Most mammals are viviparous, giving birth to live young. However, the five species of monotreme, the platypuses and the echidnas, lay eggs. The monotremes have a sex determination system different from that of most other mammals. In particular, the sex chromosomes of a platypus are more like those of a chicken than those of a therian mammal. Like marsupials and most other mammals, monotreme young are larval and fetus-like, as the presence of epipubic bones prevents the expansion of the torso, forcing them to produce small young.