Early mammals evolved from reptiles some 200 million years ago. For one thing, reptiles are cold blooded, which means that their body temperatures changes with the temperature around them.
In many cases this means that during the winter reptiles remain dormant because their body temperature is reduced to a point that precludes much activity. But even in warmer climates most reptiles must feed during the daylight hours, when it is warmer, and are less active during the night, when the temperature drops.
In contrast to reptiles, early mammals had the advantage of being warm-blooded. Because they have a constant body temperature regardless of their environment, mammals can compete with reptiles by remaining active throughout the night and throughout the year.Thus it is probable that the earliest mammals were nocturnal – that is, were most active during the night, when they would not be in danger from reptilian predators and in some cases would be able to obtain food with littlie if any competition.
Of course, being warm blooded, they could also function during the daytime hours, and this gave them an even greater advantage.
This change toward warm-bloodedness also gave mammals a geographic advantage in that they could exploit a wider range of climates and spread out over a wider territory. If climate change, either from one season to another or over a period of years, they were in a better position to survive by adapting to a new climate.
Thus mammals could adapt to a wider range of environmental conditions and yet compete successfully with reptiles in the same environment. Over many millions of years, mammals spread throughout the world, while many less adaptable reptilian species became instinct.
The Evolution of Mammals
