They bred two lots of foxes and of rats- one for for expressed aggression of any kind towards humans, one for calmness/tameness. The increased flight distance ones look like that.
The decreased ones look like this, with similar raising protocols for both (minimal handling- it's not a nice experiment, I have a lot of problems with the minimal handling and the environments from what I have seen).
They started off breeding purely for flight distance. One of the things they wanted to do was isolate nature vs nurture, so for example the fearful or aggressive foxes would be put in litters of tame foxes, and vice versa, so that others around them modelled a behaviour they were not bred for, and to look at animals of different breeding in similar environments.