Which group of mammals is known for their unique feature of laying eggs?

Monotremes are a distinct group of mammals characterized by their reproductive strategy of laying eggs, which is quite unusual among mammals. This group includes species such as the platypus and echidnas. Unlike marsupials and placental mammals, which give birth to live young, monotremes have retained the primitive feature of egg-laying that could be traced back to early mammalian ancestors.

This unique reproductive method allows monotremes to occupy specialized ecological niches. For example, the platypus has adapted to an aquatic environment, using its egg-laying strategy to reproduce in water. The eggs are leathery, similar to reptilian eggs, and the mother incubates them until they hatch.

In contrast, marsupials, such as kangaroos and koalas, give birth to underdeveloped young that continue to grow in a pouch. Placental mammals, which include humans and most other mammals, develop their young internally with a placenta supporting the fetus throughout gestation.

Therefore, monotremes stand out significally within the broader mammalian classification, making them the correct answer to the question regarding egg-laying mammals.