Mice detect social status by odor

In a complex mouse social world, newcomers can instantly reduce the social level of a stranger without seeing them. New research from the Francis Crick Institute shows that these rodents rely on a sharp sense of smell to make quick social judgments—using chemical cues to decide whether to gain a foothold or retreat in confrontation.

The study, published this month in current biology, overturned previous hypotheses about hierarchies between animals and revealed a complex chemical communication system that helps mice effectively navigate social conditions.

Chemical news reveals social status

When strange mice meet in a narrow tube, they don’t need past experience to know what to retreat. Instead, they use extraordinary sense of smell to detect invisible state signals.

“We have shown that mice use chemical cues to weigh strangers and can detect social status without the need for a history of confrontation with these specific opponents,” explains Neven Borak, a former PhD student and first author of the study.

How does the mouse recognize whose boss? The researchers found that mice rely on two separate chemical volume systems – their standard odor (olfactory system) and a dedicated system for detecting chemical signals through physical contact (Vomeronasal System).

It is worth noting that just two systems are enough – mice lose the ability to recognize social levels only when researchers blocked both systems in their experiments.

Beyond fixed behavior

This study challenges previous theories about the social hierarchy of animals. The mice did not adjust their behavior based on their opponent’s identity relative to their own identity, rather than exhibiting fixed behaviors (e.g., naturally aggressive or compliant).

When researchers tracked behavior in confrontation, they found:

• Mice push more when facing lower opponents
• They retreat more frequently when facing higher opponents
• These behaviors are not fixed characteristics, but change according to the person they face
• No results were determined for body size and body properties

The conflict between familiar mice is faster than that of strangers, suggesting that social recognition accelerates the resolution of conflicts despite the basic hierarchical information provided by chemical signals.

General grade signal

Can these findings also help explain human social behavior? Although we do not primarily use aroma to determine status, humans similarly make quick judgments about the social status of others based on visual and verbal cues.

“This is a fascinating phenomenon that humans do the same, mainly using visual cues,” Borak notes. “Our work provides an interesting perspective on social mobility: Like mice, humans can enter a new group of people, but still maintain an understanding of their own social level and measure the social status of strangers.”

The study also shows that mice can track changes in social status. When researchers artificially painted the rankings of mice with their urine with major male urine (even strangers) they were treated according to their new, higher status.

Brain decision making

What happens to the brain when rats evaluate social hierarchy? “For the first time, we demonstrate how mice integrate internal and external information about advantages. This suggests that relative hierarchy-based decisions are made in the brain, showing aggressive or compliant behavior in mice rather than resulting in aggression or recorded behavioral differences in the mouse.”

The team now plans to investigate which brain regions process information about opponent rank and mouse’s own rank before initiating a decision to retreat or progress.

Beyond the rat society

What makes these findings particularly fascinating is how they inform us about our understanding of social hierarchies across species. Many animals, including humans, live in social structures that understand relative status, help minimize conflict and establish social order.

Although we rely on different sensory systems, the underlying mechanisms of social hierarchical recognition may be evolutionarily more conservative than previous recognition.

Can future research reveal similar nervous system pathways in humans, thus helping us navigate complex social hierarchies? As scientists continue to reveal the biological basis of social behavior, we may find more similarities in various species (from mice to humans) processing information on social status.

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