Sexual Dimorphism

Definition:

Sexual dimorphism is a difference in form between male and female of the same species.

These differences may or may not be obvious. They include differences in size, coloration (e.g., plumage differences in male and female birds), or less obvious differences such as size and structure of reproductive organs or gametes.

Examples of sexual dimorphism:

• One marine animal that has obvious sexual dimorphism in size is the sperm whale. In sperm whales, males are larger than females. Male sperm whales can grow to about 60 feet in length - about 3 times the maximum length of a female sperm whale. Male sperm whales can weigh up to about 125,000 pounds, and females weigh about half of that.
• Baleen whales have the opposite situation - females are generally longer than males in baleen whales.
• Humans also display sexual dimorphism. For example, in puberty, females enter puberty earlier and go through puberty quicker than males. Males have greater muscle mass in their arms, and longer, stronger bones. Males are also usually taller than females.
• Male lions have a mane, while females do not.

Charles Darwin hypothesized that animals that were polygamous have stronger sexual dimorphism than monogamous animals, and that sexual dimorphism resulted from a type of natural selection called sexual selection - the favoring of traits that make it easier for animals to attract, find or fight for mates.

References and Further Information

• BBC Nature. Sexual Dimorphism. Accessed August 31, 2012.
• Klappenbach, Laura. What Is Sexual Dimorphism?. About.com Animals GuideSite. Accessed August 31, 2012.
• Sexual Dimorphism and Sexual Selection. Accessed August 31, 2012.
• Ridley, M. Sexual Dimorphism. Accessed August 31, 2012.
• Wells, JC. 2007. Sexual Dimorphism of Body Composition. Best Pract Res Clin Endocrinol Metab. Accessed August 31, 2012.