Most mammals do not lay eggs; instead, they give birth to live young. This reproductive strategy sets them apart from many other animal groups. While this is the typical understanding, remarkable exceptions exist, highlighting the diversity of life on Earth.
Mammals: Live Birth is the Norm
The defining reproductive characteristic for most mammals is viviparity, meaning they give birth to live young. This process involves the internal development of the embryo within the mother’s body. In placental mammals, a temporary organ called the placenta forms during gestation. The placenta facilitates the exchange of oxygen, nutrients, and other substances from the mother to the developing fetus, while also removing waste products. This internal nourishment allows for a more advanced stage of development before birth. Following birth, mammalian young are nourished by milk produced from the mother’s mammary glands, a universal trait among mammals. This contrasts with the reproductive strategies of other animal classes, such as birds and most reptiles, which typically lay eggs that develop outside the mother’s body and rely on the yolk for sustenance.
The Exception: Egg-Laying Mammals
The sole exception to the live-birth rule among mammals are the monotremes, a unique order of mammals. This small group includes the duck-billed platypus and the four species of echidnas: the short-beaked echidna, and the three species of long-beaked echidnas (Eastern, Western, and Sir David’s). These creatures are found exclusively in Australia and New Guinea. Unlike other mammals, monotremes lay eggs that are soft-shelled and leathery, similar to those of reptiles. A female echidna typically lays a single egg, depositing it into a temporary pouch on her belly for incubation, where it hatches after about 10 days. Platypuses usually lay one to three eggs, which they incubate in a specially constructed nursery burrow, often curling around them with their tail. These eggs contain a yolk, providing nourishment for the developing young, differing from the direct maternal nourishment seen in viviparous mammals.
Monotremes: Mammals Despite the Eggs
Despite their egg-laying habit, monotremes are classified as mammals due to several fundamental characteristics they share with other mammalian species. These animals possess fur or hair covering their bodies, a defining mammalian trait. They are also warm-blooded, maintaining a relatively constant internal body temperature, although their metabolic rates and average body temperatures can be lower than those of other mammals. Monotremes feed their young with milk produced by mammary glands, a hallmark of mammals.
Unlike other mammals, they do not have nipples; instead, the milk is secreted through pores onto a patch of skin, which the young then lap up. Monotremes also share specific skeletal features with other mammals, including a single bone in their lower jaw. Additionally, they possess three bones in the middle ear—the malleus, incus, and stapes—which are characteristic of mammals. These combined traits solidify their classification within the Mammalia class, demonstrating that classification relies on a combination of features rather than a single characteristic like live birth.