Experimental demonstration of alternative mating tactics of male Ptilothrix fructifera (Hymenoptera, Apidae)

Ptilothrix fructifera is a solitary, oligolectic bee dependent on the flowers of Opuntia (Cactaceae). We conducted field studies to identify the mating tactics used by P. fructifera males and to determine whether factors such as male body size and available resource flowers contribute to variation in male mating tactic. Males in a population in southern Brazil showed pronounced variation in body size, but no distinct size classes. Larger males aggressively defended territories of flowering Opuntia plants, the sole pollen source for the females. Smaller males nonaggressively patrolled nonflowering shrubs growing between the nest aggregation and patches of Opuntia. Only unmated females were attractive to males. When offered to males in patrolled shrubs, up to eight males attempted to mate with the same female, but in territories only the territory owner mated with the introduced female. About 10 min after mating, females were unattractive to males. These results identify the species as monogamous and indicate that territoriality is an adaptive mating tactic. Experimental addition of Opuntia flowers to patrolled shrubs showed that nonterritorial males can switch to territorial behaviour. Thus, the number of available flowers together with the number of competitor males determines the ratio of territorial males to patrolling males at a given site. The experiment shows that male P. fructifera show a conditional reproductive strategy in which males adopt alternative tactics according to their size.