Otters (Lutra lutra) mark territories by depositing feces on prominent spots, like rocks or large tussocks of grass. However, sometimes they defecate on intentionally mounded heaps made of mud and/or plant material. We show that in sites occupied by otters the frequency of heap-making significantly increases with the frequency of otters occurrence and is independent of the availability of naturally occurring potential marking spots. Constructing a heap is presumably costly, and thus heap-making could be an honest signal of propensity for defending territory. We propose that it should positively correlate with the quality of the territory and its owner.