Effective January 1, 2026, the Animal Care & Use Program will require that Enviropaks – the standard nesting material used in all mouse cages – be ripped into two pieces at the creation or transfer of animals to a new cage.
The Unit for Laboratory Animal Medicine (ULAM), as well as many laboratories, already perform this small but important task for enhanced animal welfare.
This change, which is outlined in the recently revised Mouse and Rat Husbandry SOP 
and the highlighted photos above, is intended to align standard program operating procedures with what is being done in practice across our animal care facilities.
Internal U-M studies have shown that opening the Enviropaks prior to cage introduction promotes healthier nesting behavior and positive outcomes for at-risk and sensitive mouse populations in addition to new weanlings. This also provides clear visualization of nesting activity, allowing for the best assessment of animal health and well-being.
This change is being announced now to allow laboratory personnel sufficient time to ask any questions they may have about implementation of the new procedure(s). All buildings and laboratories** are expected to be in compliance with the new SOP on January 1, 2026.
**Current nesting/enrichment related SToP forms will be unaffected by this change.
Should you have any questions or concerns, please contact your facility’s ULAM Husbandry Supervisor.
Links marked with a lock icon can only be accessed using valid U-M login credentials.