Relative size in the mating game

摘要

Five decades ago, the German evolutionary biologist Bernhard Rensch1 presented an intriguing rule for the differing sizes of male and female animals. He found that in several groups (clades) that each contain related species, male size relative to female size increases with the body size of the species. Rensch's rule has since been verified in animals as diverse as arthropods, reptiles, birds and mammals, including primates. The causes behind the rule, however, have remained unclear. Why are males much larger than females in many animals with large body size? And why, in the same clade, are males similar or even smaller than females in species with small body size?