Which Types of Animals Are Monotremes?

The platypus is a type of monotreme. Getty Images/Simon Foale

Monotremes (monotremataare a unique group of mammals that lay eggs instead of giving birth to live young like other mammals (such as placental mammals and marsupials). Monotremes include several species of echidnas and the platypus.

What Makes Monotremes Different?

Monotremes 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). Monotremes lay eggs and like other mammals lactate (produce milk) but instead of having nipples like other mammals, monotremes secrete milk through mammary gland openings in the skin. Adult monotremes do not have any teeth.

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.

The fact that monotremes lay eggs is not the only factor that distinguishes them from other mammal groups. Monotremes also have unique teeth that are thought to have developed independently of the teeth that placental mammals and marsupials have (even though the teeth may be convergent evolutionary adaptations due to similarities). Monotremes also have an extra set of bones in their shoulder (the interclavicle and coracoid) which are missing from other mammals.

Monotremes also 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 and rely instead only on their electroreception.


The fossil record for monotremes is rather sparse but it is thought that monotremes diverged from other mammals early on, before marsupials and placental mammals evolved. A few monotreme fossils from the Miocene are known. Fossil monotremes from the Mesozoic include Teinolophos, Kollikodon, and Steropodon.


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, 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, ant nests and mounds then licking up ants and insects with their sticky tongue. Echidnas are named after a monster of the same name, from Greek mythology.