Aggressiveness compensates for low muscle strength and metabolic disadvantages in shell fighting: an outcome of the individual's past

An individual's metabolic competence is important when escalating to costly behaviours in agonistic encounters. The use of broken shells in the wild induces hermit crabs to reduce their overall metabolic rate; however, these crabs perform better in size-symmetric encounters than crabs that used intact shells in the wild. Given this surprising finding, we explored whether carrying a broken shell increases the resource holding potential (RHP) of hermit crabs by building up their muscle strength. These shells are very heavy, with an unusable portion due to being broken; however, the crabs have to carry the complete structure. Additionally, we explored whether the use of broken shells induces long-lasting consequences in motivation (aggressiveness). We conducted body size asymmetric contests to decrease the relative effect of aggressiveness by increasing the effect of RHP. We predicted a decrease of the agonistic behaviours of crab that occupied broken shells in the wild (CBS) relative to those that occupied intact shells (CIS). Symmetric and asymmetric contests were conducted in pairs formed of a CBS and a CIS, both forced to abandon them and occupy a Nerita shell. The CBS had lower muscle strength, but fought at higher intensity in symmetric contests than CIS. The differences in body mass decreased the expression of agonistic behaviours of CBS relative to CIS. Prolonged poor conditions following the use of a broken shell seem to increase aggressiveness by affecting the long-term subjective resource value. Our findings highlight the role of the recent past associated with shell used on fighting performance. Significance statement Hermit crabs commonly fight to exchange their inadequate shell for a new one. Although high metabolic rates favour the fighting ability, crabs that occupied broken shells in the wild have low metabolism and perform better in shell fighting than crabs that used intact shells. Broken shells are heavy and larger due to having an unusable portion, but the crab has to carry the complete structure. We explored whether carrying these shells increases muscle strength and/or the aggressiveness of crabs, which might explain their fighting dynamic. Unexpectedly, crabs with broken shells were found to be weaker than those with intact ones. Instead, the use of a broken shell in the wild enhances individual aggressiveness. The results highlight that a previous history under adverse conditions can increase the likelihood of animals with functional disadvantages acquiring better resources.