Determining how cognitive ability and affective state impact assessment strategies during aggressive contests to improve pig welfare after regrouping

Globally one billion pigs are slaughtered annually and most pigs in the UK and EU are raised in intensive indoor systems. Regrouping of unfamiliar pigs is common practice and occurs several times during a pig's life. This sudden mixing of unfamiliar pigs is 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. Increased risk of infection, and reduced weight gain also occur. This proposal aims to address the problem of regrouping aggression in pigs but it is expected to be of benefit in understanding aggressiveness in a wide range of species.

Great variation exists in aggressiveness between individual pigs. At present we know little about the causes of this variation and therefore cannot prevent the expression of extreme aggression. Aggressive contests demand that animals make rapid and well-informed decisions, which requires that they assimilate and process complex information and turn it into knowledge (a skill termed cognitive ability). The mood of an animal before an aggressive encounter also likely determines its aggressive behaviour and the emotional response to winning or losing an encounter likely affect its subsequent aggressiveness. However, although cognitive ability and emotional state probably contribute substantially to differences between individuals in their aggressive behaviour and the injuries they receive, this has never been tested in any species. Here we quantify the importance of these factors in the behaviour shown during contests. In the field of behavioural ecology, understanding of contest behaviour has benefitted greatly from the use 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, without reference to the fighting ability of an opponent. After a threshold amount of energy has been spent on fighting, the individual will give up. 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 and substantially reduce the amount of injuries. This project tests the hypotheses that (i) cognitively advanced pigs win encounters and make greater use of mutual assessment with reduced injuries from fights; (ii) that a positive emotional state before a fight inflates the animal's view of its own fighting ability and buffers the effect of defeat; and (iii) that losing a fight has a more negative effect on the emotional state of an aggressive pig than a less aggressive one and this emotional response influences later aggressiveness. Finally, we will regroup pigs in a commercially-realistic way (groups of 12 animals) to test whether cognitive ability determines contest costs in the real world. Here we expect that a cognitively advanced pig is able to minimise fight costs to itself and others whilst suffering no penalty in dominance. Throughout, the project will maximise variation in cognitive ability and emotional state by varying the amount of early-life social and physical enrichment the animals receive. We will therefore test whether reductions in the costs of aggression as a result of enhancing cognitive ability and emotional state can be stimulated by management changes. If so, translating these messages to industry could benefit the majority of commercially produced pigs.

Technical Summary

Technical summary

Aggression between unfamiliar pigs is a major welfare concern. Aggressiveness varies greatly between individuals within many species, the causes of which are poorly understood, limiting our ability to reduce negative welfare impacts. Cognitive ability and affective state are hypothesised to influence information gathering and use during contests, fighting ability (termed resource holding potential; RHP) and contest costs. However, this has never been tested in any species. Here we use pigs as a model system, employing a game theoretical approach to quantify the role of cognitive ability and affective state in contest behaviour. We predict that these factors influence the assessment strategies used during contests. These consist of two broad types: In self assessment models each contestant has knowledge of its own RHP but not that of the opponent. In mutual assessment models individuals compare opponent fighting ability against their own leading to a marked reduction in fight costs. Work suggests that the mutual assessment strategy must be learnt through experience. Here we test: (Obj. 1) if cognition and affect determine success and therefore RHP in a dyadic contest, predicting that cognitively advanced pigs and those with a more positively valenced affective state will win; (Obj. 2) that cognitively advanced pigs make more use of mutual assessment and a positive affective state inflates assessment of own RHP; (Obj. 3) that the affective response to defeat is greater in aggressive pigs and reduces later aggressiveness; and (Obj. 4) that cognitive ability determines contest costs in a commercial group mixing scenario. Obj. 4 is crucial as it predicts that cognitively advanced pigs secure a position in an aggression social network under commercial conditions that minimises fight costs to itself and others but suffers no penalty in dominance. Lastly, we will demonstrate how early life social and physical enrichment can benefit such positive outcomes.