Archosaurs were initially rarer than the therapsids which had dominated Permian terrestrial ecosystems, but they began to displace therapsids in the Middle Triassic. This 'Triassic takeover' may have contributed to the evolution of mammals. It forced the surviving therapsids and their mammalian successors to live as small, mainly nocturnal insectivores. Nocturnal life probably forced the proto-mammals to develop fur and higher metabolic rates. In their burrows, keeping warm, looking after the young and feeding them with milk were keys to their later success.