Unraveling the brain mechanisms behind social hierarchies in mice

Social hierarchies are everywhere-think of high school dramas, where the athletes are portrayed as most popular, or large companies, where the CEO makes the important decisions. Such hierarchies aren't just limited to humans, but span the animal kingdom, with dominant individuals getting faster food access, higher mating priority, and bigger or better territories. Whilst it's long been thought that winning or losing can influence the position of an individual within a social hierarchy, the brain mechanisms behind these social dynamics have remained a mystery.

In iScience, researchers from the Okinawa Institute of Science and Technology (OIST) investigate the neurological basis of social hierarchy in male mice, pinpointing the neurons they believe crucial in determining these social hierarchy dynamics.

You may think that being dominant in the animal kingdom is all about physical attributes, like size. But interestingly, we've found that it seems to be a choice, based on previous experience. The brain circuitry involved in these decisions is well conserved between mice and humans, so there are likely useful parallels to be drawn."

 Professor Jeffery Wickens, head of the Neurobiology Research Unit at OIST and co-author on the study

Determining social hierarchy in mice

The researchers used dominance tube tests to determine the social structures within groups of mice. In these tests, mice are put in opposite ends of a tube and stand off against each other, with the more dominant mouse gaining right of way. Through repeated testing across multiple days, they identified which mice were dominant and which were at the bottom of their social order in their cage, based on consistent winning or losing in the dominance tests.

They then paired up the mice, with dominant mice in different cages competing against each other, as well as subordinate mice competing against each other. Depending on the results of these competitions, the mice's social standing in their home cages changed. "This is due to the winner- and loser effects", explained Dr. Mao-Ting Hsu, lead author on this paper. "Those with experience of winning become more dominant in future competitions, and those who lose become less dominant. In this study, we found that the 'loser effect' can be attributed to the activity of certain brain cells, called cholinergic interneurons."

The neuroscience of losing

The basal ganglia is an area within the brain most well-known in the context of Parkinson's Disease mechanisms and treatment. But it's also involved in regulating behavioral flexibility. This means it influences how we adapt to specific scenarios, and the decisions we make in different conditions.

The basal ganglia are split into several components and inside one of these-the dorsomedial striatum-we find a group of brain cells called the cholinergic interneurons, which have been linked previously to this flexible decision making. To understand how these brain cells are involved in social hierarchies, the researchers selectively removed these neurons from the mice and repeated the dominance tests.

Surprisingly, the researchers found differences between brain circuits for winning and losing. Removing the cholinergic interneurons disrupted the 'loser effect'-mice did not reduce in dominance based on past experiences of losing. But no change was seen in the winner effect, suggesting that different brain circuitry is involved in these social behaviors. These results also provide hints that the winner effect is likely to be a reward-based learning process, whilst the loser effect is likely to be a decision-making process when animals face different contexts or environments.

Human social dynamics

Whilst this study was limited to male mice, the findings could shed some light on human social behaviors. "Human social dynamics are obviously far more complex. The boss in one household might be at the bottom of the social ranking at work, and dominance behavior will change depending on the situation," noted Dr. Hsu. "There is little evidence of the brain circuitry involved in these kinds of flexible social behaviors in humans. However, the similarities in brain structure between mice and humans mean these types of studies might help us unlock future insights into human social dynamics."