Patrick T. Rohner, Tiit Teder, Toomas Esperk, Stefan Lüpold and Wolf U. Blanckenhorn
Being large can boost an individual’s reproductive success if larger body size increases strength in competition for mates, attractiveness to potential mates, or defense against predators and parasites. Animals should thus attain the largest size possible. In insects, however, the hard outer skeleton prevents any structural growth once the adult stage is reached, meaning that limited food quality or quantity during larval development can result in adults of suboptimal body size.
Previous research has shown that female body size is generally more strongly tied to the amount of food available than male size. However, because females are also the larger sex in most of the species examined, we cannot disentangle whether the stronger dependence of female size on food is the result of being female or the larger sex.
Taking advantage of those rare cases where males are larger than females enables investigations of whether females really respond more strongly to food shortage than males, irrespective of their relative size. If males are influenced more by food quantity than females in species where males are larger, then being the larger sex, and not being female, likely is what explains differential selection on body size.
We combined detailed within-species analyses in several fly species with between-species comparisons across a broader range of insects to study the factors underlying sex-specific body size variation. We found the larger sex to generally react more strongly to food shortage, suggesting that whichever sex is larger is also more susceptible to food limitation. In other words, the larger sex probably is under more intense selection to maximize adult body size than the smaller sex due to greater fitness consequences.
Picture credit: Dr. Rassim Khelifa