Contest versus scramble competition among males pursuing fixed or plastic alternative reproductive tactics

Reproductive and agonistic behaviours typically diverge between individuals pursuing alternative reproductive tactics (ARTs). When tactics are fixed for life, evolutionary theory predicts that the relative frequencies of alternative male genotypes are stabilized in a population by negative frequency dependence. This implies that competition is greatest between males pursuing the same tactic. The cichlid fish Lamprologus callipterus exhibits three male ARTs involving fixed and flexible tactics, and an extreme intrasexual size dimorphism determined by Mendelian inheritance. Large nest males defend territories and construct nests of empty snail shells in which females breed. In contrast, dwarf males pursuing a genetically fixed parasitic tactic enter shells surreptitiously during spawning in order to steal fertilizations. Sneaker males using another parasitic tactic, which is plastic and conditional, steal fertilizations opportunistically during spawning by quick intrusions into the nest. The variation in tactic origin and reproductive behaviour and the substantial asymmetry in body size render L. callipterus an ideal model system to study theoretical predictions regarding the types and intensity of contest behaviours among conspecific competitors pursuing ARTs. In an experiment exposing males to competitors using either the same or a different tactic, within-tactic competition was much more intense than between-tactic competition in bourgeois males, as predicted by evolutionary theory. In addition, the level of aggression displayed by bourgeois males against male intruders was apparently triggered by perceived differences in body size. In contrast to bourgeois males, parasitic males showed hardly any aggressive behaviour against other males, indicating that their contests follow the pattern of scramble competition. The conditions characterizing parasitic reproduction apparently select for rapid responsiveness when opportunities arise to fertilize eggs, whereas attacking other males in this situation seems inappropriate. Our results show that males pursuing ARTs diverge in the way they react to reproductive competition, mainly dependent on their overall resource defence strategy.