Sexual Dimorphism

The concept of sexual dimorphism received its foundational theoretical treatment in Charles Darwin's The Descent of Man, and Selection in Relation to Sex (1871), in which Darwin distinguished sexual selection from natural selection as a separate evolutionary mechanism. Darwin identified two primary pathways: intrasexual selection, involving direct competition between members of the same sex (typically males) for mating access, and intersexual selection, in which one sex (typically females) preferentially chooses mates based on particular traits. These two mechanisms, acting alone or in combination, drive the evolution of dimorphic features that range from weaponry (antlers, horns, enlarged canines) to ornamental display structures (elaborate plumage, colorful skin patches).

The degree of sexual dimorphism within a species generally correlates with the mating system. Highly polygynous species, in which one male mates with multiple females, tend to exhibit the most pronounced dimorphism—male elephant seals, for example, can weigh several times more than females. Monogamous species tend to display less dimorphism. In some taxa, reverse sexual dimorphism occurs, with females being larger than males; this pattern is seen in many raptors and numerous invertebrate species.

According to Encyclopaedia Britannica, sexual dimorphism encompasses differences in color, shape, size, and structure caused by inheritance of sex-specific genetic patterns. Size dimorphism can be extreme: male baboons (Papio) are more than twice the mass of females, and male Steller sea lions (Eumetopias jubatus) weigh approximately 1,000 kg—roughly three times the mass of females. Color dimorphism is especially prevalent in birds, where cryptically colored females remain concealed on nests while brightly colored males use plumage in courtship and territorial displays. Even feeding ecology can be sexually dimorphic, as in the mountain spiny lizard (Sceloporus jarrovi), where equal-sized males and females target different prey sizes.

Secondary sexual characteristics represent the most conspicuous expressions of dimorphism. These include the manes of male lions, cheek pads of male orangutans, the elaborate antlers and horns of male deer and antelopes, and the extravagant tail of the male peacock. Although such features may increase predation risk, their evolutionary advantages in attracting mates and outcompeting rivals outweigh this cost.

The first scientist to systematically consider sexual dimorphism in dinosaurs was the Transylvanian paleontologist Franz Nopcsa (1877–1933). According to the Natural History Museum, London, Nopcsa was the first to approach dinosaurs as biological entities rather than merely describing them as new types of animals. In 1929, he proposed that variations in the horns, frills, knobs, and crests of crested hadrosaurs and horned ceratopsians reflected differences between males and females of the same species. Nopcsa's specific male-female pairings were later shown to represent different evolutionary lineages from different time periods rather than dimorphic pairs. Nonetheless, his pioneering work established sexual dimorphism as a legitimate research question in dinosaur paleontology.

Subsequent researchers continued to investigate the subject. Peter Dodson (1976) quantitatively analyzed frill size variation in Protoceratops, suggesting sexual dimorphism as a possible explanation. Chapman et al. (1997) reviewed evidence for sexual dimorphism across Dinosauria. Various studies examined potential dimorphism in tail chevron bones, overall body robustness, and cranial ornamentation in taxa from Tyrannosaurus rex to lambeosaurine hadrosaurs.

A watershed study by Jordan Mallon, published in Paleobiology in 2017, presented the first comprehensive statistical analysis of sexual dimorphism across Dinosauria. Mallon re-examined quantitative data from nine dinosaur species for which sexual dimorphism had been previously claimed. Applying a rigorous suite of statistical tests—including normality tests, unimodality tests (Hartigan's dip test), and Gaussian mixture modeling—he found no statistically significant evidence for sexual dimorphism in any of the examined taxa.

Mallon emphasized that this result does not demonstrate that dinosaurs lacked sexual dimorphism; phylogenetic inference from their extant relatives (birds and crocodilians, both of which display dimorphism) suggests they very likely were dimorphic. Rather, the result highlights fundamental limitations of the fossil record: most dinosaur species are known from fewer than ten individuals, specimens from a single locality may span thousands or millions of years, and it is generally impossible to distinguish sex-based variation from ontogenetic, individual, or taxonomic variation without a priori knowledge of sex.

The most reliable method currently available for sexing dinosaur fossils is the identification of medullary bone (MB), a specialized endosteal tissue that female birds produce within the marrow cavities of their long bones immediately prior to egg-laying. This tissue serves as a rapidly mobilizable calcium reservoir for eggshell formation and is estrogen-dependent.

In a landmark 2005 paper in Science, Mary Schweitzer, Jennifer Wittmeyer, and John Horner reported the discovery of tissue histologically consistent with medullary bone in the femur of a Tyrannosaurus rex specimen (MOR 1125). They hypothesized that this tissue was homologous to avian medullary bone, identifying the specimen as a reproductively active female. A follow-up study by Schweitzer et al. (2016) in Scientific Reports used chemical analyses to confirm the identification, demonstrating that the tissue's chemical signature matched that of genuine medullary bone and was distinct from pathological bone or other endosteal tissues.

Medullary bone has since been reported in additional dinosaur taxa, further solidifying the evolutionary link between dinosaurian and avian reproductive biology. However, the method has inherent limitations: medullary bone is only deposited during a narrow reproductive window immediately before egg-laying. Consequently, it can only identify females that died during this specific physiological state. Males and non-reproductive females cannot be identified by this method. Furthermore, some researchers have raised concerns that environmental stress may induce the formation of histologically similar (but non-reproductive) tissues, adding interpretive complexity.

Stegosaurus mjosi: In a 2015 study published in PLoS ONE, Evan Saitta analyzed the dorsal plates of Stegosaurus mjosi from the Morrison Formation (Upper Jurassic, western USA). He identified two distinct plate morphotypes—wide and low versus tall and narrow—that co-occurred at the same fossil site and could not be explained by ontogenetic stage or species-level differences. Saitta attributed this bimodal variation to sexual dimorphism, representing the first compelling morphological evidence for dimorphism in a non-avian dinosaur species. However, which morphotype corresponds to which sex remains unknown.

Confuciusornis sanctus: Chinsamy et al. (2013), published in Nature Communications, studied this Early Cretaceous bird (~125 Ma) known from thousands of exceptionally preserved specimens from China. Approximately half of all specimens display long ornamental tail rectrices (feathers) and half lack them. Medullary bone analysis revealed that specimens lacking long tail feathers were females in reproductive condition, supporting the inference that long-tailed individuals were males. This represents one of the clearest documented cases of feather-based sexual dimorphism in a Mesozoic dinosaur (avian).

Ornithomimosaurs from Angeac-Charente: Pintore et al. (2023), published in eLife, analyzed the femora of at least 61 ornithomimosaur individuals from a mass-mortality deposit in the Early Cretaceous of southwestern France. Using 3D geometric morphometrics, they identified subtle but statistically significant differences in femoral curvature that were independent of body size. The pattern of dimorphism was consistent with sex-related femoral variation observed in extant archosaurs (birds and crocodilians). Crucially, the sample consisted of coeval individuals from a single mass-death event, eliminating temporal and geographic confounds. This study was described as the first to statistically demonstrate sexual dimorphism in a group of non-avian dinosaurs that lived and died together, and it found an approximately equal sex ratio in the herd—a result that differs from the female-skewed ratios seen in some extant archosaur populations.

Skeletal dimorphism represents only one dimension of potential sex differences. In many extant reptiles, dimorphism is predominantly color-based, and given the increasing evidence for widespread feathering among theropod dinosaurs, feather-based display dimorphism may have been common. Paul Barrett of the Natural History Museum, London, has noted that the earliest feathered theropods developed feathers on their forearms and tails—locations optimal for visual display. This suggests that small feathered theropods may have engaged in elaborate courtship displays involving feather-shaking and tail-fanning behaviors analogous to those of modern birds.

However, soft tissues including feathers, coloration, skin structures, and other integumentary features are preserved only under exceptional taphonomic conditions. For the vast majority of dinosaur species, this aspect of potential dimorphism is entirely lost to the fossil record, meaning that skeletal-based analyses inevitably underestimate the true extent of sexual dimorphism in extinct dinosaurs.

The current scientific consensus holds that non-avian dinosaurs were very likely sexually dimorphic based on phylogenetic bracketing—their closest living relatives, birds and crocodilians, both exhibit sexual dimorphism. However, statistically demonstrating dimorphism in the fossil record requires sample sizes, temporal control, and methodological rigor that are rarely attainable. Medullary bone remains the only unambiguous sex indicator in dinosaur fossils, but its applicability is limited to reproductively active females.

Recent advances in 3D geometric morphometrics, as demonstrated by Pintore et al. (2023), and increasingly sophisticated statistical methods offer new pathways for detecting dimorphism. Mass-mortality assemblages, where large numbers of conspecific individuals died together, provide the controlled samples needed for such analyses. As more such assemblages are discovered and analyzed, and as techniques for detecting chemical and microstructural sex indicators improve, the prospects for resolving sexual dimorphism in extinct dinosaurs continue to grow.