Energy density and its seasonal variation in desert beetles

Energy densities of adult beetles from the Simpson Desert, central Australia, were measured to provide a basis for understanding seasonal energy allocation, prey switching and selection among insectivores, and variation among taxa. The mean energy density of adult beetles is 21.14±2.29 J mg−1 ash-free dry weight (AFDW), energy density varying significantly among families. The energy density of beetles exhibits significant seasonal variation, and is maximum in autumn (22.13±2.24 J mg−1 AFDW) and minimum in spring (19.84±2.03 J mg−1 AFDW). Also, energy density is higher in the abdomen (21.98±2.68 J mg−1 AFDW) than the thorax (20.56±2.81 J mg−1 AFDW), head (20.11±2.88 J mg−1 AFDW) and legs (19.62±2.91 J mg−1 AFDW); each body region changes seasonally, as it does for the whole beetle. Energy density is correlated negatively with body water content (r=−0.559, P<0.001). Seasonal shifts in energy density among taxa and body components indicate different strategies for conservation and use of energy among desert beetles. Such shifts may also explain prey switching and selection among insectivores.