Voluntary Group-Housing Compatible Fecal Sample Collection in Mice Using a Custom-Made Cage System.

Reliable measurement of fecal corticosterone metabolites requires fecal samples that can be assigned to individual animals and collected within a defined time window. This is challenging in group-housed mice, where pooled home-cage samples do not allow individual assignment, and temporary separation may interfere with animal welfare and study outcomes. The goal of this protocol is to present a simple, inexpensive cage system that enables voluntary, simultaneous, and individual fecal sample collection from group-housed mice. The system consists of a wire-topped, polycarbonate home cage connected to three individual collection boxes. After habituation and training, mice voluntarily enter the boxes to access a small food reward. The doors can then be closed without chasing, handling, or forced transfer. Fecal pellets are collected from each box separately, allowing samples to be assigned to individual animals while maintaining social housing outside the sampling period. This protocol supports non-invasive longitudinal sampling and is suitable for studies measuring fecal corticosterone metabolites or other fecal biomarkers. The design is easy to build from standard cage material and acrylic glass, can be disassembled for cleaning, and allows several cages to be sampled in parallel. By reducing handling-related disturbance and improving standardization of the sampling window, the method provides a practical refinement for endocrine monitoring in group-housed mice.