1. Egg-laying: Monotremes are the only mammals that lay eggs. This feature makes them distinct from all other mammals, which give birth to live young after internal gestation.
2. Unique Reproductive System: Monotremes have a reproductive system that combines features found in both reptiles and mammals. Male monotremes possess spurs on their hind legs, which are used during mating and are similar to the spurs found in some reptiles.
3. Absence of Nipples: Unlike other mammals, monotremes do not have nipples. Instead, they have mammary glands that produce milk. The young echidna and platypus lap up the milk from the surface of the mother's fur.
4. Cloaca: Monotremes have a cloaca, which is an opening that serves multiple functions. It serves as the outlet for the digestive, urinary, and reproductive systems, similar to what is observed in reptiles and birds.
5. Electrorelocation: The platypus possesses a unique feature known as electrolocation. It has electroreceptors located in its bill, which allow it to detect electrical signals emitted by its prey. This ability helps the platypus locate small invertebrates in the water.
6. Venomous Spur: Male platypuses have venomous spurs on their hind legs. These spurs are used during mating rituals and are capable of delivering a painful sting. The venom can cause severe pain in humans.
7. Body Temperature Regulation: Monotremes are semi-aquatic (platypus) and terrestrial (echidna), which influences their body temperature regulation. They are considered "mesotherms" or "protothermic" animals, meaning their body temperatures are not as well-regulated as in other mammals. This feature allows them to tolerate variations in environmental temperature.
8. Limb Structure: Monotremes have unique limb structures. The platypus has webbed feet, well-suited for swimming, while the echidna has strong limbs and claws adapted for digging.
9. Ear Morphology: Monotremes have soft and flexible ears that can retract to cover the ear openings while swimming or digging, ensuring no water or dirt enters the ear.
10. Longevity: Monotremes have relatively long lifespans compared to other mammals of similar sizes. The platypus can live for up to 25 years in captivity, and the echidna's lifespan can exceed 50 years.
These unique characteristics make the platypus and echidna fascinating subjects of study and conservation, as they provide valuable insights into the evolution and diversity of mammals.