They have two small limbs for slow locomotion, but the creatures rarely move, and never swiftly. They will stay in roughly the same area for days on end, pouring over the rocks in the immediate area, looking for lichens and mosses for nourishment. They have acute senses of hearing, and upon hearing other animals approach, hold still - they can maintain their position for hours, if necessary. If, by chance, they are discovered, Fenner's rocks will open their jaws wide, baring a massive looking set of teeth while releasing a piercing screech. However, the teeth are purely for show - they are soft, cartilage growths that would be ineffective against a predatory animal (assuming the Fenner's rock would be able to move swiftly enough to bite, which is doubtful)

Fenner's rocks are believed to be largely silent animals - this is not true. Due to their hearing, they can detect predators from quite a distance and cease their activities. However, observation by remote sensors has shown that groups of Fenner's rocks often "sing" to each other, and the songs change depending upon the size of the group, availability of food, the weather and the season of the year. Scientists aren't sure exactly what function these communications serve.

Fenner's rocks have become a problem on some worlds where food is abundant and predators are rare. They can multiply so quickly that within a season or two they can choke off natural waterways. In these massive numbers, they also produce high levels of certain dangerous strains of bacteria which have been known to infect animals and sentients.