The Direct Physiological Costs of Natal Dispersal of the Desert Isopod (Hemilepistus Reaumuri)

Abstract

Terrestrial isopods, commonly known as woodlice, are one of the very few land-living groups of the Class Crustacea. Furthermore, the xeric-adapted isopod Hemilepistus reaumuri (Audouin & Savigny, 1826) in particular, is considered the most successful herbivore and detritivore of the macrofauna inhabiting many Old World deserts. The life cycle of this semelparous and monogamous isopod includes a dispersal phase, during which the young isopods emerge from their subterranean burrows and may travel up to one kilometer while dispersing from their natal burrows to the sites where they will attempt to breed. They potentially benefit from this movement by locating new habitats, by finding a mate, and by avoiding inbreeding with conspecifics. However, the direct physiological costs of dispersal have not been studied as thoroughly as other behavioral and life history traits in terrestrial isopods, let alone other organisms. In the present study, the costs of this important dispersal phase of Hemilepistus reaumuri were measured by comparing those isopods which emerged from their subterranean burrows, with which actively dispersed. The dispersers are defined as those isopods which emerged from their burrows, but were then allowed to disperse. The condition at different distances dispersed and the time spent dispersing was analyzed, as was the amount of water and energy lost while dispersing. Ash (minerals and inorganic matter) content of these isopods was determined utilizing an ashing oven. Furthermore, water content was determined with the use of a drying oven, while organic matter and caloric content were determined through bomb calorimetry. In these experiments, it was found that isopods lose energy and water while dispersing, and are generally in worse condition the longer they disperse. However, they actually gain organic material while dispersing. Analysis revealed that the primary metabolic need of emerging isopods is food (organic material), while that of the dispersers is water. Since the emergers have just come out of their humid burrows and did not obtain organic material throughout their growing process, there is a trade-off between time spent eating and time spent desiccating throughout this dispersal period. As a result of this study, a more complete understanding of the physiological costs of this behavioral phenomenon was reached.