three weeks after re-grouping, show that regrouping aggression may retain an important function in domesticated pigs and potentially in other species. [ABSTRACT FROM AUTHOR]

Understanding animal contests has benefited greatly from employing the concept of fighting ability, termed resource-holding potential (RHP), with body size/weight typically used as a proxy. However, victory does not always go to the larger/heavier contestant and the existing RHP approach thereby fails to accurately predict contest outcome. Aggressiveness, typically studied as a personality trait, might explain part of this discrepancy. We investigated whether aggressiveness forms a component of RHP, examining effects on contest outcome, duration and phases, plus physiological measures of costs (lactate and glucose). Furthermore, using the correct theoretical framework, we provide the first study to investigate whether individuals gather and use information on aggressiveness as part of an assessment strategy. Pigs, Sus scrofa , were assessed for aggressiveness in resident–intruder tests whereby attack latency reflects aggressiveness. Contests were then staged between size-matched animals diverging in aggressiveness. Individuals with a short attack latency in the resident–intruder test almost always initiated the first bite and fight in the subsequent contest. However, aggressiveness had no direct effect on contest outcome, whereas bite initiation did lead to winning in contests without an escalated fight. This indirect effect suggests that aggressiveness is not a component of RHP, but rather reflects a signal of intent. Winner and loser aggressiveness did not affect contest duration or its separate phases, suggesting aggressiveness is not part of an assessment strategy. A greater asymmetry in aggressiveness prolonged contest duration and the duration of displaying, which is in a direction contrary to assessment models based on morphological traits. Blood lactate and glucose increased with contest duration and peaked during escalated fights, highlighting the utility of physiological measures as proxies for fight cost. Integrating personality traits into the study of contest behaviour, as illustrated here, will enhance our understanding of the subtleties of agonistic interactions. [ABSTRACT FROM AUTHOR]

Abstract: Chronic social stress in pigs compromises immune function, reduces ADG, increases activity and skin lesions and affects feeding behaviour but to different extents in individuals and contemporary groups housed in the same way. Assessing the animals’ perception of chronic social stress is difficult. A large inter-individual distance has been suggested to indicate acute stress immediately following regrouping. The current study hypothesised that large inter-individual distances between grower pigs in groups whose composition was stable for 6 weeks would also be predictive of the expression of other traits sensitive to chronic social stress. Group sizes (20 and 80) and feeder space allowances (32.5 and 42.5mm/pig) were studied in a 2×2 factorial design replicated four times to simulate commercially relevant social environments. Inter-individual distances were calculated for individual resting pigs and defined as the mean distance to the nearest resting neighbour (MNND) and mean distance to all other resting group members (MD). Relationships between MNND or MD and potential stress response variates were examined by appropriate linear or generalised linear mixed models. The mean growth rate was poorer in large groups and the mean number of skin lesions was greater in pens with a small feeding space (P <0.05), suggesting that these treatments may have increased social tension, but the effects on the mean proportion of activity (P <0.001) and feeding behaviour (P <0.05) were not in the expected direction. Pigs which spaced at large distances from other group members had high ADG (P <0.05). However, large distances between pigs were associated with a higher mean level of activity (P <0.001) and a lower mean frequency of feeding bouts (P <0.05) and total feeding duration (P <0.05), as would be predicted if spacing behaviour was indicative of social stress. Furthermore, fewer lesions were found when pigs rested at a large distance from their nearest neighbour on days 3 and 21 post-mixing (P <0.05) but, on average, maintained close proximity to other group members in general on day 3 (P <0.05), suggesting that the group as a whole was more tightly clustered. Spacing behaviour, even in the confined conditions of commercial indoor environments, appears to relate to some measures of chronic stress, although the relationship with skin lesions was complex and that with ADG was not in the expected direction. Measuring spatial responses to activation of neuroendocrine stress response pathways or modifying affective state through anxiolytic drug administration could help to validate how spacing behaviour reflects the perception of chronic social stress. [Copyright &y& Elsevier]

Abstract: Harmful social behaviours are prevalent in commercial farming environments and their reduction through economically feasible husbandry changes is challenging. Selective breeding may offer a complementary approach to reduce the expression of these traits. This article explores the progress made in estimating the genetic contribution to harmful social behaviours, the likely consequences of selection on these for production or welfare relevant traits, and the barriers that constrain uptake of selection and how these may be overcome. Significant but often low heritabilities have been reported for all of the major harmful social behavioural traits which may reflect the difficulty of their measurement (pig aggression 0.04–0.43, tail biting 0.05–0.27 and savaging of piglets 0.02–0.9; feather pecking in layers 0.11–0.20). The need for sensitive, easily measured indicators of the likelihood of showing harmful behaviour and estimation of the economic and non-economic benefits of reducing the expression of these traits remain significant constraints to their selection. Routine genotyping and the use of this information for genome-wide selection is advancing rapidly for some species. This methodology is particularly attractive for traits that are difficult or costly to measure and may provide a way of selecting against harmful social behaviours. However, past selection on economic traits may have contributed to the expression of harmful behaviours which could be accelerated by the use of genomic information to more rapidly improve economic traits. Genome-wide selection therefore poses both a threat to welfare and significant possibilities to improve welfare depending on how it is used. Alternatively, selection on associative effects could allow the development of animals that coexist without depressing the growth performance of group members. As effects on group member growth probably operate through social interactions, this may offer an alternative route through which social behaviour can be improved without the need for behavioural phenotyping. Understanding precisely how behaviour will be affected by this approach is crucial and now beginning. Harmful social behaviours ought to respond to selection, albeit slowly, and are the kind of trait likely to benefit most from the carefully managed use of emerging breeding methods, with potential benefits for welfare. [Copyright &y& Elsevier]

Abstract: Wild pigs (Sus scrofa) are only territorial during the peri-parturient period and when young compete for access to teats. In other species, territorial defence is affected by contextual factors including resource quality. Territoriality could be stimulated in species such as pigs by artificial mixing and intensive housing. Test pigs were introduced to a novel opponent for 5min on consecutive days (days 1 and 2) in two locations (home pen of the test pig (P) or a neutral arena (A)) in a randomised block design. On day 1, animals tested in their home pen were more likely to initiate an escalated attack (proportion of pigs 0.43 vs 0.19 in the pen and neutral arena, P <0.01). A shorter latency to attack on day 2 in the PP (pen on both days) and PA (pen then arena) treatments suggested that a previous encounter in the home pen affected later aggressiveness. The time spent engaged in non-escalated aggressive behaviours (parallel walking, pushing, knocking and biting) with the opponent was similar in the two environments. Some capacity for territorial defence through escalated aggression and the enhancement of subsequent aggressiveness appears to occur in pigs when in their home pen. The suppression of escalated aggression in the neutral arena by handling and environmental exploration cannot be discounted, but appears not to have affected the occurrence of non-escalated aggression. The mixing of pigs in an occupied pen is likely to enhance escalated aggression from the occupants without reducing that from the intruders and should be avoided. [ABSTRACT FROM AUTHOR]

Immature domestic pigs are increasingly housed in large social groups (>50 pigs) without an understanding of how they adapt to these conditions. At a conventional group size (typically 10–30 animals), pigs exhibit dominance relationships which are frequently established using physical assessments of strength and fighting ability and may include the use of aggressive behaviour. The relationships also rely on individual identification of group members and the recall and reinforcement of previous fight outcomes. This paper reviews the implications of a large group size for the establishment and long-term maintenance of dominance relationships in immature pigs. Evidence suggests that, in large groups, less reliance is placed on aggression during the immediate post-mixing phase than would be expected if the same proportion of dominance relationships are established using aggression in both large and small groups. This occurred despite an ability to distinguish between pen mates and non-pen mates. The energetic costs to a pig of establishing dominance relationships with a given proportion of group members is likely to increase with group size. It is considered whether these elevated costs place a ceiling on the number of fights a pig can engage in or whether they are able to assess dominance ability using passive means and thereby reduce the proportion of group members with which they fight. It is probable that a combination of these two routes is responsible for the limitation of aggression observed. Sub-grouping behaviour, investigated in large poultry groups, appears not to be adopted by pigs in response to a large group size. There is a need for further fundamental research to investigate the way in which pigs adapt behaviourally to a large group size at the level of the individual pig and several potentially fruitful investigations are suggested. [Copyright &y& Elsevier]

Simple Summary: Aggression between pigs in pig husbandry is common during regrouping but rare in stable social groups. Farmers report the occurrence of lethal gang aggression in stable groups of pigs, whereby the group attacks one group member until it is dead. Our aim was to document this extreme type of aggression and to identify potential causes. Forty-two farmers, experiencing lethal gang aggression or not, filled out a survey about their farm management. From 91 victims, information was obtained on their injuries and body condition. Gang aggression occurred more on farms with deep straw bedding, a housing type commonly associated with better animal welfare. However, the presence of straw may also be related to other factors, which could not be disentangled here. Gang aggression did not relate to the genetic line, breeding company or feed type. It equally occurred between females and males and tended to occur more in winter. Victims were covered in injuries, but had a healthy body condition, whereas survivors had a lower body condition. Overall, the cause seems multi-factorial, and further research on the occurrence of lethal gang aggression is needed. Intraspecific coalitional aggression is rare among all species, especially within stable social groups. We report here numerous cases of intraspecific lethal gang aggression within stable groups of domestic pigs. The objective was to describe this extreme aggression and to identify potential causes. Management data were collected from farms with (n = 23) and without (n = 19) gang aggression. From one farm, 91 victims were assessed for skin injuries and body condition score. Lethal gang aggression was significantly associated with deep straw bedding, which may be related to various other factors. Gang aggression tended to occur more in winter, and was unrelated to genetic line, breeding company, group size or feed type. It occurred equally in female-only and mixed sex groups (male-only groups were not represented), from around eight weeks of age. Injuries typically covered the whole body and were more severe on the front of the body. Victims who survived had a lower body condition score and fewer injuries than victims found dead. There are still many unknowns as to why this abnormal social behaviour occurs and it deserves further research attention, both for its applied relevance to animal welfare as for the evolutionary background of lethal gang aggression. [ABSTRACT FROM AUTHOR]

Simple Summary: Aggression between pigs is a major animal welfare issue in commercial farming, however only a minority of farmers believe that aggression is a problem that needs to be addressed. We investigated whether the farmers' reluctance to reduce aggression is linked to desensitization as a result of their frequent exposure to the behavior. We showed farmers video clips of pigs during and immediately after a fight and they judged through a questionnaire the severity of what they saw. These judgments were compared to (a) animal-based measures of injury (skin lesions) and exhaustion (blood lactate), and (b) human observers with and without experience of working with pigs. Farmers perceived fights as severe and were motivated to prevent them continuing. They were not desensitized to aggression as their judgments were similar to those of participants who had never worked with pigs. When farmers (and comparison groups) did not see the fight occurring, they judged exhaustion and injuries to be lower than indicated by the animal-based measures. Farmers could benefit from information on how to better assess the impact of aggression by scoring lesions and from evidence of the economic and welfare impact of these lesions. Several animal welfare issues persist in practice despite extensive research which has been linked to the unwillingness of stakeholders to make changes. For example, most farmers do not perceive pig aggression to be a problem that requires action despite the fact that stress and injuries are common, and that several solutions exist. Frequent exposure to animal suffering could affect farmer responses to distressed animals. This study investigated for the first time whether this occurs, using pig aggression as a focus. Using video clips, 90 pig farmers judged the severity of aggression, level of pig exhaustion and the strength of their own emotional response. Their judgments were compared to objective measures of severity (pigs' skin lesions and blood lactate), and against control groups with similar pig experience (10 pig veterinarians) and without experience (26 agricultural students; 24 animal science students). Famers did not show desensitization to aggression. However, all groups underestimated the outcome of aggression when they did not see the fight occurring as compared to witnessing a fight in progress. We suggest that farmers be provided with evidence of the economic and welfare impact of aggression as indicated by lesions and that they be advised to score lesions on affected animals. [ABSTRACT FROM AUTHOR]

Simple Summary: Pig welfare is a societal concern, partly due to the intensive rearing conditions. One welfare concern is aggression between newly regrouped pigs. Aggression at weaning is reduced by putting several groups of piglets together when young; termed socialization. Information is missing about how socialization affects sow health and if the behavioural changes in piglets are long-lasting. We aimed to address these questions by studying sow udder quality and pig behaviour and growth in socialized and control groups. Pigs were socialized by either joining two litters (32 sows; 16 groups) at 14 days of age or not joining them (33 sows). At weaning, the sows of socialized groups had more udder damage than the controls. Socialized piglets had double the amount of bite injuries (skin lesions) than controls the day after socialization, but had 19% fewer skin lesions at regrouping at eight weeks old when injuries are more numerous and severe. At 11 weeks old, there was no difference between the groups. In a test for aggressiveness, socialized pigs attacked more often and quicker, showing greater confidence in agonistic skills. Socialization means additional work for farmers and may cause more udder damage, but has beneficial effects for pig behaviour and welfare at later regrouping. Early life socialization of piglets has been shown to reduce piglet aggression at weaning, but information on sow health and long-term benefits is lacking. We aimed to assess how socialization impacts sow udder quality and long-term pig behaviour and growth. At two weeks of age, 65 litters either experienced socialization with one other litter (SOC) or did not (control; CON). Sows (housed in farrowing crates) were scored for teat damage and piglets were observed for aggressive behaviour (resident-intruder test) and growth and skin lesions up to 11 weeks under conventional farm conditions (including weaning and regrouping). At weaning, SOC sows had more teat damage than CON sows (p = 0.04). SOC piglets had double the number of lesions 24 h post-socialization compared to the control (19 versus 8; p < 0.001). In the resident-intruder test, more SOC pigs attacked the intruder (SOC 78%; CON 66%; p < 0.01), and attacked more quickly (p = 0.01). During regrouping (week 8), SOC pigs had 19% fewer lesions (SOC 68; CON 84; p < 0.05), but three weeks later, groups did not differ. Growth was unaffected by treatment. Overall, socialized piglets seem to be equipped with greater confidence or agonistic skills, leading to fewer injuries from fighting up to at least six weeks after socialization. [ABSTRACT FROM AUTHOR]

Animal contests are costly and predicted to be won by the individual with the higher resource-holding potential (RHP). Weight is often used a proxy for RHP; however, victory does not always go to the heaviest competitor, indicating that other traits may also contribute to RHP. Here we investigated the effect of preweaning play-fighting experience on postweaning contest behaviour in the domestic pig, Sus scrofa. We predicted that individuals that played more would win contests later in life. Play-fighting experience was also predicted to influence contest escalation behaviour, on both an individual and a dyadic level. Lastly, a pre-established framework and eight contest cost measures were used to establish whether competing individuals gather/use information regarding play-fighting experience as part of an assessment strategy. Play-fighting experience was recorded for socialized and control litters before weaning and contests were staged between unfamiliar pigs of the same treatment after weaning. Controlling for competitor weight difference revealed that increased play-fighting experience was linked to contest success in females, while the opposite pattern was found in males. Play-fighting experience did not influence which individual within the dyad escalated contest behaviour, but dyads containing more experienced losers were more likely to perform a stage of nondamaging aggression. When we used skin lesions in losers as a measure of contest cost, we found evidence for the role of play-fighting experience in a novel mutual assessment strategy in socialized dyads, whereas control dyads performed opponent-only assessment. We suggest that while assessments of RHP can be made using a physical correlate of play-fighting experience (such as skillfulness), early life socialization is required for individuals to gain an understanding of their own RHP. • Contest outcome is influenced by preweaning play-fighting experience. • Female winners had more play-fighting experience than female losers. • Male winners had less play-fighting experience than male losers. • Play experience can be used to determine assessment strategy. • Socialized pigs use novel mutual assessment; controls use opponent assessment. [ABSTRACT FROM AUTHOR]

Infrared thermography (IRT) or thermal imaging is increasingly being used as a non-invasive method to gain information on animals' physiological and emotional state. IRT has the potential to serve as a non-invasive quantitative assessment method but few studies have examined its utility in predicting welfare-relevant outcomes of dynamic scenarios relevant to commercial farming. This study used 1284 thermal images taken from 46 pigs in a controlled test environment while they engaged in an agonistic encounter (dyadic contest) at 13 wk. of age. Images were taken of the complete body from a dorsal perspective. A pilot study indicated that a rectangular thermal window on the back region was the most suitable and reliable area for obtaining temperature data in this situation. From this thermal window, the average, minimum and maximum temperature, standard deviation and coefficient of variation (CV) were obtained. These were analysed in relation to contest phase (from non-contact assessment, through escalated fighting to retreat), fight occurrence, contest duration, contest outcome (winner/loser status) and changes in blood glucose, blood lactate, and skin injuries. Variables showed a strong change in response to the moment of contest resolution (retreat of the loser); temperatures reduced sharply and CV increased, but did not differ between winners and losers. Contests that included a fight showed lower temperatures. Contest duration, body weight and sex only had minor influences on the temperatures. As the drop in temperature at contest resolution was irrespective of contest intensity, and the pattern was similar in winners and losers, this data potentially reflects vasoconstriction as a result of psychological stress rather than solely a physiological change. The study shows that peripheral temperature, as recorded by IRT, responds to the intensity and phases of a contest and may allow new insight into the physiological and welfare outcomes of aggressive behaviour. • Thermal images of pigs in dyadic contests were related to physiology and behaviour. • Temperature dropped at contest resolution, regardless of fight duration. • Results are in line with the hypothesis of a thermal response to emotional state. • Winners and losers did not differ in thermal profile, suggesting both are affected. • The possibility of emotional arousal rather than valence deserves further research. [ABSTRACT FROM AUTHOR]

Animal contests vary greatly in behavioural tactics used and intensity reached, with some encounters resolved without physical contact while others escalate to damaging fighting. However, the reasons for such variation remains to be fully explained. Aggressiveness, in terms of a personality trait, offers a potentially important source of variation that has typically been overlooked. Therefore, we studied how aggressiveness as a personality trait influenced escalation between contestants matched for resource-holding potential (RHP), using detailed observations of the contest behaviour, contest dynamics and escalation levels. We predicted that winner and loser behaviour would differ depending on personality. This was tested by examining 52 dyadic contests between pigs, Sus scrofa . Aggressiveness was assayed in resident–intruder tests prior to the contest. Contests were then staged between pigs matched for RHP in terms of body weight but differing in their aggressiveness. In 27% of the contests a winner emerged without escalated physical fighting, demonstrating that a fight is not a prerequisite between RHP-matched contestants. However, the duration of contests with or without fighting was the same. In contests without a fight, opponents spent more time on mutual investigation and noncontact displays such as parallel walking, which suggests that ritualized display may facilitate assessment and decision making. Winners low in aggressiveness invested more time in opponent investigation and display and showed substantially less aggression towards the loser after its retreat compared to aggressive winners. Aggressiveness influenced contest dynamics but did not predict the level of escalation. Prominent behavioural differences were found for the interaction between personality and outcome and we therefore recommend including this interaction in models where personality is considered. Analyses based on contest duration only would miss many of the subtleties shown here and we therefore encourage more detailed analyses of animal contests, irrespective of the level of contest escalation. [ABSTRACT FROM AUTHOR]

Aggression between pigs causes injuries and production losses and is a long standing animal welfare issue. Although the physiological impact of aggression has been well described, little is known about the emotional experience of aggressive interactions. Our aim was to investigate the emotional expression of winners and losers after a fight and how this relates to costs of fighting. Emotions were studied through use of Qualitative Behavioural Assessment (QBA), a method where participants qualitatively assess the emotional expression of animals seen live or on video. Eighteen pig farmers watched 28 short video clips of pigs which had just won (n = 14) or lost (n = 14) a fight. Farmers rated the pigs’ emotions based on a pre-existing list with 21 descriptors of emotions, while being unaware of the contest outcome (winner/loser). Scores were analysed by a Principal Component Analysis (PCA), which resulted in two factors combining the 21 descriptors into four expressive quadrants. Factor 1 ranged from relaxed/content to tense/frustrated, thereby describing valence (explaining 43% of total variance), and factor 2 ranged from active/lively to listless/indifferent, describing arousal (explaining 16%). Winners (W) and losers (L) did not significantly differ in their expression of valence (W −0.19 ± −0.20;  L 0.16 ± 0.17; P = 0.16) or arousal separately (W −0.07 ± 0.22; L 0.06 ± 0.18; P = 0.51), but did in the valence-arousal interaction ( P = 0.02). In winners a high valence related to low arousal whereas in losers high valence related to high arousal. In addition, winners were observed as more negatively affected than losers by a high number of skin lesions ( P < 0.01). QBA scores significantly correlated with skin lesions (skin lesions positively correlated with 12 descriptive QBA terms reflecting impaired welfare), blood lactate (curious r = −0.41; lively r = −0.44; playful r = −0.40; positively occupied r = −0.39), blood glucose (distressed r = 0.40; fearful r = 0.39; playful r = −0.38) and the contest duration (sociable r = −0.39) (all P < 0.05). This shows that skin lesions not only reflect physical injury but can also be associated with a negative emotional state, which adds value to their use as a welfare assessment tool. The use of QBA in this study sheds light on the complex ways in which animals emotionally perceive aggression and physical injury. Further studies of this kind will enable better understanding of the true welfare impact of aggressive interactions. [ABSTRACT FROM AUTHOR]

Aggression between unfamiliar commercial pigs is common and likely invokes strong emotions in contestants. Furthermore, contest outcomes affect subsequent aggressive behaviour, suggesting a potential lasting influence on affective state. Here we used a combination of qualitative and quantitative methods to assess the emotional expression of pigs in agonistic encounters. We investigated how recent victory or defeat influences emotions expressed in a subsequent contest, and the role of aggressiveness as a personality trait in emotional expression. We observed the pre-escalation contest behaviour (second contest; age 13 wks) in animals of different aggressiveness (categorised using two resident intruder tests as Agg+ or Agg-, age 9 wks), which had recently won or lost a contest (first contest; 10 wks). We measured gaze direction and ear position. Observers watched video clips of the initial 30 s of the second contest and evaluated the emotional expression of 57 pigs (25 contest 1 winners, 32 contest 1 losers) using qualitative behavioural assessment (QBA) with a fixed list of 20 descriptive terms. QBA identified three principal components (PCs), accounting for 68% of the variation: PC1 (agitated/tense to relaxed/content), PC2 (fearful/aimless to confident/enjoying) and PC3 (listless/ indifferent). Agg- pigs and males showed a more positive emotionality (PC2). PC1 and PC3 were unaffected by first contest outcome and aggressiveness. Agg+ pigs were more likely to hold their ears back (X2 =7.8, p = 0.005) during the early contest period. Differences in attention were detected in the contest outcome × aggressiveness interaction (χ24.3, p = 0.04), whereby approaching the opponent was influenced by winning and losing in the Agg- pigs only. QBA and gaze behaviour reveal differences in emotional valence between pigs of different aggressiveness: less aggressive pigs may be more susceptible to the emotional impact of victory and defeat but overall, more aggressive pigs express more negative emotionality at the start of agonistic encounters. [Display omitted] • The emotional expression of pigs meeting a new opponent is influenced by past contest experience. • Prior losers gazed towards the exit gate more than winners. • Aggressive personality influenced ear posture, gaze direction and emotional expressivity. [ABSTRACT FROM AUTHOR]