Researchers at the Francis Crick Institute have shown that mice use chemical cues, including odours, to detect the social rank of an unfamiliar mouse and compare it to their own, using this information to determine their behaviour.
Like many mammals, mice live in a social hierarchy where some are more dominant than others. This helps to avoid conflict and establish breeding partners.
It has been suggested previously that some mice might display fixed behaviour regardless of who they are interacting with, or that physical properties can give cues about social ranking. However, new research published today in Current Biology shows that mice instead infer an unfamiliar mouse's rank through chemical cues transmitted in the air (odours) or through direct contact (non-volatile scent cues).
The Crick team worked this out using a test where male mice enter a transparent tube at opposite ends, meeting in the middle. In this type of confrontation, a more submissive animal will typically retreat1.
The researchers first looked at interactions in mice who shared the same cage, using this to rank each mouse on a hierarchy, before observing how the mice responded to a set of unfamiliar opponents.
They found that the strangers could recognise each other's rank, compare it to their own, and either retreat or force the other mouse to retreat.
The team then tested the mice in the dark, finding that this did not affect rank recognition, suggesting that cues like physical size or behaviour don't determine recognition of a more aggressive opponent. Similarly, castrating the mice to remove their sex hormones had no impact.
Finally, the team experimentally blocked the two chemosensory systems that mice use – one for odours in the air (olfactory system) and one for chemical signals transmitted by physical contact (vomeronasal system).
They found no effect when just one of these systems was removed; both needed to be ablated before the mice couldn't recognise opponent rank. This showed that mice use both olfactory and vomeronasal systems to recognise rank and can compensate if one is missing.
Like mice, people can also infer the social status of others around them relative to their own, also using sensory cues, including language, facial expression or clothing.
The next step for the researchers is to investigate which areas of the brain process the information on opponent rank and own rank and initiate a decision to retreat or advance.
Neven Borak, former PhD student in the State-Dependent Neural Processing Laboratory at the Crick and first author, said: "We've shown that mice weigh up strangers using chemical cues and can detect social status without needing an extensive history of confrontations with those specific opponents. This is a fascinating phenomenon that humans do too mostly using visual cues. Our work offers an interesting perspective on social mobility: humans, like mice, can enter a new group of people but still maintain understanding of own social rank and gauge the social status of unfamiliar people."
Jonny Kohl, Group Leader of the State-Dependent Neural Processing Laboratory at the Crick and senior author, said: "We've shown for the first time how mice integrate internal and external information about dominance. This shows that a decision based on relative ranks is made in the brain before mice show either aggression or submissive behaviour, rather than there being fixed differences in behaviours leading to an aggressive or docile mouse."
The State-Dependent Neural Processing Laboratory studies how processes within the brain are impacted by the state of the body. By studying how physiological states, such as pregnancy, stress or sleep, impact neural circuits in mice, the researchers hope to advance a more integrative view of brain physiology in health and disease.
