Mice, like us, are social animals. As social animals they like to hang out with each other. Also like us, they dont just hang out with anyone. Who a mouse chooses to hang out with will depend on number of factors, such as how old the other mouse is, what its social status is and what environment they are in.
In mice as in humans, the first step to forming a social bond with someone is approaching them. However, mice dont just approach any old mouse when wanting to make a new friend.
One thing that can make one mouse approach another mouse to begin with is if that other mouse appears to be in distress. One study found that female mice were more likely to approach another familiar female mouse if that mouse was in pain (but this wasnt found in male mice). People have suggested that perhaps the mouse is just curious or perhaps even wants to rescue the other mouse from whatever is causing the distress.
However, other studies have found that mice will avoid cage mates that seem to be in pain. This makes sense as their distress could signal danger or disease, and generally speaking it's best to look out for your own survival first.
A recent study looked to see whether the social status of the mouse in pain might affect a cagemates tendency to approach it. Using male mice, the researchers found that when a subordinate mouse was in a cage with a dominant and mid-status mouse, that mouse was equally likely to hang out with either mouse. However, if the dominant mouse was in pain, he was more likely to approach this mouse than the other one. However, when a dominant mouse was housed with subordinate and mid-status cagemates, he spent equal time with both of them, even when the subordinate one was in pain.
Why might mice only approach a dominant mouse that seems to be in pain? The researchers suggest that it could be that when a mouse becomes sick it disrupts the social hierarchy, so they are no longer perceived as dominant. The previously subordinate mouse might then no longer be 'intimidated' by it, or at least be willing to check out what's going on in case there's the chance that it may now become the top mouse.
First mouse: Davide Santoni
Mice: Alois Staudacher
Watanabe, S. (2014). The dominant/subordinate relationship between mice modifies the approach behavior toward a cage mate experiencing pain.Behavioural processes, 103, 1-4.