How Sex Killed Tyrannosaurus rex

Sunday, February 10, 2019

AP Photo/Michel Euler

Tyrannosaurus rex, king of the tyrant lizards, may have been the most fearsome land predator to ever exist on planet Earth. Yet despite its lofty position and regal name, T. rex did not live a pampered, kingly lifestyle. True, no other dinosaur species directly predated upon T. rex after it reached the age of two, when an individual would have grown large enough to dissuade any would be attacker, but that doesn't mean that tyrannosaurs cruised through life, succumbing only to death from old age. In reality, life at the top was not a walk in the (Cretaceous) park.

The most obvious source of peril was prey. Tyrannosaurs would try to pick off juvenile, smaller, or sickly dinosaurs, but would still have to reckon with an angry Ankylosaur, Triceratops, or Edmontosaur in the process. A swipe from a clubbed tail could shatter bones, rendering a T. rex unable to hunt and susceptible to starvation. A stab from a horn could result in infection and eventual death. Herbivores may have had to contend with being hunted, but at least they didn't have to do battle every time they wanted a meal.

Infant T. rex suffered the highest mortality, endangered by predators and disease, but upon becoming a juvenile around age two, life was fairly safe, with nearly three-quarters of individuals surviving to their 13th birthday. Here was where things started to get hairy, however

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. Ericksonover 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.

Tyrannosaurus rex confirms that life at the top of the food chain is not the easiest, quite the opposite in fact. As few as one in fifty Tyrannosaurs may have reached their maximum attainable body sizes, achieved after living for more than twenty years. Firmly outliving the mighty T. rex were the massive, herbivorous Apatosaurus, Brachiosaurus, and Supersaurus, which could live as long as a century, almost four times longer than the lifespan of Sue, the oldest-known T. rex.