Food competition, but not other common assays, reveals social dominance in female and male CD1 mice
This study demonstrates that while traditional social dominance assays fail to distinguish rank in female CD1 mice, a food competition task reliably identifies linear dominance hierarchies in both sexes, establishing it as a superior method for studying social organization across genders.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
In the animal kingdom, social life is rarely a flat landscape where everyone is equal. Instead, most social groups organize themselves into hierarchies, a system where some individuals hold higher status than others. This social ranking, often called dominance, is not just about who gets to lead; it shapes how an animal eats, how it handles stress, and even how its body fights off illness. For decades, scientists have studied these hierarchies in male rodents, using specific tests to figure out who is the boss and who is the follower. These tests have helped researchers understand the brain and behavior of males, but a significant gap remained: we knew very little about how female mice establish their own social orders. Because female mice often behave differently than males, the old tests designed for males might not work for them, leaving scientists without a reliable way to study the social lives of half the population.
To solve this puzzle, researchers at the University of Florida set out to find a method that could accurately reveal social rank in both male and female mice. They chose to work with CD1 mice, a common outbred strain known for being socially active, and they put four different behavioral tests to the ultimate challenge. The goal was simple but critical: determine which test could consistently identify a clear leader and follower in both sexes. The team first tried the methods that had worked well for males in the past. They watched the mice in their home cages to see who started fights or who chased whom. They also observed how much urine the mice left behind, a behavior often linked to marking territory and claiming status. Finally, they used a "tube test," a classic experiment where two mice are placed in a narrow tube and must decide who backs down to let the other pass.
The results of these traditional tests were revealing, but not in the way the researchers hoped. When they watched the female mice in their home cages, they found almost no fighting or chasing. While the male mice frequently engaged in these aggressive interactions, the females did not, making it impossible to tell who was dominant based on who started a fight. Similarly, the urine marking test failed to distinguish rank among the females. Although the male mice left distinct patterns of urine that clearly separated the leaders from the followers, the female mice left very few marks, and those that were left did not correlate with a clear social order. The tube test proved equally unhelpful. In this setup, the mice would push against each other in a narrow space, but the behavior of the female mice did not produce a consistent pattern of winners and losers. The test simply could not separate the dominant from the subordinate in either sex when using this specific strain of mice.
The researchers then turned to a different approach: a food competition task. They understood that in the wild, animals often compete for limited resources, and this competition can reveal who holds the power. In the lab, they placed pairs of mice in an arena with a single, desirable sugar pellet waiting on the other side of a gate. When the gate opened, the mice raced to get the treat. The first mouse to reach and eat the pellet was considered the winner of that round. By running these competitions over several days, the team could see who consistently won and who consistently lost. This method worked perfectly. In both male and female groups, the food competition test revealed a clear, linear hierarchy. The mice that won the most races were consistently identified as the dominant individuals, while those who lost were the subordinates. The test showed that the dominant mice reached the food faster than the subordinate ones, a difference that held true regardless of whether the mouse was male or female.
What makes this finding so important is that it finally provides a reliable tool for studying female social behavior. The study showed that while female mice do not use aggression or territorial marking to establish their rank in the same way males do, they do form strict social hierarchies when it comes to accessing food. The researchers found that these hierarchies were just as organized and predictable in females as they were in males. This discovery suggests that the way female mice express dominance is different from males, relying on priority access to resources rather than overt fighting or scent marking. By proving that food competition is a robust way to measure social rank in both sexes, the study opens the door for future research. Scientists can now use this method to explore the brain mechanisms behind social dominance in females, a field that has been difficult to navigate without a test that actually works. The work confirms that to understand the full picture of social life in mice, researchers must look beyond the old rules designed for males and find the methods that speak to the unique behaviors of females.
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