Experts at Cheshire Wildlife Trust are keeping an eye on dormice like this one to make sure they're coping with being released into the wild.
They were reintroduced to a secret forest location 15 years ago using 250 little wooden boxes as homes.
Numbers of dormice have gone down in the last couple of decades because there aren't as many hedgerows and forests - their natural habitats.
Any adult mice found are microchipped so scientists can see how their population has developed since they were reintroduced.
Each mouse is checked by a vet who weighs and scans them, sometimes in cotton bags so they don't escape, as well as taking DNA samples from their fur.
Dormice spend three quarters of their lives fast asleep, so it's quite easy for the vets to check them.
Lots of volunteers have given up their free time to help with the study.