Understanding assessment strategies during aggressive encounters in pigs to improve welfare following regrouping.

Global demand for pig meat is increasing, with over one billion pigs slaughtered annually. Production has become increasingly intensive with most pigs in the UK and EU being raised indoors and reaching slaughter weight within 6 months. During this production cycle, regrouping of unfamiliar pigs is common practice, typically occurring several times during a pig's life. This sudden mixing of unfamiliar pigs represents a major animal welfare concern.

The social structure of domestic pigs is based on a dominance hierarchy. In the wild, migration between social groups occurs gradually, and hierarchies are formed with minimal aggression. In contrast, when pigs are mixed into new groups (regrouped) under commercial conditions, dominance hierarchies are formed through vigorous fighting, with many pigs receiving 100 or more skin scratches caused by biting. This is stressful for pigs, resulting in injuries, increased risk of infection, and reduced weight gain. This proposal is aimed at addressing the problem of regrouping aggression in pigs.

The information gathering and decision making processes used by pigs to resolve aggressive encounters are poorly understood. However, there is a large body of research on contest behaviour in numerous other animal species. This work has benefitted greatly from the application of theoretical models that are based on particular information gathering rules. Two classes of model have been developed that differ in the strategies used during contests. In the first class, termed self assessment, animals make fight decisions based purely on their own fighting ability and stamina, using internal cues and without reference to the fighting ability of an opponent. After a certain (threshold) amount of energy has been spent on fighting, the individual will give up, and the opponent with the lower threshold will be the loser. In the second class of model, termed mutual assessment, animals self assess but also use information about the fighting ability of an opponent. Although more complex, it has greater benefits as an animal can quickly withdraw from a fight it is likely to lose.

Whether pigs settle fights using a self or mutual assessment strategy has major implications for the degree of escalation. With self assessment animals will always fight up to a threshold, whereas with mutual assessment the use of information about the opponent will allow the weaker animal to quickly abandon a fight it is likely to lose. By examining aggressive interactions between pairs of unfamiliar pigs and employing a suitable statistical framework, this proposal will identify the assessment strategy used.

Similar to human personalities, pigs show individual differences in aggressiveness, with some individuals being more aggressive than others. How these differences influence fight outcomes and the types of assessment detailed above is currently unknown. This will be revealed using a technique to measure aggressiveness. Additionally, the importance of fight experience for developing mutual assessment ability will be examined by comparing pigs that have experienced regrouping in comparison to individuals with no prior fighting experience.

An important aspect of this project is aimed at optimising the assessment abilities of pigs to minimise levels of aggression when regrouped. We will investigate whether socialising piglets during early life, by allowing adjacent litters to mix prior to weaning, equips pigs with the necessary skills for mutual assessment during aggressive encounters later in life.

Finally, the above research outcomes will be translated to the regrouping situation. Group composition will be manipulated to optimise the capability of individuals for mutual assessment. This will facilitate hierarchy formation with minimal fighting. Achieving this objective and translating this knowledge to industry has the potential to improve the welfare of the vast majority of commercially produced pigs.

Regrouping of unfamiliar pigs occurs routinely during production, resulting in aggressive behaviour, with dominance hierarchies formed through vigorous fighting. This practice is a considerable welfare concern. However, the information gathering and decision making processes used by pigs to resolve such encounters are poorly understood and there is little knowledge regarding how to facilitate hierarchy formation in a manner that could be commercially adopted.

We detail a game theoretical approach to identify the assessments made by aggressively interacting pigs. Contest models fall into two broad categories: Self assessment models, presume that each contestant has some knowledge of its own fighting ability, termed resource holding potential (RHP) but gathers no information about the opponent. Mutual assessment models involve individuals gathering information concerning the fighting ability of the opponent and comparing this against their own ability. Existing work on pig aggression has failed to consider self assessment. The assessment strategy used has major implications for the costs of fights, with self assessment predicting that animals should always fight up to a threshold of costs whereas mutual assessment predicts that the use of information about the opponent will markedly reduce fight costs. Moreover, consistent individual differences in aggressiveness are well recognised in pigs but how these differences interact with RHP and assessment strategies is unknown. We will: (i) examine how aggressiveness influences RHP, (ii) determine the assessment strategy used, (iii) examine how fight experience influences assessment ability, and (iv) optimise assessment ability through early life socialisation. Finally, (v) we will translate these research findings to the regrouping scenario, manipulating group composition at mixing with respect to RHP, aggressiveness and previous socialisation to facilitate rapid hierarchy formation at minimal fight cost.

Who will benefit from this research?

Managed animals and animal welfare: The aggression that follows regrouping of unfamiliar pigs is a significant welfare concern, and addressing this problem will be of major benefit to individual animals.

Pig Producers: By engaging with industry representatives, both nationally (e.g. BPEX, QMS) and internationally (e.g. Danish Pig Research Centre) as well as targeting individual producers with specific events, research findings will be effectively translated.

Welfare accreditation schemes: The findings of this proposal will help deliver the goals and expectations outlined in a number of welfare accreditation schemes and codes of practice.

Research staff: The named researcher and technicians will gain valuable transferable generic skills related to the project.

Wider public: Consumers are increasingly aware of animal welfare problems associated with the housing and management of animals in intensive agriculture.

How will they benefit from this research?

Managed animals and animal welfare: This proposal addresses fundamental knowledge gaps in the understanding of the information gathering and decision making processes underlying aggressive interactions between unfamiliar pigs. Translating this acquired knowledge to reduce fighting amongst regrouped pigs has the potential to improve the welfare of the vast majority of commercially produced pigs, given that regrouping is common practice and involves animals at all stages of the production cycle.

Pig Producers: Negative consequences of regrouping include; injuries, reduced weight gain, poorer food conversion efficiency, immunosuppression, a heightened risk of infection and lameness and, in sows, compromised foetal implantation. These are not only welfare concerns but represent a significant economic burden for producers. By reducing regrouping aggression, the present proposal will therefore enhance the economic performance of production systems. This is especially important in an industry with traditionally low and variable profit margins that does not receive subsidy support. Moreover, the associated improvements in animal welfare can be promoted by the industry to improve public attitudes towards this agricultural sector.

Welfare accreditation schemes: A number of farm and abattoir accreditation schemes that certify welfare standards measure outcomes of aggression as a key component (e.g. Red Tractor, RSPCA Freedom Foods, all major retailer codes). The Defra Codes of Practice on Welfare of Pigs and the EFSA Report of the Scientific Veterinary Committee also require actions to be taken to minimise fighting. None of these auditing schemes or codes provide adequate advice on the actions that can best minimise fighting, either to farmers, abattoir managers or legislators. The impact of such schemes will be strengthened by our findings, providing advice on how the goal of reducing fighting can be achieved in a manner that is commercially applicable. Research findings will also be of interest to animal welfare charities (e.g. RSPCA), pressure groups (e.g. CIWF), expert groups that advise policy makers (The Farm Animal Welfare Committee, European Food Standards Agency) and policy makers themselves (e.g. Defra).

Research staff: Will gain useful skills in experimental design, data collection, analysis and manuscript preparation, as well as developing oral communications skills through presentations to a variety of audiences. Benefits will also be gained from working for SRUC and being based within the Roslin Institute Building. Career development will benefit from being exposed to this environment, which provides the ideal opportunity for networking and gaining knowledge.

Wider public: By reduced regrouping aggression, this proposal will have a positive effect on pig welfare which will be of interest to citizens concerned with the welfare of animals in intensive agriculture.