[Animals] When do mammals turn into warm-blooded animals?

[#Wittels-]

An investigation based on the study of the fossilized inner ears of the ancestors of mammals suggests that endothermy in this type of vertebrate could have originated in the late Triassic, about 233 million years ago

The poet Jesús Lizano said in one of his most po[CENSORED]r poems that, wherever he looked, they only saw mammals. Mammals, the animal class to which we belong to Homo sapiens, respond to the description of being vertebrate animals; amniotes, that is, in whose embryonic development 4 envelopes are developed that protect the embryo; they have milk-producing mammary glands with which they feed the young; and perhaps that characteristic that has guaranteed mammals high rates of evolutionary success: endothermy.

 

In simple terms, endothermy is the ability of some animals to maintain their body temperature without resorting to an outside heat source. That is, endothermic animals can maintain a high body temperature, between 31°C and 45°C, as a product of their metabolism.

In contrast, ectotherms, or cold-blooded animals, rely on ambient temperatures to regulate their temperature. Thus, compared to ectothermic animals, endothermy has allowed mammals to have a more active behavior, in addition to a greater aerobic capacity, to travel greater distances or reach higher speeds, albeit at the expense of higher energy costs.

 

Consequently, mammals and birds have been able to occupy a wide variety of ecological niches for which other vertebrates do not have the ability to compete, so in a certain way, endothermy has been one of the guarantors of the evolutionary success of mammals after the extinction of the dinosaurs. However, both the emergence and evolution of endothermy remains, to this day, one of the great unsolved mysteries of paleontology.

Now, however, to try to find out when the jump occurred in which mammals began to regulate their body temperature autonomously, the team of scientists led by the researcher from the University of Lisbon, Ricardo Araújo, has studied the ear internal of various ancestors of modern mammals.

 

The ear canals contain a fluid called endolymph, whose viscosity changes depending on the animal's temperature, and which is essential for motor coordination, navigation and spatial awareness. As the authors explain, the increase in body temperature during the ectothermic-endothermic transition of the ancestors of mammals would have decreased the viscosity of the endolymph, which would negatively affect the biomechanics of the semicircular canals, causing this mechanism to have maintained a similar performance, these conduits were to increase in size. In Araújo's words "to maintain optimal functionality during this transition, morphological changes in the membranous ducts and oscillating bone canals would have been necessary".

Thus, to track these changes, the authors developed an indicator based on the morphology of the bone canal, which they refer to as the thermomotility index, and which they studied in 56 extinct species of sipnapsids, the group of reptiles from which the first mammals would evolve. The results suggest that endothermy evolved abruptly during the late Triassic period in early mammalomorphic reptiles, accompanied by a sharp increase in body temperature of between 5 °C and 9 °C, as well as an improvement in aerobic and anaerobic capacities.

 

Link: https://www.nationalgeographic.com.es/naturaleza/cuando-se-convirtieron-mamiferos-animales-sangre-caliente_18554