Male size-dependent dominance for burrow holding in the semiterrestrial crab Neohelice granulata: multiple tactics used by intermediate-sized males

Abstract

Dominance and the establishment of social hierarchies are frequently related to size: the larger individuals gain greater reproductive success, while the smaller ones display alternative mating strategies. We studied the existence of dominance and the alternative mating strategies among Neohelice granulata small (SM) and large (LM) males competing for burrows. LM construct burrows with copulation chambers while SM do not. Field studies showed the existence of a SM's size-range of 30-32 mm carapace width when they change behavior and occupy burrows with copulation chambers (hereafter referred to as the Bswitch size-range^). We found a restricted size-range in mating pair formation. Laboratory experiments showed that LM dominate SM because SM did not construct burrows in the presence of LM, and LM displaced SM from their burrows. When given the chance, recently mated SM occupied burrows without copulation chambers while not recently mated SM occupied chambered burrows. This is evidence that these males may be displaying a cheating mating strategy to copulate with females looking for these burrows: they occupy but do not own these burrows. SM can also intercept and mate females on the surface. Given the size restriction in pair formation, intermediatesized males in the switch size-range (30-32 mm carapace width) may copulate with a broader female size-range, larger and smaller than themselves. In this way, SM in the switch size-range may be acquiring a higher reproductive success by adopting multiple tactics. Male dominance hierarchies have been well documented in crustaceans, except for crabs. Here, we demonstrate male dominance related to the construction and defense of burrows and male size in the crab N. granulata. We found that small males of a certain size range adopt an alternative Bcheating^mating strategy that can enhance fitness. Therefore, although they are not large and dominant, small males can nevertheless achieve high reproductive success as a result of this alternative reproductive tactic.

Neohelice granulata burrow fidelity behaviour related to landscape heterogeneity

“…The organic matter in the sediment and penetrability are very low [39]. As occurs in MCH, crab burrows occupy both the upper-and mid-intertidal zones, but burrows in SAO reach a maximum depth of 25 cm and do not include a mating chamber at the entrance [32].…”

“…Crabs occupy the upper-and mid-intertidal zone. Burrows are deep with maximum depth 70 cm, and include a mating chamber to protect the pair during mating [32,33].…”

“…Burrows appear to be key resources to these crabs, providing shelter against adverse environmental conditions [24,25] and predatory attacks from several species of birds and fish [26][27][28][29][30]. Burrows also provide an area for sexual encounters and a safe place where females breed their embryos [31,32]. N. granulata build burrows differentially depending on the environmental characteristics: while in some populations males build burrows with a mating chamber to protect the pair during mating [32,33], in others, they build burrows with a widened entrance but without a mating chamber [31].…”

“…Burrows also provide an area for sexual encounters and a safe place where females breed their embryos [31,32]. N. granulata build burrows differentially depending on the environmental characteristics: while in some populations males build burrows with a mating chamber to protect the pair during mating [32,33], in others, they build burrows with a widened entrance but without a mating chamber [31]. Burrows with a mating chamber facilitate pre and post-copulatory behaviours, that in turn allow greater sperm transfer.…”

“…Winners of agonistic encounters for the possession of burrows with mating chambers are typically those largest male individuals. Therefore, burrows with mating chamber are always occupied by males larger than 30 mm of carapace width [32], which are consequently more likely to be selected by females during mate searching [31,[34][35][36]. Recent studies reported that N. granulata individuals often make short-term explorations to find food and then use their own footpath to return to the burrow [36].…”