Movement Patterns in Sphaeridium: Differences between Species, Sexes, and Feeding and Breeding Individuals

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Abstract
The within-pasture and between-pasture movements of 2 closely related and abundant beetles, S. lunatum and S. scarabaeoides (Coleoptera: Hydrophilidae), feeding and breeding exclusively in cow pats is described. S. lunatum occurs, on average, in older pats that S. scarabaeoides and in both species, mature females occur in older pats than males or immature females. The difference in the colonization pattern between the sexes results from differences in the rates of immigration to and emigration from pats. Females arrive at fresh pats earlier than males, but mature females, unlike immature ones, are likely to leave the pat rapidly; if they stay, they will stay a long time. Density-dependence in the rate of emigration was shown experimentally and was observed in the field. The proportion of individuals with an empty gut was lowest amongst resident immature females, which were probably mostly feeding, higher amongst resident males and mature females and highest amongst arriving individuals. Feeding and breeding are partly conflicting activities in the time budget of these beetles. In S. scarabaeoides, immature females predominate amongst long-distance migrants (the oogenesis-flight syndrome), but in S. lunatum, males and mature females also frequently move long distances. S. lunatum showed more long-distance movements. Other differences exist between mature and immature females. Such differences in movements probably decrease the spatial coincidence between species, which decreases the intensity of interspecific competition, and may allow ecologically similar species to coexist in the same community.