Operational sex ratio and aggression within alternative strategies in male Atlantic salmon, Salmo salar.
Weir, Laura*,1, Grant, James2, Hutchings, Jeffrey1, 1 Dalhousie University, Halifax, Nova Scotia, Canada2 Concordia University, Montreal, Quebec, Canada
ABSTRACT- Alternative mating strategies exist in many animal populations and are usually expressed among males who differ in behaviour, physiology and/or morphology. These strategies often include larger individuals who fight for access to females, and those who sneak matings from the fighter males. Although a substantial amount of research has focussed on comparing behaviour and reproductive success among male strategies, few studies have examined interactions among males both within and among strategies. Though sneaker males do not interact aggressively with fighter males, they may fight with other individuals using the same strategy. Levels of aggression within both fighter and sneaker males may increase with an increase in the operational sex ratio (OSR: potentially reproducing males to fertilisable females) as competition for mates becomes more intense. However, at very high OSR values, aggression may decrease because females are not economically defendable when there are many competitors. We carried out spawning experiments involving large anadromous males and mature male Atlantic salmon parr to examine the relationship between OSR and aggression within male strategies over different spatial and temporal scales. As expected, OSR varied over time and differed between male strategies. Anadromous males established dominance relationships whereby OSR could explain aggression at larger spatial and temporal scales, but not at individual spawning events. By contrast, OSR was a good predictor of aggression among mature male parr during encounters at a nest. Our results suggest that the relationship between aggression and OSR is consistent within alternative male strategies, though OSR best predicts aggression at different spatial scales with respect to male strategy. Furthermore, they suggest that males adopting a sneaker strategy may be more behaviourally plastic than previously thought.
Key words: operational sex ratio, aggression, Atlantic salmon, mating strategies
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