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SNIFFING OUT FRIENDS AND FOES: HOW OLFACTORY SIGNALS INFLUENCE THE SOCIAL ENVIRONMENT OF MALE LABORATORY MICE
Home cage aggression in male laboratory mice continues to challenge preclinical researchers. It reduces animal welfare and can alter research parameters, potentially reducing the validity and reliability of study data. While simply reducing aggression would be beneficial, promoting socio-positive, affiliative behaviors would greatly improve mouse welfare as mice are a social species. Mice also use olfaction to communicate, so this sensory modality could be used as a tool to improve social interactions in the home cage. A scoping review of the literature on how mammalian odor signals impact same sex social behavior found that studies are dominated by rodent subjects, treatments from urine, and aggression measures (Chapter 1). As a whole, urine treatments had a variable effect on aggression. This review highlights that treatments from non-urinary sources are not often tested, and affiliative behavior is rarely measured.
One murine odor source worth exploring is found in used nesting material. Mice build complex nests for insulation and it has been speculated that the nest holds odor signals that appease home cage aggression, particularly aggression triggered by cage cleaning. It has been suggested that the nest contains secretions from plantar sweat glands, but the chemical content of neither nesting material nor plantar sweat have been examined. The main goals of this dissertation are to identify the odors stored in used nesting material, determine the sources of those odors, and test them for a behavioral role.
Samples of used nesting material were collected from cages of group housed male mice. Further, plantar sweat, saliva, and urine were collected from the dominant and subordinate mouse in each cage as plausible odor sources. All samples were analyzed for protein and volatile organic compound content. Home cage aggression and affiliative behavior were also recorded to compare to odor profiles. Protein profiles showed that used nesting material contains a variety of proteins that primarily originate from plantar sweat, saliva, and urine sources (Chapter 2). A large proportion of these proteins contain messages about individual identity and bind volatile compounds that further contribute to identity cues. This suggests that the nest aids in maintaining a familiar odor environment. Analysis of volatile content showed that small compounds in the nest are also traced back to plantar sweat, saliva, and urine sources (Chapter 3). Few of the compounds have a known behavior role. However, one compound detected in nest, sweat, and saliva samples had a negative correlation with home cage aggression and three compounds (two from sweat and one from urine) had a positive correlation with affiliative behaviors, making them potential candidates for controlled studies on social behavior.
Before testing the four candidate compounds, a challenge from the correlation study needed to be addressed. Body fluid samples were collected from individual mice based on social status, as this factor impacts production of known murine pheromones. Further, aggression is typically directed from a dominant to a subordinate mouse for territorial reasons. An aggression appeasement signal is likely to be produced by a subordinate to mitigate the dominant mouse’s perceived threat. Data from the correlation study showed no odor profile differences based on social status, and the pheromones that are known to vary with social status did not differ between dominant and subordinate mice. Therefore, Chapter 4 assesses the convergent validity of several dominance measures. Over one week, home cage interactions were observed in group housed male mice. For every aggression occurrence, the aggressor and target mouse was recorded to calculate individual dominance rankings in each cage. Then, individual mice were evaluated for the following measures known to correlate with dominance: levels of urinary darcin (a murine pheromone); scores from three rounds of the tube test; and ratio of preputial gland weight to body length. Postmortem wounding was also compared. Results showed that urinary darcin and preputial gland ratio have strong convergent validity with dominance ranking based on home cage aggression.
Finally, the four candidate compounds (identified in Chapter 3) were developed into treatment solutions to assess their effect on home cage social behavior (Chapter 5). Cages of group housed male mice were randomly assigned one of five treatments (four compounds + control) and home cage aggression and affiliative behavior were recorded for one week. Postmortem wounding was recorded as a secondary aggression measure and social stress was measured through fecal corticosterone metabolites from each cage’s dominant and subordinate mouse (rank based on preputial gland ratio). Treatment did not predict changes in most measures. This may be due to limitations in application or from the original correlation study, which are further discussed.
Although the final study showed null results, future research is still warranted to fine tune application methods and gain a better understanding of how odor signals impact interactions other than aggression. The relationship between olfaction and affiliative behaviors is largely unexamined and this dissertation is a first step in filling that gap.