When chasing, also commonly referred to as “pursuit,” a mouse is running after something, usually after a cagemate or target. Chasing can be observed in various contexts, including predatory aggression, sexual pursuit, during agonistic encounters, and playtime.
Chasing may occur in a social context and if the animal gets close enough to the target, it may nip it playfully.
In an aggressive context, one mouse will chase another and upon reaching it more aggressive behaviors will ensue. Aggressive chasing typically ensues right after an attack. An opponent will flee and the dominant mouse will aggressively chase them.
In a natural setting, a mouse will often be observed chasing insects or other prey. There may be a relationship between predatory aggression and intermale aggression.
Chasing may also be observed amongst the other sexual behaviors exchanged between a female and male mouse.
Therefore, depending on the context, chasing is a versatile behavior that may be either affiliative, agonistic, sexual, or utilized as a means for feeding.
Chasing Resembles Following
Theoretically, chasing is similar to the behavior following, because both behaviors involve one mouse being ahead as the other is shadowing along in its trail. The difference is that chasing is often tinted with aggressive connotations. When measured during an experiment, following and chasing can be lumped under the same category, “Social Interactions,” (or chasing may be in the “Aggressive Behaviors” category, based on the researcher’s discretion) and the cumulative scores across categories will determine whether the animal is being social or aggressive.
Function of the Chasing Behavior
- To build relationships: When a mouse is still young, it will chase other mice playfully, in order to build relationships with other mice and familiarize. Playful chasing is important for normal development.
- To acquire food: A mouse will chase its food aggressively, running quickly behind insects and other smaller invertebrates, in order to eat and acquire energy.
- To pursue females: Dominant mice are more likely to chase female estrous mice than non-dominant mice, thus increasing their chances of reproduction.
- To establish social hierarchy: Mice establish social dominance aggressively. Mice with a higher status will chase other mice more frequently than subdominant mice.
- To increase chance of survival: Chasing is an ingrained behavior in mice because it is quite important for increasing survival. Without chasing, mice would be extremely limited in their feeding and socializing options, thus lowering their chances of survival.
Application of Chasing
- In a new environment: When mice are placed together in a novel environment for the first time, chasing is bound to occur for the purpose of establishing social hierarchy.
- In the presence of an estrous female: Rate of chasing only amongst dominant males will increase upon the introduction of an estrous female.
- In agonistic situations: During agonistic encounters, dominant mice are more likely to exhibit chasing behaviors than non-dominant mice.
Behavioral Tests for Assessing Chasing Behavior
Chasing behavior does not occur in isolation. Rather, it happens in junction with other behaviors.
- Cross-Category Comparisons: In an experimental setting, chasing is often counted and classified under a broader category, such as “Aggression” or “Social Interaction,” along with other behaviors and then the cumulative score is taken (and subsequently compared across experimental conditions). For example, “Social Interaction” would include instances of chasing, but also of mounting and wrestling.
- Specific Behavioral Comparisons: Another common method for assessing chasing behaviors is simply to compare it as an individual behavior (rather than a part of a category) and specifically across conditions. Sometimes, chasing is its own category, especially when trying to typify the particular kind of aggressive behavior a mouse is most likely to express.
- Open Field: Pursuit or predatory behavior can be observed using an Open Field apparatus combined with video technology and by placing a mouse and an insect, such as a cricket, in the same arena together. It is also possible to place a pair of mice in the Open Field and observe any spontaneous aggressive interactions.
- Social Interaction Test: In this test, mice are placed in a novel cage together for an allotted period of time and their interaction is recorded. Then, a computer program is used to classified the types of behaviors observed, including aggressive behaviors such as chasing and exploratory behaviors such as digging.
- Resident-Intruder Task: Chasing can be readily observed in the Resident-Intruder Task. A mouse is placed in a cage and allowed to acclimate, becoming the “resident.” Then, a few days later, an unfamiliar or “intruder” mouse is placed in the same cage. Chasing typically ensues as the two mice face each other to establish dominance.
- Modified Resident-Intruder Task: Some researchers use a Modified Resident-Intruder Task which is similar to the classic task but differs in that social behaviors are measured in mice that were reared in grouped housing conditions. In this task, chasing is typically classified as a social behavior.
- Video Analysis: Video analysis makes it possible to record interactions between mice, observe their interaction, and correctly record the frequency of the behaviors which were elicited during an encounter.
Pharmaceutical Effects on Chasing
(-)-Pindolol Decreases Chasing
(-)-Pindolol, a β-adrenergic 5-HT1A/1B antagonist, may be used to decrease agonistic behaviors. A dose of 20 mg/kg of pindolol is associated with a decrease in chasing behaviors in mice. Such a finding may suggest that serotonin receptors are involved, somehow, in the expression of aggressive behaviors such as chasing.
Mouse Strains Exhibiting Chasing Behavior
Chasing behavior, be it aggressive or playful, can be commonly observed in mice. Since chasing is a behavior crucial for survival (it is performed when mice are trying to capture prey), it will be ingrained in mice.
Aggressive Strains Are More Likely to Chase
However, based on the mouse strain, some mice will exhibit chasing behavior more frequently. This is particularly obvious in aggressive mice which chase other mice actively in order to exert force on them and create hierarchy within the cage. For example, mice that rank high in the Tube Dominance Test (which measures aggression levels based on confrontation) are more likely to exhibit chasing behavior during the Social Interaction Test.
ArKO Male Mice Have High Chase Frequency Towards Females
Some mice, such as the homozygous aromatase knockout (ArKO) mice may be drastic and display very aggressive chasing combined with biting towards estrous females. This type of behavior will be seen in 2 out of 10 ArKO mice and, by comparison, is rarely observed in wild-type mice interacting with estrous females.
MAO A/B KO Mice Display Aggression Through Chasing
The MAO A/B KO mouse is most likely to display chasing when performing an aggressive behavior, therefore chasing characterizes the type of aggression displayed by this mouse strain.
- When chasing, a mouse is running after a cagemate or target.
- Chasing can be observed in various contexts, including predatory aggression, sexual pursuit, during agonistic encounters, and playtime.
- Although chasing is similar to the behavior following, it is typically more aggressive in nature.
- Dominant mice are more likely to display chasing behaviors than non-dominant mice.
- The purpose of chasing behavior is to build relationships in young mice (when expressed playfully), to acquire food, and to pursue estrous females for reproduction. All of these reasons explain why chasing is important for survival.
- Chasing may be observed when mice are placed in a new environment (for the purposes of establishing hierarchies), during agonistic situations, and in the presence of an estrous female.
- Chasing may be compared to other specific behaviors in terms of frequency or it may be tallied under a general category together with other category-related behaviors.
- Certain behavioral tests exist in which chasing can be observed and measured, including the Open Field Test, the Social Interaction Test, and the Resident-Intruder Task.
- Some substances, such as pindolol which is a serotonin antagonist, can directly increase chasing behaviors.
- Aggressive mice strains are more likely to display chasing behavior.
- The MAO A/B KO mouse strain expresses aggression through chasing, not fighting.
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