How adaptation mechanisms interact across functional levels to control the adaptability of an individual during its own lifetime and also across generations is a core complex question of contemporary research. This work belongs to a series of first efforts in our team looking for gaining insights in identifying and understanding the adaptation mechanisms of ruminants through decrypting functional levels effects in a series of environmental and physiological conditions changes. We consider the evolution of energy body reserves (BR) mobilization or accretion process as an indicator of adaptability while evidencing individual differences responsive of internally or environmentally driven changes in grazing ruminants. Forty-one Romane ewes reared in a natural extensive rangeland of Center France were allocated in homogeneous groups according to body weight (BW) and body condition score (BCS), and distributed by parity [multiparous (MULT), n = 20; primiparous (PRIM), n = 21] and litter size (LSi) [having singletons (SING), n = 21 or TWIN, n = 20]. Feeding was restricted to grazing and other fibrous sources (hay), thus avoiding luxury energy intake and enhancing BR mobilization in function of requirements. Individual BW, BCS (1 to 5 scale), plasma NEFA and glucose (GLU) were monitored across a complete physiological year at –44, 0, 24, 61, 88, 119, 168, 207, 227, 257 and 312 d relatives to lambing (DIM). Blood sampling were performed early morning before the first meal. Data were analyzed by mixed procedure of SAS (2003), considering parity, litter size and its interactions as fixed effects. MULT (59.8 ± 1.21 kg) were systematically heavier (P < 0.0001) than PRIM (51.7 ± 1.18 kg). Beginning of lambing, LSi affected (P < 0.05) BW in MULT [where ewes with SING (46.5 ± 0.95 kg) were heavier than TWIN (43.3 ± 0.92 kg)] but not in PRIM. BCS was not affected by parity but, from lambing and throughout the experiment, MULT with SING (2.8 ± 0.06) expressed consistent higher (P < 0.0001) BCS than those MULT with TWIN (2.5 ± 0.06). NEFA was higher for ewes having more than 1 fetus (TWIN; 0.61 ± 0.040 vs. 0.49 ± 0.041 mmol. L−1) just 1.5 mo before lambing, afterward this difference disappeared probably because of a lack of sensitiveness of this parameter for the fluctuating feeding regimens in extensive grazing conditions. This was coherent with the higher concentration of GLU at lambing in MULT with one fetus (SING; 0.83 ± 0.047 vs. 0.68 ± 0.043 g.L−1, P < 0.0385) but not in PRIM. Results indicates that MULT are able to mobilize BR easier than PRIM in situations of negative energy balance (e.g., TWINS vs. SING; peri-lambing days), confirming that flexibility in BR utilization comes with experience, as evidence of an environmentally driven character. Trying to unravel how interactions between heritable and environment-dependent differences between individuals lead to interindividual differences in BR dynamics is our main research goal in the next future.