# How many Tyrannosaurus rex have come to Earth?

Dinosaurs are probably the most popular “traffic stars” in the animal world. Although no one has ever seen them live, it does not prevent people from loving them. Unfortunately, people know very little about the reality of dinosaurs. We can only speculate on the life of idols through a little bit of clues like “fans” who are chasing stars. Today, the question we want to speculate is, how many of our favorite Tyrannosaurus rex ever lived on Earth?
simple biological calculation questions

At first glance, you may feel that there is no way to start. After all, dinosaurs no longer exist, and even fossils are scattered, and not every dinosaur can leave fossils. How to know the exact number of dinosaurs? But in the eyes of biologists, this is just a simple calculation problem, and the answer can be obtained by referring to known data and laws.
The number of species is estimated by multiplying the number of generations by the number of individuals per generation. Among them, the number of generations is relatively easy to know. It is enough to divide the total living time of the entire species group on the earth by the average duration of each generation. Based on the fossil information, we estimate that the Tyrannosaurus Rex group lived on the earth for 1.2 million to 3.6 million years. For the convenience of calculation, we take the median of 2.4 million years.
So how long does each generation of Tyrannosaurus rex last on average? In fact, this is the difference between the average reproductive age and the average lifespan. From the beginning of breeding offspring to the end of life, the Tyrannosaurus rex has continued on the earth from generation to generation. If there are still individuals of the species alive, then it is very easy for us to obtain this data, but the Tyrannosaurus rex is extinct, and we can only rely on limited fossils to calculate. Biologists deduced from the microstructure of the Tyrannosaurus rex’s skeleton that its first mating age was generally 15 years old, and then based on the lifespan of each individual reflected in the fossils, drawn its survival curve and estimated the average longest lifespan of Tyrannosaurus Rex. Based on these data, biologists eventually estimated that each generation of Tyrannosaurus rex lasted an average of 19 years. In this way, we know that the number of generations of Tyrannosaurus rex is 126,000.
Next, if we can estimate the number of individuals in each generation, then we can know how many Tyrannosaurus rex appeared on Earth. To estimate the number of individuals of a species, an ecological law needs to be used, that is, Darmouth’s law. It describes the relationship between body weight and population density—larger animals need more living space, and the population density is smaller. For example, if other factors are not considered, the number of rabbits that can be supported in a 1 square kilometer of grassland is definitely much more than the number of elephants that can be supported in the same area. In the same way, the higher the metabolic rate, that is, the species that consumes more energy, the more living space is required.
Biologists have made estimates of T. rex’s weight, arguing that it had a slightly higher metabolic rate than the modern large lizard Komodoosaur. In this way, according to Darmouth’s law, the average living space of each Tyrannosaurus rex is about 109.9 square kilometers, that is to say, the population density of Tyrannosaurus rex is 1/109.9. Multiplying this data by the area of ​​​​the Tyrannosaurus rex living area shows that there are about 20,000 individuals in each generation.
Finally, we can get the number of “superstar” Tyrannosaurus rex on earth: 126,000 × 20,000 = 2.52 billion.
Unknown conditions are hidden in the title

However, it is not difficult to find that the result of 2.5 billion Tyrannosaurus rex is not so accurate, because when we do the calculation, many numbers are estimated, and sometimes we need to take the middle value of the estimated range for calculation, so the results are naturally biased. . The reason why we can only get an estimate range is because the relevant data will change with the changes in the living habits and living environment of the Tyrannosaurus Rex.
For example, we estimate the living space of each T. rex to be about 109.9 square kilometers based on its weight and metabolic rate, which is actually based on a premise: T. rex is a solitary predator. Even when species are in similar ecological niches, population densities can vary widely depending on whether they live alone or in groups. Just like lions and tigers, which are also at the top of the food chain, in the same living area, the number of lions living in groups is much higher than that of solitary tigers. In fact, biologists estimate T. rex population densities ranging from one per 7 square kilometers to one per 1,724 square kilometers, accounting for the different effects of living alone and in groups.
So, did Tyrannosaurus rex live alone or in groups? There is still no accurate answer to this question, because archaeologists have excavated both single Tyrannosaurus rex fossils and group fossils of Tyrannosaurus Rex. In 2021, a group of tyrannosaurs (T. rex is the largest tyrannosaur species) fossils were unearthed in southern Utah, USA. These tyrannosaurs lived at the same time, they were probably hunting in groups and then encountered a sudden flood. , is buried to this day.
In addition, differences in living environment also have a significant impact on species population density. For example, the Arctic fox living in the Arctic and the Tasmanian devil living in Australia have the same weight, but the population density of the Tasmanian devil is 6 times that of the Arctic fox, which is closely related to various environmental factors such as climate and food. Considering that during the survival period of Tyrannosaurus Rex, the climatic conditions changed drastically, and the number of prey also fluctuated accordingly, and the population density of Tyrannosaurus Rex will not be fixed in different periods. It can be seen that in order to accurately judge the population density of Tyrannosaurus rex and even calculate its specific number, many factors must be considered.
Regardless, this estimation method is still a great help in determining the number of extinct species. In an ecosystem, organisms form an intricate network of predators. If we can excavate more dinosaur fossils in the future, especially the fossils of Triceratops, the main predator of Tyrannosaurus rex, and use the same method to calculate the number of Triceratops, the two sets of data will be mutually verified, which will help to estimate the Tyrannosaurus rex more accurately. quantity. Similarly, the number of other extinct creatures in the biological network can also be estimated based on this, and the more data is known, the more conducive to mutual verification.
As the number of excavated fossils continues to increase and research methods continue to improve, our speculation about extinct creatures will become more evidence-based. At that time, a clearer and vivid picture of ancient ecology will unfold before our eyes.