Final answer:
Species that exhibit sexual dimorphism often live solitary lives and have large differences in body size between males and females, driven by reproductive competition, as seen in animals like peacocks, elephant seals, and gorillas.
Step-by-step explanation:
Sexual dimorphism is a pronounced difference in size or appearance between the sexes of an animal. This phenomenon is especially noticeable in species with solitary lifestyles and significant variance in male reproductive success. Some of the most striking examples of sexual dimorphism are found in species like peacocks, where males have elaborate tail feathers, and elephant seals, where males are much larger than females. In these animals, larger size or elaborate traits can play a critical role in mating success, either through male-male competition or female choice.
In the case of species referred to in the question, such as primates like the gorilla, or marine animals like the elephant seal, males often have significantly larger body sizes compared to females. This largely solitary lifestyle associated with high male competitive success for mating opportunities can drive the evolution of the differences in size and attributes between males and females of these species.