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

Abstract

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 1.1 M mice die annually in the EU before scientific use. We hypothesize that pre-weaning mortality in laboratory mouse breeding is associated with cage social and macro/micro-environment conditions. Here we count pups from 509 C57BL/6J litters daily for accurate detection of mortality, and monitor cage micro-environment for 172 C57BL/6J litters. Probability of pups to die increases with the increase in dam age, number and age of older pups in the cage (of overlapped/cohabitating litters), and in small (<6 pups) and large (>11 pups) focal litters. Higher temperatures (>23.6 °C) and nest scores (>3.75) compensate 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.

A laboratory mouse breeding study suggests that new pups born in the presence of older pups and older dams are more likely to die before weaning, but these social risks can be alleviated by proper thermal and light environment.