Same resource,different benefits: hermit crab shell structure advantages owners,but not intruders in agonistic interactions

Resources may confer advantages by enhancing their owners’ fighting ability (resource holding potential; RHP). While the resource-correlated RHP hypothesis has been recognized as a determinant of agonistic success in different taxa, this has mostly been based on assessment of either the intruder or the owner, but only rarely in both contestants. We tested whether the internal structure of shells affects hermit crabs’ RHP, both as owners defending the shell against eviction and as intruders attempting to gain access to an occupied shell. We conducted contests (n?=?60) to compare the success in shell eviction by intruders in intact shells vs. in shells with the columella artificially reduced, and the success of shell retention by owners in intact shells vs. shells with the columella reduced. The internal configuration of the shell showed different resource-correlated RHP effects depending on the individual’s role in the fight. The presence of a columella in the intruder’s shell did not affect the likelihood that they would evict their opponents. However, owners resisted more evictions in shells with intact columella than those in shells with reduced columella. Our results demonstrate that the same resource can offer different RHP advantages to owners and intruders during an agonistic interaction.