Monotremes

Monotremes are an ancient, small group of egg-laying mammals represented today by the platypus and the echidnas. They inhabit Australia and New Guinea and form the most primitive living lineage of mammals, retaining several features uncommon in other mammals, such as a cloaca, the possession of eggs, and milk secretion without nipples.

Monotreme reproduction and development are distinctive. They are oviparous: females lay eggs—platypuses typically lay one to three eggs, while echidnas usually lay a single egg per breeding cycle. Eggs incubate in a nest or burrow for about 10 days before hatching. The young, called puggles, are nourished by milk secreted from mammary glands and delivered without nipples; the milk either pools on the skin or is secreted onto fur for the young to lap. Male echidnas have venomous spurs on their hind limbs, a rare mammalian trait.

Anatomy and sensory biology distinguish monotremes as well. The platypus has a highly sensitive bill equipped with electroreceptors to detect prey in murky water; echidnas rely more on a combination of smell and touch. Both groups have a single opening, the cloaca, for excretion and reproduction, and they generally exhibit lower body temperatures than many placental mammals. The platypus and echidnas retain other primitive mammalian features, reflecting their early split from other mammals.

Ecology and conservation: Platypuses are semi-aquatic carnivores that feed on aquatic invertebrates and small vertebrates, while echidnas are primarily insectivorous and occupy a variety of habitats from forests to deserts. They tend to be solitary and often crepuscular or nocturnal. IUCN assessments vary by species and region, with the platypus often listed as Near Threatened in parts of its range and echidnas generally categorized as Least Concern, though local threats from habitat loss and climate change exist.