monotreme

Monotremes are an order of egg-laying mammals that represent one of the earliest diverging lineages of living mammals. The order contains two families: Ornithorhynchidae, represented by the platypus (Ornithorhynchus anatinus), and Tachyglossidae, which comprises the echidnas. They are native to Australia and New Guinea.

Reproduction and anatomy: Monotremes are oviparous; they lay eggs that hatch outside the mother's body. The young then feed on milk secreted by mammary glands, but monotremes lack nipples; milk is secreted onto the fur and is absorbed by the hatchlings. The platypus has a broad, duck-like bill with electroreceptors to detect prey underwater; echidnas have a long, slender beak with tactile sensors. Adult echidnas are covered in spines and dense fur; platypuses have dense fur without spines. Males of the platypus have a spur on the hind limb that can deliver venom; some echidnas have spurs as well during the breeding season.

Distribution and ecology: Platypuses inhabit rivers and streams in eastern Australia and Tasmania; echidnas occur across Australia and in New Guinea. They feed on invertebrates, with platypuses hunting aquatic invertebrates and echidnas foraging for ants and termites. They have relatively low body temperatures for mammals and exhibit traits that reflect their early evolutionary position among living mammals.

Evolution and taxonomy: Monotremes diverged from other mammals early in mammalian evolution and retain several primitive traits, such as a cloaca. They are less diverse today, with only a handful of living species grouped into two families.

Conservation: Monotreme populations face threats from habitat loss, climate change, and introduced predators in Australia; some species have restricted ranges and are sensitive to environmental changes.