Why Mammals Didn’t Grow as Humongous as Dinosaurs?


One of the things fascinating features of Dinosaurs that made us look in awe was their gigantism (of course most dinosaur species weren’t giants: there were chicken-sized and dog-sized ones). But, those large stature creatures didn’t go on for long. Their number thinned at some point getting replaced by creatures only half their size, and later that fateful asteroid struck our Earth 65 million years ago, extinguishing all of these humongous animals. The extinction allowed mammals to rapidly evolve, but the land-dwelling mammals didn’t evolve bodies as large as the dinosaurs. The ecological dominance of the dinosaurs may have kept the mammals, but what prevented them in attaining those enormous sizes?

Between 37 and 23 million years ago, there lived a land mammal called ‘Paraceratherium’, which stood 18 feet tall and weighed 17 tons. It would have looked like a large-scaled rhinoceros without horns. They are part of entirely-extinct group called ‘hyracodonts. Paraceratherium were the largest land mammal of all time, but still if that mammal stood next to a Sauropod dinosaurs (herbivorous dinosaurs), it would have resembled a dwarf. An average sauropod stretched over 100 feet long from its puny head to the tip of its tail and weighed more than 75 tons. We have seen everything from gorilla to arthropod growing to a giant size in famous Hollywood movies, but why didn’t in reality an African Elephant or a Paraceratherium didn’t grow like a sauropod? The question has many answers and of course every convincing answer is scrutinized by scientists as we learn more about the evolutionary trends from one decade to another.

A restored Paraceratherium next to a White Rhinoceros

A restored Paraceratherium next to a White Rhinoceros

A 2010 research by Biologist Felisa Smith (and her 19 colleagues) provided some valid points on the question. Many dinosaur fossils show that these creatures had air-sacs extending from their lungs. These air sacs provide an efficient supply of oxygen and the rapid airflow would have offered a great cooling mechanism, which is essential for such huge but active animals to get rid of all the excess heat through their skins. So, a dinosaur would have weighed less than a similar-sized solid-boned mammal. And this mechanism could also allow dinosaurs support a body weight of about 80 tons with their four legs (whereas African Elephants could only reach about six tons).

Feeding also worked to the advantage of these giant dinosaurs. These creatures didn’t chew food like today’s giant mammals, but rather clipped branches and twigs and swallowed wholly. It means that they took in huge amount of food quickly. Since there was no need for bulky teeth to chew, the sauropods’ heads were lighter and so the necks could also be longer, reaching for a wider range of plants. The mammals are termed as ‘endotherms’, which means that they regulate their own body temperature. A mammal of considerable size uses ten times more energy than a giant dinosaur.


Mammals weren’t able to evolve as big as those Jurassic Period titans because they have to use large part of their physical energy to keep their bodies warm. For example, we humans must maintain a temperature of 98.6 degrees Fahrenheit to stay alive. But, dinosaurs didn’t regulate their body temperature and used that extra energy to grow larger. But, then why today’s reptiles or the birds, which is said to have descended from dinosaurs, didn’t grow big? For this the scientists point to the environmental conditions, which plays a huge role in an organism’s evolution.

Mammals didn’t evolve much in its first 140 million years out of the 210 million years. But, after dinosaur’s extinction, the mammals diversified rapidly to various sizes and expanded dramatically. Once the old Earth was reset, an enormous ecosystem chucked out the dominance of those 100 feat creatures. Before 42 million years, mammals’ body sizes, across all continents (as per the research conducted with fossil records) hit a barricade due to temperature constraints. Reproductive strategies is also said to have the answers to the growth questions (as mammals gestate). The above mentioned points by famous biologists may not have yet provided the full picture. One thing is for sure: that it is hard to all the reasons behind the evolutionary pressures which continue to change life on this planet.


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