Monotremes, the Unique Egg-Laying Mammals

Monotremes (monotremata) are a unique group of mammals that lay eggs, unlike placental mammals and marsupials, who give birth to live young. Monotremes include several species of echidnas and the platypus.

Monotreme's Most Obvious Differences From Other Mammals

The most striking difference from other mammals is that monotremes lay eggs. Similar to other mammals, they do lactate (produce milk). But instead of having nipples like other mammals, monotremes secrete milk through mammary gland openings in the skin.

Monotremes are long-lived mammals. They exhibit a low rate of reproduction. Parents take close care of their young and tend to them for long periods of time before they become independent.

Monotremes also differ from other mammals in that they have a single opening for their urinary, digestive, and reproductive tracts. This single opening is known as a cloaca and is similar to the anatomy of reptiles, birds, fish, and amphibians.

Differences in Bones and Teeth

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Platypus Facts

By Anne Marie Helmenstine, Ph.D.

There are a number of other less salient characteristics that distinguish monotremes from other mammal groups. Monotremes have unique teeth that are thought to have evolved independently of the teeth that placental mammals and marsupials have. Some monotremes have no teeth.

Monotreme teeth may be an example of convergent evolutionary adaptation, however, because of similarities to other mammals' teeth. Monotremes also have an extra set of bones in their shoulder (the interclavicle and coracoid) which are missing from other mammals.

Brain and Sensory Differences

Monotremes differ from other mammals in that they lack a structure in their brain called the corpus callosum. The corpus callosum forms a connection between the left and right hemispheres of the brain.

Monotremes are the only mammals known to possess electroreception, a sense that enables them to locate prey by the electric fields generated by its muscle contraction. Of all monotremes, the platypus has the most sensitive level of electroreception. Sensitive electroreceptors are located in the skin of the platypus's bill.

Using these electroreceptors, the platypus can detect the direction of the source and the strength of the signal. Platypuses swing their head from side to side when hunting in water as a way of scanning for prey. Thus, when feeding, platypuses do not use their sense of sight, smell, or hearing: They rely only on their electroreception.

Evolution

The fossil record for monotremes is rather sparse. It is thought that monotremes diverged from other mammals early on, before marsupials and placental mammals evolved.

A few monotreme fossils from the Miocene epoch are known. Fossil monotremes from the Mesozoic epoch include Teinolophos, Kollikodon, and Steropodon.

Classification

The platypus (Ornithorhynchus anatinus) is an odd-looking mammal with a broad bill (that resembles the bill of a duck), a tail (that resembles the tail of a beaver), and webbed feet. Another oddity of the platypus is that male platypuses are venomous. A spur on their hind limb delivers a mixture of venoms that are unique to the platypus. The platypus is the only member of its family. 

There are four living species of echidnas, named after a monster of the same name, from Greek mythology. They are the short-beaked echidna, the Sir David's long-beaked echidna, the eastern long-beaked echidna, and the western long-beaked echidna. Covered with spines and coarse hair, they feed on ants and termites and are solitary animals.

Although echidnas resemble hedgehogs, porcupines, and anteaters, they are not closely related to any of these other mammal groups. Echidnas have short limbs that are strong and well-clawed, making them good diggers. They have a small mouth and do not have any teeth. They feed by ripping apart rotten logs and ant nests and mounds, then licking up ants and insects with their sticky tongue.