Evolutionary causes and ecological consequences of sexual differences in hermit crabs were examined through their life history patterns, habitats, behavioral responses to seasonal and size differences in the utilization of vacant shell resources and intraspecific competitive ability. The sand-dwelling hermit crab, Diogenes nitidimanus, and the sand snail, Umbonium moniliferum, were extremely abundant in the study area, comprising a virtually single resource and utilizer system. Size-frequency distributions of the crabs were nearly flat for males but had a prominent peak for larger females, suggesting different patterns of growth and mortality for the two sexes. No conspicuous peak in the size range of small crabs was obvious in size-frequency histograms for each sex from September to the following June. Males were larger than females.
The number and size of vacant shells produced by the population of U. moniliferum, estimated from their survivorship and growth curves, indicated that both extremely small and large shells were readily available, whereas medium-sized shells suitable for small crabs were relatively rare. This resulted in small crabs using shells that were damaged, much smaller than the sizes they preferred, and of less common species or species that did not inhabit the sand flat.
Habitat use by males was more diverse than by females. The males used shells of U. moniliferum and other snails inhabiting the sand flat and shells of preferred size more frequently than did females, and also tended to use shells that were larger and more variable in size, Shells that were broken or epibiota covered were less preferred by both sexes, although epibiota-covered shells were used more frequently by males. Broken shells were used at similar frequency by both sexes.
Crabs supplied with shells of a similar size distribution to that available in the field were cultured for 1 yr. Males tended to grow faster than females, exhausting the limited supply of medium-sized shells. The smaller crabs restrained their growth until such shells became available, explaining the absence of a peak of small crabs in the field population. Although the sex ratio of the field population was female biased, that of the laboratory population was 1:1, indicating that males had a higher mortality rate in the field.
These results suggest that males are better intraspecific competitors than females in the acquisition of vacant shells. This quality enables males to grow larger, which is advantageous in the acquisition of females. However, shell scarcity produced greater stress in males resulting in their higher mortality rate.