Monkeys, which are primates, do not lay eggs because they are mammals. This reproductive strategy, known as viviparity, is a distinguishing characteristic of the vast majority of the class Mammalia. All monkeys belong to the infraclass Placentalia, which refers to the specific way their offspring develop internally before birth.
Mammals and Live Birth
Most mammals, including all monkey species, are viviparous, meaning they give birth to live young that develop inside the mother’s body. This internal development is supported by a complex, temporary organ called the placenta. The placenta forms from both maternal and fetal tissues within the uterus, acting as a lifeline between the mother and the developing embryo.
The placenta’s function is to facilitate the exchange of oxygen, nutrients, and waste products between the mother’s bloodstream and the fetus’s blood without the two blood supplies directly mixing. This period of internal development, known as gestation, allows the infant to grow substantially and be born relatively well-formed. Primates, particularly the higher primates like monkeys and apes, typically have a highly invasive type of placenta known as hemochorial, which maximizes nutrient transfer efficiency.
The young are nourished solely by the mother during this time and are born at a stage of development where they can survive outside the uterus. After birth, all mammalian young are further nourished by milk produced by the mother’s mammary glands. This combination of internal gestation and postnatal lactation is the standard reproductive pattern for most mammals.
Contrasting Egg-Laying Animals
The ability to lay a shelled egg is characteristic of a different evolutionary path, most notably seen in reptiles and birds. This reproductive method, called oviparity, involves the embryo developing outside the mother’s body within a protective shell. The eggs of these animals are classified as amniotic eggs because they contain specialized membranes, including the amnion, which creates a protective, aqueous environment for the embryo.
The shell of a bird or reptile egg provides protection while allowing for gas exchange, and the large yolk sac supplies all the necessary nutrients for the developing young. The evolutionary shift to the amniotic egg was a major adaptation that allowed reptiles and birds to become fully terrestrial, independent of water for reproduction.
Other animal groups, such as amphibians and most fish, also lay eggs, but these are non-amniotic and generally lack a hard shell, requiring them to be laid in water. This reproductive strategy is fundamentally different from the viviparity of monkeys, where the mother’s body provides the protection and resources that an eggshell and yolk sac would otherwise offer.
The Monotreme Exception
While monkeys and the vast majority of mammals give birth to live young, there is a small, distinct group of mammals that does lay eggs: the monotremes. This group is represented by only five species, including the platypus and four species of echidna, all found in Australia and New Guinea. Monotremes are still classified as mammals because they possess hair, are warm-blooded, and produce milk to feed their young.
However, their reproductive system retains primitive features, such as a single opening called a cloaca for waste and reproduction, which is a trait shared with reptiles and birds. The eggs they lay are small, with a leathery shell, and contain a large yolk to sustain the developing embryo. The mother incubates the egg outside her body, often in a temporary pouch or a nest, for a period of about ten days.
After hatching, the young monotreme feeds on milk secreted from mammary glands onto a patch of skin, as these mammals lack nipples. Monotremes represent an ancient branch of the mammalian family tree, illustrating a unique combination of traits that diverged from the lineage leading to all other modern mammals, including the primates.