The effect of a high fibre diet on energy and water balance in two Namib desert rodents

The effect of dietary fibre content on food consumption, digestive ability and water balance was studied in two Namib desert rodents, Aethomys namaquensis and Gerbillurus paeba. In addition, changes in these factors were monitored when free water was withheld. 1. Daily energy expenditure (DEE) for A. namaquensis remained fairly constant and similar to that predicted by mass despite changes in food and water regimes. The DEE in G. paeba was more variable. On a millet (low-fibre) diet with ad lib. water DEE was 35% higher than the expected value. This increased still further within the first week of water deprivation before dropping back to pre-water deprivational levels once mass had stabilized. On the high fibre diet, the DEE of G. paeba was similar to that predicted by mass when water was freely available. This was reduced by half when the animals were water stressed, suggesting that G. paeba might employ torpor under these circumstances. 2. Water balance was assessed by measuring water intake and loss. Faecal water content when maintained on a millet diet was similar in both species, irrespective of whether water was freely available or withheld. However, urine concentrations and estimated evaporative water losses were higher in G. paeba. Observed differences in water loss are largely a function of variables related to weight specific metabolic rate. Differential water losses were offset by differences in metabolic water production, enabling both species to attain a positive water balance and survive indefinitely on a millet diet. 3. When provided with a more fibrous diet, low assimilation efficiency and the resulting increase in faecal production, coupled with increased faecal water content, increased faecal water loss to such an extent that it precluded the maintenance of a positive water balance in either species. Despite increased urine concentration and a decline in evaporative water loss, both species rapidly succumbed to chronic water deprivation.