The effect of group size, age and handling frequency on inter-male aggression in CD 1 mice

Paulin Jirkof,Letty Medina,Paige Ebert,Donna Strasburg,Natalie Bratcher,Brianna N Gaskill

doi:10.1038/s41598-020-59012-4

Abstract

Aggression in male mice often leads to injury and death, making social housing difficult. We tested whether (1) small group size, (2) early age of allocation to a group decreases aggression and 3) manipulation increases aggression in male mice. A 14wk study was performed to assess the following conditions in male CD-1/ICR mice: group size (1, 2, or 3), age at grouping (5 or 7wks), and manipulation (daily scruffing or minimal weekly handling). Wounds, body weights, food consumption, nest scores, sucrose consumption, fecal corticosterone and blood for hematology were collected. At the end of the study, mice were euthanized and pelted to assess wounding with the pelt aggression lesion scale (PALS). No signs of acute or chronic stress were observed in any of the groups. Trio housed mice showed less bite wounds than pair housed mice. In general, mice in larger groups ate less but weighed more. Individually housed mice, however, had high nest scores, low body weights, and increased sucrose and food consumption. These results suggest that even when nesting material is provided, individual mice may be experiencing thermal stress. Based on this data, CD-1 mice can successfully be housed for up to 14wks and groups of 3 may be the best for reducing even minor levels of aggression (i.e. wounding).

Highlights

Aggression in male mice often leads to injury and death, making social housing difficult
Depending on the availability of resources, a single male mouse might defend a territory against other male mice, or share a territory with a few other male mice; these latter male groups are organized by a dominant-subordinate relationship
Male CD-1/ICR mice were housed under different conditions for 14 weeks and behavioral, clinical, endocrinological and hematological indicators were assessed during the course of the study to test our hypotheses

Summary

Introduction

Aggression in male mice often leads to injury and death, making social housing difficult. These results suggest that even when nesting material is provided, individual mice may be experiencing thermal stress Based on this data, CD-1 mice can successfully be housed for up to 14wks and groups of 3 may be the best for reducing even minor levels of aggression (i.e. wounding). Laboratory mice are housed in same sex groups in small enclosures, are often mixed at various ages with unfamiliar cage mates, undergo experimental procedures and face frequent disruption of odor cues due to cage cleaning procedures All of these common procedures may be stressful and may bear the potential to lead to escalated fighting, severe injury, or even death. Only one of these previous studies conducted in-depth behavioral observations along with physiological measures[12] It is unclear if singly housed mice were compared to harmonious or aggressive male groups. Despite the controversy of the available studies, if male mice, irrespective of dominance status, are given a choice, they seem to prefer social contact with other males over being alone[6,21,22]

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