Watch who you’re calling childless

Illustration: Álvaro Bernis

Dec 18th 2025|5 min read

IMAGINE a colony of sentient lizards, living on a distant planet. It comprises 100 females and 100 males. Each lives for 100 years. There is only one male and one female of any age (eg, one 15-year-old female, one 74-year-old male). The planet is safe and peaceful: no lizards die from disease or violence. But every year the two 100-year-olds pass away. And every year two babies are born, the fruits of an unusual reproductive cycle. At age 24 every female produces a male baby; at 26, a female. She will never have any more. The population of Planet Lizard is thus fixed at 200.