The platypus breeding season varies widely depending on location. Mating is thought to occur:
It took 93 years from when platypuses were discovered to get scientific proof that they were oviparous (meaning that they lay eggs) (Griffiths, 1978).
Females produce 1-3 eggs annually, usually 2 (Carrick et al., 2008).
After fertilisation the first shell layer is laid down and the egg passes down the oviduct into the uterus where second and third layers of the shell, secreted by glands in the walls of the uterus, are added.
The uterus also supplies nutrients to the egg, facilitating a size increase to about 14mm in diameter and 17mm in length by the time it is laid.
Incubation time is unknown but is possibly 10 days (Grant, 2007).