Personality and predictability in fallow deer fighting behaviour: the relationship with mating success

•We studied a free-ranging fallow deer herd during the rut.•We investigated intraindividual variability (IIV) in escalating to fighting.•Individuals differed in tendency to escalate but little support for ‘personality’.•IIV of individual males was similar over consecutive ruts.•Intermediate levels of IIV were associated with higher mating success.

Individuals often differ from each other in how they respond to environmental events: a feature of behaviour often termed animal personality. Furthermore, animals often show unpredictability in how much they respond to these events over time leading to the suggestion that personality and intraindividual variability (IIV) might have important fitness consequences. We investigated this hypothesis by focusing on the tendency for individually identifiable male fallow deer, Dama dama, to escalate low-level (noncontact) agonistic interactions to fighting during the rut. Males differed in their tendency to escalate noncontact interactions to fighting; however, repeatability in escalation rates was unstable over the rut suggesting that escalation rate is a poor measure of personality. There was no difference in the level of IIV in escalation rate shown by males. A comparison of IIV recorded over two consecutive annual ruts showed that IIV was highly correlated between years; therefore, over an extended time period individuals were consistent in their willingness to engage in fighting. There was a nonlinear relationship between IIV and mating success; specifically, individuals that showed either low or high IIV in escalation rate had lower mating success than individuals that showed intermediate levels of IIV. Aggression and the willingness to commit to fighting are important fitness correlates; nevertheless, our understanding of how personality and IIV are related to aggression is poorly understood. This study shows that although escalation rate appears to be a poor measure of personality in the fallow deer, IIV is related to increased fitness in individuals that show intermediate levels of predictability in their willingness to fight over the rut.