The incubation period is the time from when an animal egg is laid to when it hatches. The term incubation period is used to refer to all egg laying mammals like fish, birds and reptiles but also to the platypus and the echidna which are the only egg laying mammals on earth. The incubation period varies between animal species from the incubation period of a penguin egg which is around 60 days to the incubation period of the an iguana egg which is between three and four months.