Gerbils sometimes like to groom each other. To carry out this bonding session they sit on top of each other and comb through their companion's fur, searching for parasites and dirt and smoothing out tangles.
It's a good sign when your pets do this, as the behavior means they’re getting on well. In fact this is one of the signs to look out for when introducing gerbils for the first time. It shows that they trust each other.
Mutual grooming reinforces gerbils' social bonds
In addition to being a bonding exercise, grooming also helps gerbils reach otherwise inaccessible parts of their anatomy. This is one of the many reasons why these animals should not be kept on their own - they need each other!