The role of body size in mating success of Sphenarium purpurascens in Central Mexico

1. The effect of body size on the assortative mating and reproductive behaviour of the univoltine grasshopper Sphenarium purpurascens (Charpentier) was studied in Central Mexico. 2. Assortative mating by size was observed in the field. Evidence of positive assortative mating in relation to body size was found in laboratory experiments. Female fecundity and male success in contests were also correlated with body size. 3. Larger females had a higher number of eggs per pod. Larger males usually won fights and were able to take over females from other males, and to resist takeovers by other males while guarding. 4. Individuals of both sexes were observed copulating with more than one sexual partner in the field, suggesting polygamy. Male–male contests determined access to females, and males exhibited a postcopulatory prolonged mate-guarding behaviour lasting up to 18 days. 5. In a 2-year study, sex ratio was male-biased at the beginning of the reproductive season and decreased to 1:1 by the end of the season, suggesting that the population is protandrous. 6. The results of this study indicate that assortative mating results from male–male competition and female availability, and suggests that body size is a potential target of natural and sexual selection.