Evolutionary biologist Olivia Judson speculates about the Tyrannosaurus rex's sexual equipment.
We now have a robust understanding of how sexual pressures -- the pressures to find, impress, and seduce a mate -- influence the evolution of males and females. So much so that if you tell me a fact, such as the average size difference between males and females in a species, or the proportion of a male's body taken up by his testes, I can tell you what the mating system is likely to be. For example, where males are much bigger than females, fighting between males has been important - which often means that the biggest males maintain a harem. If testes are relatively large, females probably have sex with several males in the course of a single breeding episode.
(thx, bill)