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

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).

Figure 1

(a) False positive PALS (Photo ID 4885, animal ID 102, single, 7 weeks, minimal handling). Spots on the inside right flank, some small spots on the back and neck area. (b) True positive PALS (Photo ID 4766, animal ID 7, pair, 5 weeks, minimal handling). Spots everywhere but not the tail area, vessels, no pigmentation like seen in B6. (c) Average weighted PALS LSM ± SE are plotted. Data on the y-axis is represented as a log₁₀ transformed scale but values have been backtransformed to be meaningful in the real world. Different letters above bars indicate significant differences by post-hoc Tukey tests (P < 0.05).

Figure 2

Sucrose consumption per mouse over time. Asterisks indicate significant differences pair vs single; x indicates significant differences trio vs. solitary.

Figure 3

Food consumption per mouse per week. (a) Comparison of treatment groups. Different letters above bars indicate significant differences by post-hoc Tukey tests (P < 0.05); (b) Development over time. Data on the y-axis is represented as a log₁₀ transformed scale but values have been backtransformed to be meaningful in the real world.

Figure 4

Average body weight per mouse. (a) Comparison of treatment groups. Different letters above bars indicate significant differences by post-hoc Tukey tests (P < 0.05); Bodyweight change per mouse over the experiment: (b) group size, (c) age at allocation and (d) manipulation frequency. (e) Body weight per mouse according to health status. Asterisks indicate significant differences.

Figure 5

Fecal corticosterone metabolites from baseline data only. Baseline was the only time point that showed significant differences via test slices. Therefore, only baseline data has been graphed. Data are represented on the y-axis as a square root transformed scale but values have been backtransformed to be meaningful in the real world. Different letters above bars indicate significant differences by post-hoc, Bonferroni corrected, T-tests (P < 0.0055).

Figure 6

Experimental timetable. a = body weight measurements, b = cage change, food intake, c = nest score, d = sucrose consumption, e = fecal samples.