Sexual dimorphism--a difference in size and appearance between adult males and adult females, apart from their genitalia--is a common feature in the animal kingdom. It's not unusual for the females of some species of birds to be larger and more colorful than the males, for instance, and we're all familiar with the giant claw of the male fiddler crab.
When it comes to sexual dimorphism in dinosaurs, though, the evidence is much more uncertain. First, the relative scarcity of dinosaur fossils--even the best-known species are represented by only a few dozen skeletons--makes it perilous to draw general conclusions about the relative sizes of males and females. And second, bones alone don't have much to say about secondary sexual characteristics, much less the actual sex of the dinosaur in question.
Girl Dinosaurs Had Bigger Hips
When it comes to distinguishing between male and female dinosaurs, paleontologists can consult one distinctive feature: the size of the fossil's hips. The females of large dinosaur species like T. Rex laid relatively large eggs, so their hips were configured in a way to allow for easy passage (in an analogous way, the hips of adult human females are wider than those of males, to allow for easier childbirth).
Oddly, T. Rex appears to have been sexually dimorphic in another way: many paleontologists now think that the female of this species was larger than the male What this implies, in evolutionary terms, is that females competed for the right of mating with available males. This contrasts with modern species like the walrus, in which the (much bigger) males compete for the right to mate with smaller females.
Boy Dinosaurs Had Bigger Frills
T. Rex seems to be one of the few dinosaur species whose females asked (figuratively, of course), "Do my hips look big?" Lacking extensive fossil evidence about relative hip size, scientists have no choice but to investigate secondary sexual characteristics--which, as mentioned above, don't tend to be preserved well in the fossil record.
Protoceratops is a good case study in the difficulty of inferring sexual dimorphism in long-extinct dinosaurs. Some paleontologists believe that the males of this species had larger, more elaborate frills on the backs of their heads, which were used as mating displays (fortunately, there's no shortage of Protoceratops fossils, meaning there are a large number of individuals to compare). Lacking more evidence, though, other scientists are unconvinced.
Sometimes, Gender Can Be Hard to Determine
As stated above, one major problem with establishing sexual dimorphism in dinos is the lack of a representative population. Ornithologists can easily collect evidence about hundreds of males and females of a single bird species, but a paleontologist is lucky if his dinosaur of choice is represented by more than a handful of fossils.
Lacking this statistical evidence, it's always possible that the variations noted in dinosaur fossils have nothing to do with sex: perhaps two different-sized skeletons both belonged to males, but from widely separated regions, or perhaps dinosaurs simply varied individually the way humans do. In any case, the onus is on the researcher to provide conclusive evidence of sexual differences; otherwise we're all just fumbling in the dark.
