Small and nocturnal, the first mammals jump, climb, swing and swim through the dinosaur world. Obliged to inhabit small niches in a world of giants, mammals will discover that their diminutive size opens a proverbial window of opportunity.
In the first waves of mammal diversification, some rodent-sized insect-eaters evolve lactation, enabling mothers to spend more time in the nest keeping their young both fed and warm. Some mammal species evolve even smaller bodies.