Water economy in rodents: evaporative water loss and metabolic water production

Studies on water balance in desert-dwelling granivorous rodents use evapora tive water loss (EWL) and metabolic water production (MWP) to evaluate the efficiency of water regulation, expressed by the model Ta @ MWP = EWL, defi ned by an ambient temperature (Ta) value at (@) which MWP/EWL = 1. Here we evaluate and apply this model (1 - r(2)) determining water regulation effic iency, based on the energetic cost (MR) to maintain water balance (WB), tha t is MR-WB. To test the model, EWL was measured and MWP estimated in nine s pecies of rodents from different localities of northern and north-central C hile (II and IV Regions): Octodon degus (Od) and O. lunatus (Olu) (Octodont idae), Abrothrix olivaceus (Ao), A. longipilis (Al), A. andinus (Ad), Phyll otis darwini (Pd), P. rupestris (Pr), P. magister(Pm), Oligoryzomys longica udatus (Ol) (Muridae) and Chinchilla lanigera (Cl) (Chinchillidae). Literat ure 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), followe d by Od < Olu < Ao < Pd < Pm < Pr < Ad < Al < Ol. 2) EWL and body mass (M) are related through Independent equations considering two distinctive habit ats: EWL (xeric)= 5.968 M-0416 (r = -0.89; P < 0.001) and EWL (mesic)= 17.2 72 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 intrasp ecific level, MWP/EWL and Ta are related through a negative exponential equ ation: MWP/EWL=a 10(-bTa) (r = -0.95; P < 0.001). 5) Ta @ MWP = EWL and hi 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 ac cording to the prevailing animal's habitat : MR - WE (xeric) = 34.627 M-0.3 39 (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 prev ious ones because they include rodents with different dietary habits, are a ble to discriminate patterns in the water regulation efficiency as a functi on of different habitats (xeric and mesic), and enable to evaluate the ener getic cost of water balance.