The Universe 25 Experiment

A mouse’s perfect paradise… or not.

Catherine Rasgaitis
5 min readApr 13, 2021
photo by Alexas_Fotos, courtesy of Pixabay

Between the late 1960s and early 1970s, American ethologist John B. Calhoun created a seemingly perfect utopia for mice. Calhoun’s predator-free, disease-free enclosure was furnished with limitless food and even an upper level with miniature mouse condos. Essentially, the mice would enjoy all the modern comforts people have come to expect today.

But the rodent society, dubbed the Universe 25, would quickly prove to be far from paradise.

The Experiment

To begin the experiment, Calhoun introduced four pairs of healthy mice into the enclosure. For the first 104 days, the mice explored their new habitat, marked their territory, and began nesting. Then, the population began to increase, doubling every 55 days.

Interestingly, even when the population was well under 1/4 of the enclosure’s capacity, most of the mice still crowded together in select areas. Eating, for example, was a shared activity so mice would group together during feeding times even though there was plenty of space to eat by themselves.

By the 315th day, the population reached 620 mice. Crowding behavior discouraged mating, heavily contributing to dropping birthrates. Universe 25 would now begin its slow but steady decline.



Catherine Rasgaitis

2x Top Writer — Space & Innovation | Enthusiast of all things tech and science!