THE MATING CHANCES OF SMALL MALES OF THE CERAMBYCID BEETLE TRACHYDERES-MANDIBULARIS DIFFER IN DIFFERENT ENVIRONMENTS (COLEOPTERA, CERAMBYCIDAE)

Some males of the cerambycid beetle Trachyderes (Dendrobias) mandibularis gained access to mates by defending a patchily distributed food resource, the fruits of saguaro cactus (Cereus giganteus). Male beetles differed greatly in fighting ability because of extreme variation in body size and a striking dimorphism in mandibular weaponry. As is typical in resource defense mating systems, larger males had an advantage in combat. Major males with their large pincer jaws invariably defeated minor males with small cutting jaws, and larger majors usually defeated smaller majors. However, although minor males were at a competitive disadvantage on saguaro fruits, they did not suffer a great penalty in terms of mating probability. In contrast, minor males have a considerably lower probability of mating at desert broom (Baccharis sarothroides) where sap ooze sites are few in number and effectively monopolized by major males (Goldsmith, S. K., Behav. Ecol. Sociobiol. 20, 111-115, 1987). On saguaros, minor males successfully obtained mates through scramble competition while avoiding direct physical competition with larger, territorial major males. Smaller males of either morph may have succeeded in acquiring mates in part because there were many more ripe saguaro fruits than beetles, which made it impossible for larger major males to monopolize females effectively under these conditions.