Dataset. Proper micro-environment alleviates mortality in laboratory mouse breeding induced by litter overlap and older dams.

The ongoing worldwide effort to reduce animal numbers in research often omits the issue of pre-weaning mortality in mouse breeding. A conservative estimate of 20% mortality would mean approximately 1.1M mice die annually in the EU before scientific use. We hypothesized that pre-weaning mortality in laboratory mouse breeding is associated with cage social and macro/micro-environment conditions. Here we counted pups from 509 C57BL/6J litters daily for accurate detection of mortality, and monitored cage micro-environment for 172 C57BL/6J litters. Probability of pups to die increased with the increase in dam age, number and age of older pups in the cage (of overlapped/cohabitating litters), and in small (11 pups) focal litters. Higher temperatures (>23.6°C) and nest scores (>3.75) compensated for some of the socially-associated risks for pup death. These findings can be implemented in strategies for reducing pre-weaning mouse mortality, a more welfare-friendly and sustainable approach for science.