Aggressive personality as a component of fighting ability (#531)
In animal conflict, individuals with superior fighting ability, i.e. resource holding potential (RHP), usually win, but sometimes apparently inferior rivals win. RHP is typically reflected in measures of body size. We propose that aggressiveness could help explain why sometimes smaller rivals win. Since consistent individual differences in aggressiveness have been observed in numerous species, we investigated whether aggressiveness as a personality trait forms a component of RHP. Contests (n=52) were staged between pairs of pigs of similar age (10wk) and body weight, but which differed in their aggressiveness. Aggressiveness was determined from latency to attack (s) during the established resident-intruder test. Contests lasted until a clear winner was apparent. Durations, behaviours and outcome were recorded. By modelling aggressiveness as component of RHP we examined whether aggressiveness was used as part of an assessment strategy, as predicted by game theory. The individual aggressiveness of the winner and loser did not influence the contest duration, fight duration, or other phases of the contest (all P>0.05), suggesting that aggressiveness does not form part of a self- or mutual assessment strategy. Contrary to predictions, a greater difference in aggressiveness between the opponents increased the contest duration (b=0.37±0.1s/sec difference in attack latency; P=0.04), duration of display behaviour (b=0.13±0.0s/sec; P=0.003) and of mutual pushing (b=0.11±0.0s/sec; P=0.02), but not the fight duration (P>0.05). This indicates that aggressiveness does have a role in contest resolution. Contest outcome was strongly related to which individual initiated biting (P<0.001). More aggressive individuals initiated biting more often, but aggressiveness was unrelated to contest outcome (P=0.98). We suggest aggressiveness offers an honest signal of intent (willingness to fight) but does not provide an honest signal of RHP, since it is unrelated to contest outcome. Viewed in this light, it is unsurprising that aggressiveness does not contribute to a self or mutual assessment strategy.