LIFE × EVOLUTION

Endothermy: the Origin of Warm Blood

~233 million years ago

Phanerozoic · Mesozoic · Triassic

The ancestors of mammals became warm-blooded about 233 million years ago, in the Late Triassic period, when they raised their body temperature by five to nine degrees and began holding it constant.

Endothermy is the maintenance of a body temperature above that of the surroundings by internal heat production. The ancestors of mammals acquired it in the Late Triassic, the last part of the Triassic period: they raised their temperature by five to nine degrees and began holding it constant. The evidence is indirect and comes from the inner ear. The semicircular canals are fluid-filled tubes of the inner ear that serve as the organ of balance. Warmer blood lowers the viscosity of that fluid, its resistance to flow. The organ of balance would then work less well, unless the shape of the canals changes to compensate. Araújo and colleagues (2022) measured that shape in fifty-six extinct species, and it changes abruptly in the Late Triassic. Earlier relatives on the mammal branch were most probably ectothermic, with a body temperature set by the surroundings. On the bird lineage the shift to endothermy appears to have occurred separately.

Why it matters

Endothermy costs about ten times more food, and it gives an animal access to the night, the winter and high elevation. Two lineages, that of mammals and that of birds, took on that cost, each apparently independently. Those two lineages occupy terrestrial environments today.

Dating & uncertaintyDisputed

Researchers have interpreted bone growth marks, isotopes, which are variants of a chemical element, and posture as evidence of endothermy, and none of the three settles when it appeared. The shape of the inner ear gives a clearer indication, an abrupt change in the Late Triassic, and the date displayed follows it. Other researchers read the same evidence as a slower emergence of endothermy. In their reading it begins in the Late Permian, the end of the period before the Triassic. It then accelerates in the Early Triassic, after the end-Permian extinction, a mass extinction between the two periods. In that case no single date can be given.

Earth history as one day

22:46:051 h 14 min before midnight

if Earth’s 4.54 billion years were compressed into a single day