In recent years, laboratory mice have increasingly been kept in research facilities for extended periods, allowing them to reach advanced ages. As the phenotype of mice changes with age, adjustments in evaluation criteria during daily inspections are necessary to accurately detect and assess age-related conditions. The criteria typically applied to younger animals tend to overestimate age-related abnormalities, often resulting in the erroneous culling of healthy mice solely based on their age.
A nuanced evaluation of age-related phenotypes without mistaking them for pain, suffering, or harm poses a challenge for all involved personnel and requires both training and experience. It is essential to define when aging begins, what phenotypic traits an aging or elderly mouse exhibits, and how aging mice should be housed, cared for, and treated. To assist with this, image and video resources illustrating the phenotypic differences between young and old mice are provided at Norecopa's Refinement Wiki. Additionally, a care sheet for geriatric mice will be referenced once it is published (the care sheet was first submitted for publication in January 2025 and resubmitted in April 2026).
A female C57BL/6J mouse using the running plate for the first time in her life.
A female C57BL/6J mouse at 19 months of age using a running plate after having access to the enrichment item for 2 weeks. This is the same mouse as displayed in the other video, showing the mouse using the running plate for the first time in her life.
Example for environmental enrichment provided to geriatric mice
Example for environmental enrichment provided to geriatric mice.
Example for environmental enrichment provided to geriatric mice.
Female C57BL/6J mice at 15 months of age were housed in groups of 12 animals. In addition to age-related changes in the fur, symptoms of barbering were also observed.
Female C57BL/6J mice at 15 months of age were housed in groups of 12 animals. In addition to age-related changes in the fur, symptoms of barbering were also observed.
Female C57BL/6J mice at 15 months of age were housed in groups of 12 animals. In addition to age-related changes in the fur, symptoms of barbering were also observed.
Age-related changes in the coat of C57BL/6J mice. Due to individual ageing processes, age-related coat changes may be more or less pronounced in mice of different ages. The pictures shown are deliberately not sorted by real age but by signs of ageing. Each picture shows different individuals of C57BL/6J mice. A and B: female, unremarkable appearance, smooth, close-lying, completely black and full coat, C: male, smooth, close-lying black coat, first light spot visible (red arrow), D: male, coat not lying flat but standing up slightly (no piloerection), light spots visible (red arrows), E: male, coat grey mottled, coat not smooth but slightly protruding (no piloerection), F: female, coat colour rather greyish than black, coat dull and slightly protruding (no piloerection). Foto: Paul Friedemann Pohlig
24-month-old female C57BL/6J mice with less dense coat and barbered coat. Foto: Pia Kahnau
C57BL/6J mice with different degrees of age hump (from A - mild age hump to C - severe age hump). Foto: Paul Friedemann Pohlig
C57BL6/J mouse with cloudy eye. Foto: Jan Baumgart
Example of an enriched cage (type IV). The home cage of the 24-month-old female mice is equipped with various structural elements such as A: wooden cube, wooden angle, climbing opportunities, B: a running disc, various houses or a resting plate. Foto: Pia Kahnau
