Housing of castrated and intact laboratory mice in pairs as a solution for aggressive behaviour

Aggression between laboratory mice is a common problem and can result in welfare issues such as mice hurting or killing each other and also stress. Swedish laws and regulations clearly states that mice must be housed with a companion due to their natural social structure. Many studies have investigated the effect off enrichment, group size, cage size and other potential changes in in their environment on the occurrence of aggression. Even with these changes aggression can exist and therefore a new solution was tested in this paper. The proposed solution was to house an intact mouse with a castrated one and thus fulfill laws and regulations while still maintaining the physiology of one of the mice for the purpose of medical experiments. The mice used in the study was transgene males, C57BL/6 males, C57BL/6 femlaes och Balb C females. They were housed in makrolon 3 elevated cages with 6 cm aspen sawdust substrate, unbleached autoclaved paper jams as nesting material, nestlets, one paper pulp house, two aspen gnaw sticks and water and food (R3, manufacturer Lactamin) was provided ad libitium. The animals who was to be castrated was so at the age of 4 weeks. Depending on the particular mouse the study started at the age of 5 or 6 weeks with them being paired one intact mouse with one castrated. 6 combinations were formed with differentiating strain and sex with 5 pair in each combination. Evaluation of aggression was performed with two methods, observational studies and continous monitoring of injuries and aggressive behaviour with imminent risk of injuries. Observational studies was carried out at the point of pair formation as well as cage changing for 15 minutes where the chosen behaviours were recorded by an observer. If aggressive behaviour were to intense and was threatening to lead to injuries the pairs were separated. The continous monitoring was carried out at least once a day by a animal technician or veterinary and it’s function was to abort pairs in which injuries was present or if aggressive behaviour took place which had a high risk of resulting in injuries. We found that pairs with an intact male and a castrated male were unfitting because all of these pairs was separated. In contrast only 10 % of groups with an intact male and a castrated female were separated. No strong relation between strain and abortion was found. Observational studies showed that a much higher grade of the chosen behaviors were carried out at pair formation than cage changing. The only behaviour with a explainable cause was mating attemps since it was only carried out by the transgene males we used in this study. The conclusion of this study is that pair housing with an intact male and a castrated male is not fitting for maintaining the social need of mice whilst not altering the physiology of one mouse due to high prescence of aggression while pair housing with an intact male and a castrated female can be