We have had 3 cavies, the ones we now have are 9 yrs and the other is 12 months old... They all pee on the grass and cement. The older one started to pee on trees and lift his leg only out on walks when he was 7yrs old. But will not do it at home (except when we got the new dog and he started to lift his leg to pee on him) (we put a stop to that) Maybe a cavie thing.