BODY-SIZE, GENDER, SEED HUSKING AND ENERGY-REQUIREMENTS IN 2 SPECIES OF DESERT GERBILLINE RODENTS, MERIONES-CRASSUS AND GERBILLUS-HENLEYI

1. Small mammals require more energy per unit gut capacity than large mammals and, as a result, are forced to be more selective in their die tary habits. We predicted that when offered seeds, Meriones crassus (a dult body mass [m(b)] = 50-100 g) would husk less and consume proporti onately more husks than the smaller Gerbillus henleyi (m(b) = 8-12 g). Furthermore, in the sexually dimorphic M. crassus, larger males would consume proportionately more husks than females but in G. henleyi, wh ich does not show size difference between sexes, there would be no dif ference in husk intake. 2. Only male M. crassus (m(b) = 86.2 g) consum ed husks; female M. crassus (m(b) = 57.0 g) and both sexes of G. henle yi (m(b) = 9.20 g for males and 8.85 g for females) did not. Therefore , our hypotheses were partly supported. 3. Digestibilities of dry matt er and energy for male M. crassus were lower than for females and thos e of G. henleyi males and females. This was a result of the,greater pr oportional consumption of low digestible, low energy husks. 4. We also predicted that the average daily metabolic rates (ADMR) of these spec ies would be (1) similar to each other when compared allometrically an d (2) lower than those predicted for rodents of their body masses. The se hypotheses were based on the coexistence in deserts and similar gra nivorous diets of the two gerbil species. ADMR for each species was ca lculated from the regression equation of body mass change on metaboliz able energy intake at the point of zero change in body mass. 5. Their ADMRs, 8.23 kJ g(-0.54) day(-1) for M. crassus (m(b) = 72.6 g) and 8.6 5 kJ g(-0.54) day(-1) for G. henleyi (m(b) = 9.05 g), were similar and therefore this hypothesis was confirmed. However, in contrast to our hypothesis, both these species had ADMRs that were similar to predicte d values, that is, 96.1% and 101.1%, respectively, of rates predicted for their body masses.