The pre-teens heralded sexual maturity. Combat for mates and nesting sites would turn T. rex against T. rex. Females would also likely experience extreme stress from laying lots of eggs. Between the ages of 13 and 18, mortality for T. rex might have spiked to as high as 23 percent a year. According to Florida State paleontologist Gregory M. Erickson, over half of the known T. rex specimens seem to have died within six years of reaching sexual maturity.

As T. rex battled each other, they might also have been spreading parasites. In 2009, a group of paleontologists theorized that the dinosaurs commonly suffered from Trichomonas gallinae infections, which afflicts modern birds to this day. The parasite eats away at the back of the throat, inflaming nearby tissues and even leaving telltale holes in the back of the lower jaw, which are conspicuously present in many notable T. rex fossils.