Because the eggs are laid on land, internal fertilisation is needed. Thus, from the initial step of a cleidoic egg comes a number of changes in behaviour. With the changes in behaviour comes better and more flexible brains. Some reptiles, for example, crocodiles and birds, look after and protect their young. Mammals are an extreme case: they carry the protection and nursing of the young much further. All mammals provide milk for their young, and many smaller mammals raise their young in burrows, providing a meaasure of security. Humans go even further still, with the long period of learning before their young are independent. Also, in human evolution, there is a reduction in instinctive behaviour, which is inherited. Instead, there is a great increase in learned behaviours and social life. These later adaptations were made possible by the earlier evolution of the cleidoic egg.