May 8, 2013
Sexual dimorphism is the difference in morphology between male and female members of the same species. Sexual dimorphism includes differences in size, coloration, or body structure between the sexes. Dimorphism means two forms. “Sexual dimorphism” is two sexes of a species differ in external appearance or other features. Males and females may differ in size, color, shape, the development of appendages (such as horns, teeth, feathers, or fins), and also in scent or sound production. Species in which male and females are identical in appearance or other features are said to be “monomorphic.” There are different types of dimorphic traits found in marine mammals and explains some of the reasons why these traits might have evolved and what can be inferred about the lives of males and females in a particular species from the pattern of sexual dimorphism. The quality of the information available on sexual dimorphism varies widely across marine mammal species. We know quite a lot about a few species, which are used repeatedly as examples, and virtually nothing about others. Despite some of the difficulties of observing marine mammals, understanding of the evolution of sexual dimorphism is increasing steadily as studying of rarely encountered species accumulates and new techniques are further developed. Sexual dimorphism has fascinated biologists since before the time of Darwin. Darwin considered that most sexual dimorphism was due to sexual selection, in which evolutionary forces acted separately on the sexes (Darwin, 1871). For example, females might choose to mate with highly ornamented males or males might develop characteristics or traits useful for fighting with other males to win in contests for access to females. Today, these two processes are often referred to as female choice and contest competition. More recently, scientists have learned that males compete not only by physical fighting and display but also, in species where females mate with more than one male, by sperm competition within the female reproductive tract. The adult males and females of a species may differ in size, color, shape, and the development of appendages such as horns, teeth, feathers, or fins, scent, or vocalizations. In marine mammals, one of the most striking sexually dimorphic characters is size. In some species, males are dramatically larger than females. For example, in southern elephant seals (Mirounga leonina), adult males (maximally at 3700 kg) weigh 4–10 times as much as the adult females (which weigh 350–800kg). Males in some species also possess greatly enlarged teeth that are lacking in females and are used in fights between males(Wells). The best-known example is the unicorn-like tusk of the Narwhal. The tusk, which is actually greatly enlarged left upper tooth, usually erupts only in males and can grow to an extraordinary size, exceeding 3m in length and 10 kg in weight. The noses of males are sometimes bizarrely modified. For example, the most distinctive feature of the male hooded seal is an inflatable hood and bright red nasal sac that may function in attraction and courtship displays(Wells). The appendages of males (flippers, flukes, and dorsal fins) are sometimes greatly enlarged. The best-known example of dorsal fin enlargement is seen in male killer whales. In adult males, the dorsal fin is erect and may grow to 1.8 m in height whereas the dorsal fins of females are less than 0.7m and distinctly falcate. Sexual dimorphism is shown also in body size, flipper size, and genital pigmentation pattern.
Although sexual dimorphism traditionally referred to differences in morphological traits, the sexes can also produce different vocalizations or odors or be differently colored or patterned. Among marine mammals, differences in color are usually limited to fairly minor differences in pattern or density of pigmentation. For example, in ribbon seals the banding pattern is similar in both...