Under which condition would a body of rock be most likely to exhibit brittle behavior?

A body of rock would most likely exhibit brittle behavior under the condition of LOW TEMPERATURE.
At low temperature, the water in the crevices of the rock will freeze up, the freezing up of this water causes fractures in the rocks  and as time goes on the rocks become brittle and parts of it will start falling away.

Used to water and cool climates, probably not thick coated or fluffy. they most likely have thin coats and have to survive to find food. sorry if this wasn't the answer you were looking for..: )