Why Terrestrial Mammals Will Never Achieve the Sizes of Their Giant Ancestors

Brachiosaurus, for example, was more than 20 meters long and weighed up to 50 tonnes. By way of comparison, the African elephant, the largest current terrestrial mammal, caps around 4 meters at the tourniquet and does not exceed 6 tonnes. This size difference is explained by several biological and environmental factors specific to dinosaurs.

One of the major elements that has favored their gigantism is their metabolism. Unlike mammals, which are endotherms and regulate their internal temperature by consuming a considerable amount of energy, the dinosaurs were probably mesotherms. This intermediate operating mode allowed them to grow quickly while spending less energy than modern mammals. Felisa Smith, a researcher in biology, underlined from Earthsky that mammals use on average ten times more energy than a reptile or a dinosaur of equivalent size, thus limiting their potential for gigantism.

The mesozoic offered an ideal framework for the evolution of dinosaurs. They lived under a warm climate, surrounded by lush vegetation. This abundance favored the rapid growth of herbivores and, therefore, that of their predators. A study by the University of New Mexico, published on Phys.org, has shown that the increase in the size of terrestrial vertebrates follows similar patterns worldwide. Comparable ecological conditions have thus enabled dinosaurs to reach impressive sizes without major constraint.

Thanks to this warm climate and these vast territories, the dinosaurs had access to the resources necessary to grow. On the other hand, giant and smaller terrestrial mammals have evolved in more limited environments. These conditions limited their development after their appearance.

The evolution of giant terrestrial mammals

The extinction of dinosaurs, 65 million years ago, released many ecological niches. The mammals, hitherto small, quickly took advantage of it. During the mesozoic, they often lived at night to escape the great predators. After the disappearance of dinosaurs, their diversification accelerated. Over the millennia, some species have reached increasingly imposing sizes.

Several species of terrestrial mammals have thus reached impressive dimensions, although none has competed with the largest dinosaurs. The paraceratherium, an ancestor of the rhinoceros, weighed between 15 and 20 tonnes and measured five meters at the tourniquet. This giant herbivore, which lived around 34 million years ago, represented one of the largest terrestrial mammals never listed. According to John Gittleman, a biologist at the University of Georgia, “the fact that such a large number of different lines have evolved independently to such similar maximum sizes suggests that giant mammals fulfilled similar ecological roles worldwide. The consistency of the scheme strongly implies that the biotes of all the regions responded to the same ecological constraints ”, quoted by Iflscience.

The Indritherium, another colossus of the tertiary era, has also reached extraordinary proportions, occupying the vast plains of Eurasia before disappearing. The analysis of the fossils conducted by the researchers of the study, made it possible to reconstruct its evolution and to demonstrate that its gigantism resulted from a combination of favorable environmental factors, such as a temperate climate and the presence of large meadows rich in vegetation.

However, despite these cases of gigantism, no mammal has ever approached the size of the greatest dinosaurs. The reasons for this limit are not only evolving, they are also deeply rooted in the biology of mammals themselves.

Biological and ecological limits

After the disappearance of dinosaurs, terrestrial mammals experienced phases of gigantism. However, several constraints have limited their growth. Their endotherm metabolism is therefore one of the main causes. It requires significant energy expenditure to stabilize body temperature. Dinosaurs seemed to benefit from a more economical metabolism. Mammals must produce heat permanently. Felisa Smith stresses that this requirement greatly increases their food consumption. This necessity then reduces their ability to reach extreme sizes.

The environment also plays a decisive role. The amount of food necessary to support an animal of several tens of tonnes is immense and requires a large enough habitat to ensure continuous supply. Dinosaurs lived in a time when abundant vegetation and hot climate favored increased food production. On the other hand, mammals have evolved in more varied contexts, sometimes marked by glacial periods or prolonged droughts, thus limiting their extreme growth potential.

The geographic conditions also influence these evolutionary dynamics. The study published on Phys.org highlights the correlation between the living area of ​​a species and its evolution towards large sizes. The more large a territory is free from natural barriers, the more it can accommodate large animals. However, the continents of the cenozoic were more fragmented than those of the mesozoic, reducing the opportunities of expansion of the great mammals.

The evolution of mammals has imposed limits that neither their metabolism nor their anatomy can exceed. For 140 million years, they remained small and discreet to avoid competition with dinosaurs. After their extinction, mammals experienced rapid diversification. However, this expansion stopped 42 million years ago. Felisa Smith demonstrated this by studying the fossil archives. Consequently, their size seems to have reached a scalable ceiling. Energy and ecological constraints have slowed their development.