Being small may have helped animals to replenish their numbers. In modern animals, "the bigger the animal is, the longer the gestation time is going to be", says Ornella Bertrand, a postdoctoral researcher in mammal palaeontology at the University of Edinburgh. For instance, the African elephant gestates for 22 months, whereas a mouse"s pregnancy lasts around 20 days. Faced with the apocalypse, the mouse has the better odds of keeping its population up.