Aggression in social animals: Effects of group size resource holding potential and costs of fighting on the outcome of battles

Lead Research Organisation: University of Plymouth
Department Name: Sch of Marine Science & Engineering


Animals of the same species often come into conflict with one another over ownership of essential resources, such as food, shelter, territory or mates, which are essential for ensuring their survival and chance to reproduce. These conflicts may occur between pairs of individual animals and result in a 'fight', or they may occur between groups of individual animals and result in a 'battle'. This is likely to happen when groups of social animals, such as humans, apes or social insects come into conflict, usually over the ownership of territory. Many studies have investigated the resolution of fights but far less is known about the factors that determine victory in battles. In fights an individual's fighting ability is the main factor that determines whether it will win. In battles the number of individuals in each group should have a strong effect on which group wins, but the effect of differences in the fighting ability of the individuals that make up each group is not well understood. Also, the way in which individuals engage one another within the battle might influence the contribution that group size and fighting ability make to the chance of winning. The aim of this study is to address these questions by investigating battles in a social insect, the red wood ant Formica rufa. These ants are common in the southern UK but their status is less secure across the whole of their European range. To investigate the ways that fighting ability and fighting behaviour influence how strongly differences in group size should affect the outcome of battles, we will use approaches that have been developed during studies of fights between pairs of individual animals. These will include measuring body size, weapon size and the energy status of combatants in opposing groups. By studying this species of ant we aim to draw conclusions of general relevance that will improve our understanding of aggressive behaviour in animals in general and particularly of aggression between groups. The species chose is also classed as 'vulnerable' by the IUCN and knowing more about its biology and behaviour may aid in the conservation of this species that is native to the UK.

Technical Summary

Much theoretical and empirical effort has been devoted to understanding how factors such as RHP and motivation should influence aggressive behaviour in animals, from the view-points of both contest dynamics and contest outcomes. The vast majority of this effort has been focussed on understanding pair-wise agonistic encounters or 'fights'. Empirical studies have focussed on analysing the effects of variation in resource value and RHP on the outcome of these encounters. Variables analysed include body size, strength, weapon size and energetic status of the opponents. However, contest behaviour is not restricted to pair-wise encounters and in social animals may occur between opposing groups. Models predicting the effects of group size and average RHP of group members were developed for human conflict and have been applied to cases of animal group conflict. These models were recently extended with the aim of accounting for key differences in the effects of RHP expected in human and animal conflicts. The primary aim of the proposed work is to test the assumptions and predictions of the updated theoretical framework. The secondary aim is to advance our understanding of group-contests by extending approaches developed for the analysis of pair-wise contests to the case of group contests. This will involve analysis of key components of individual RHP, including weapon size and energetic status, and characterisation of individual agonistic encounters within the overall group-contest in terms of key theoretical models of pair-wise encounters. We will also generate new ESS models of group conlifct, which will be informed by the findings of these studies.


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Description When groups of animals fight one another there are various different ways in which the numerically larger group could use its size advantage. If they co-ordinate their attacks against the outnumbered opponent they will be able to use their larger number to great effect. Our experiments on wood ants show that this level of coordination is only partially present. Although larger groups are more likely to win, small groups offset this to an extent by fighting harder. Furthermore, the total advantage of the larger group is simply the sum of the individual fighting abilities (determined mainly by body size) of its extra members - not the square of its size advantage as we would expect if attacks were well coordinated. This project is relevant to our understanding of the evolution of combat between groups - colonies of ants invest in large numbers of individuals but our results indicate that this should be balanced against investing in individuals that are strong enough to fight well. These results could impact on the theory of combat as well as on how we view inter-group combat in other animals including sea anemones, other species of ant, primates and perhaps even humans. The work is also relevant to forestry and agriculture as wood ants have been proposed as a biological pest control species. Understanding how rival colonies interact with one another will be useful in this regard.
Exploitation Route We are using our findings on empirical experiments on ant contests to parameterise some new models of multi-party contest behaviour with a collaborator.
Sectors Aerospace, Defence and Marine,Agriculture, Food and Drink

Description Giacomo Santini 
Organisation University of Florence
Country Italy 
Sector Academic/University 
PI Contribution We have provided data for the parameterisation of new models for multi-party / intergroup combat in animals. We will contribute to a joint publication on this (in prep)
Collaborator Contribution The partner has expertise in mathematical modelling and has helped us to refine our experimental designs so that they will yield useful results in this regard. The partner is a co-author on one of the papers arising from this project.
Impact Batchelor TP, Santini G & Briffa M. 2012. Size distribution and battles in wood ants: Group resource holding potential is the sum of the individual parts. ANIMAL BEHAVIOUR 83: 111-117. Santini G, Frizzi F, Bagnoli F, Chelazzi G, Briffa M. In prep. Group combat in ants: testing Lanchester's laws in battles between native and invasive species
Start Year 2010