monjungle Dinosaurs Archive

Sexual Dimorphism

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Sexual dimorphism occurs when male and female individuals of a single species exhibit different anatomical traits. Such traits can be determined reliably in dinosaurs only when an abundance of specimens from the same species can be compared. Such traits helped dinosaurs distinguish male individuals of their species from females, and may have also been related to particular behaviors. For example, male African elephants have tusks, while females do not. These tusks are used during combat or jousting with other males. Male deer use antlers to wrestle with rivals to win the favor of females during the mating season.

Suggested sexual dimorphism has been observed in at least three kinds of ornithopods.

One of the earliest ornithopods, Heterodontosaurus, is usually found with a quartet of tusklike teeth in its jaws. Another heter-odontosaurid specimen, however, has been found with an identical skull, except that it lacked the tusks. It has been suggested that the tusks were only found in males, although the tuskless variety may have also been a juvenile whose tusks had not yet formed.

The basal ornithopod Hypsilophodon shows variation in the spine that may denote an anatomical difference between males and females. In this case, the number of vertebrae fused to the pelvis, known as sacrals, may be either five or six, and varies from individual to individual. This is a highly unusual occurrence because the number of sacral vertebrae is usually the same for all members of a taxon. It has been suggested that this is the result of sexual dimorphism.

Among hadrosaurs, scientists have discovered an abundance of flamboyant skulls that help distinguish species of duckbill from one another. These same skulls may demonstrate traits associated with male and female members of the same species. Specimens of Parasaurolophus have been found with short and long versions of their elaborate head crest, suggesting a gender difference as the cause. Dodson made a case for sexual dimorphism in Lambeosau-rus based on numerous specimens with otherwise identical skeletal elements.