Which mammal lays eggs?

The platypus, and the four species of echidna - the monotremes, the oldest surviving branch of the mammal family. The first platypus specimen sent to London in 1799 was assumed to be a hoax, and George Shaw took scissors to the pelt looking for the stitches where somebody had sewn a duck's bill onto a beaver.

It is a genuinely peculiar animal by any measure. It lays eggs but produces milk, and having no nipples, sweats it through patches of skin for the young to lap. The males carry venomous spurs on their hind legs, one of very few venomous mammals, and the venom causes pain severe enough to resist morphine. It has ten sex chromosomes where most mammals have two.

The bill is the real instrument. Platypuses hunt with their eyes, ears and nostrils shut, detecting the faint electrical fields of muscle contractions in shrimp and larvae buried in the riverbed - electroreception, which is common in fish and almost unknown in mammals. They also glow greenish under ultraviolet light, which was noticed only in 2020 and which nobody can yet explain.