Rating enrichment items by group-housed laboratory mice in multiple binary choice tests using an RFID-based tracking system

There is growing evidence that enrichment of housing conditions of laboratory animals has positive effects on behavior, growth, and health. Laboratory mice spend most of their lives in their housing rather than in experimental apparatus, so improving housing conditions is a first-choice approach to improving their welfare. Despite the increasing popularity of enrichment, little is known about whether it is also perceived as being beneficial from the animals point of view. This is especially true due to the fact that enrichment has become an umbrella term that encompasses a wide variety of different elements. Therefore, we categorized enrichment items according to their prospective use into the categories structural, housing, and foraging. In multiple binary choice tests we let 12 female C57BL/6J mice chose and rank 5 enrichment items per category. All possible pair combinations of enrichment items within each category were presented counterbalanced for a 46-hour period in a home cage based system consisting of two interconnected cages. A new analyzing method combined the binary decisions and ranked the enrichment items within each category by calculating worth values and consensus errors. Mice ranked the lattice ball (foraging), the rope (structural) and the second plane (structural) in upper positions. No clear preferences were determined for different types of housing enrichment during inactive times (light phase) whereas these objects were actively explored during the dark phase. Here the floorhouse and the paperhouse revealed high worth values. Overall, a high consensus error in ranking positions was observed reflecting strong individual differences in preferences. This highlights the importance of a varied enrichment approach as not all mice prefer the same item at all times. Given the known overall beneficial effects of enrichment, these data will help to provide appropriate enrichment elements to improve animal welfare and refine animal experimentation.