Water economy in rodents: evaporative water loss and metabolic water production
Studies on water balance in desert-dwelling granivorous rodents use evaporative water loss (EWL) and metabolic water production (MWP) to evaluate the efficiency of water regulation, expressed by the model Ta @ MWP = EWL, defined by an ambient temperature (Ta) value at (@) which MWP/EWL = 1. Here we evaluate and apply this model (1 - r²) determining water regulation efficiency, based on the energetic cost (MR) to maintain water balance (WB), that is MR-WB. To test the model, EWL was measured and MWP estimated in nine species of rodents from different localities of northern and north-central Chile (II and IV Regions): Octodon degus (Od) and O. lunatus (Olu) (Octodontidae), Abrothrix olivaceus (Ao), A. longipilis (Al), A. andinus (Ad), Phyllotis darwini (Pd), P. rupestris (Pr), P. magister (Pm), Oligoryzomys longicaudatus (Ol) (Muridae) and Chinchilla lanigera (Cl) (Chinchillidae). Literature information on rodents from xeric and mesic habitats was also analyzed. Results indicate that: 1) Cl has the lowest EWL (0.58 mg H2O/g h), followed by Od < Olu < Ao < Pd < Pm < Pr < Ad < Al < Ol. 2) EWL and body mass (M) are related through independent equations considering two distinctive habitats: EWL (xeric)= 5.968 M-0.416 (r = -0.89; P < 0.001) and EWL (mesic) = 17.272 M -0.532 (r = -0.85; P < 0.001). 3) MWP and M are related through the equation: MWP = 14.256 M--0.539 (r = - 0.98; P< 0.001). 4) At the intraspecific level , MWP/EWL and Ta are related through a negative exponential equation: MWP/EWL=a 10-bTa (r = -0.95; P< 0.001). 5) Ta @ MWP = EWL and M are related through the equation: T @ (MWP = EWL) = 26.799 M-0.142, (r = - 0.49, P < 0.02). 6) MR-WB and M, are related through independent equations according to the prevailing animal's habitat : MR - WB (xeric) = 34.627 M-0.339 (r = - 0.93; P < 0.001) and MR-WB (mesic) = 68.132 M-0.381 (r = -0.86; P < 0.001). These last two equations have comparative advantages to the previous ones because they include rodents with different dietary habits, are able to discriminate patterns in the water regulation efficiency as a function of different habitats (xeric and mesic), and enable to evaluate the energetic cost of water balance.