How to Beat Extinction: Live Fast, Die Young
When you don’t know if you have much of a future, you focus more on the now—there’s no point in biding your time and waiting when you could die any day. It seems that evolution follows this rule too—a recent study published by Field Museum scientists in Scientific Reports reveals that for Lystrosaurus (pygmy hippo-sized mammal relatives that lived with the dinosaurs), when the going got tough, the tough got busy. These animals lived during the most catastrophic extinction event the world has even seen, and in light of the unpredictable times, it is hypothesized that they began reaching sexual maturity earlier in their shortened life cycles.
“Basically, our research indicates that before the extinction event Lystrosaurus individuals could live for more than a decade. In the aftermath of the extinction, however, their life expectancy was very low, usually only about two to three years. One way they could compensate for their shortened life spans was to begin mating at younger ages instead of living long lives and taking years to reach sexual maturity” explains Ken Angielczyk, Associate Curator of Fossil Mammals. “If you’re an animal like a turtle and it takes you twenty years to reach sexual maturity, there’s a very good chance that you’ll die before you reproduce if your environment is unstable.”
Before the Permian mass extinction event hit 252 million years ago, lystrosaurs lived around thirteen to fourteen years, but following the extinction individuals only lived two or three years. Despite the far shorter lifespans, there’s still an abundance of Lystrosaurus fossils from the Early Triassic Period, and earlier reproduction might have been the key to their success.
But hold up—how are scientists able to tell how long Lystrosaurus lived in the first place? It’s in their bones. All bones bear records of how they grew and changed during an animal's life. These growth records, kind of like the growth rings in a tree trunk, are still present in the fossilized Lystrosaurus bones 252 million years later. Scientists take thin slices of the bones and use a microscope to analyze them for growth patterns. In addition to looking at growth features, which formed when the animal periodically slowed or stopped its growth, scientists also examine the microscopic structure of the bone tissue itself. The tissue, and the canals for minute blood vessels that penetrate it, are less organized when the bone was growing quickly. More organized tissues and blood vessels grew more slowly. Compared to Lystrosaurus fossils from before the extinction event, Lystrosaurus fossils from afterwards show many fewer growth lines (meaning they were alive for fewer years), and more disorganized tissue structure and canals for blood vessels, meaning that they were growing rapidly but died young. In short—lots of young Lystrosaurs living short lives.
Once the scientists had information about the life histories of Lystrosaurus before and after the extinction event, they used a computer model to investigate whether the observed shift to shorter lifespans might have contributed to its success in the unpredictable environments following the extinction. The answer: have lots of babies, and start early. Daryl Cordon, one of the study’s coauthors at the National Museum in South Africa, explains, “We used simulations to test how a shift in reproductive mode may have enabled Triassic therapsids like Lystrosaurs to compensate for losses due to environmental instability. We adjusted the models so that populations were allowed to breed at younger ages, thereby shortening the generation times. Under this scenario, populations were found to be able to survive for longer even in highly unpredictable environments, despite the high death rates of adults.” According to the statistical model, Lystrosaurus could have increased its chances of survival by 40% by adopting this “mate early and often” policy.
This discovery not only helps shed light upon the Permian-Triassic mass extinction, but it also gives us insights into how species will cope with the mass extinction event the Earth is undergoing today. “In the past century, we’ve seen some species do the same thing. For example, when fishing rates were low, Atlantic cod had long life spans and reached large sizes, with the biggest individuals producing the most offspring. But due to human overfishing, the average size of the cod is going down, because all the big individuals have been harvested, and the remaining smaller ones are being forced to reproduce as early as possible,” says Angielczyk. “Learning about the Permian-Triassic extinction can tell us about extinction today.”
