In seasonal environments, herbivorous mammals employ physiological, morphological, and behavioral strategies to meet their nutritional demands despite fluctuating environmental conditions. Biochemical parameters that represent nuanced indicators of nutritional status and physiological function can provide a framework to understand animal-habitat interactions at a population level. Here, we assessed variation in Guanaco, Lama guanicoe, nutritional physiology in a temperate, semiarid region (Monte Le & oacute;n National Park, in southern Patagonia, Argentina), testing the hypothesis that their nutritional state responds to the seasonal variation in resources and energy demands of their annual cycle. We captured 36 free-ranging, adult guanacos between austral early spring and early autumn, and measured biochemical variables in blood serum that are proxies of nutritional intake and mobilization of stored energy (glucose, urea, creatinine, urea:creatinine, albumin, total protein, alkaline phosphatase, nonesterified fatty acids). Urea was highest in early spring, following winter food deprivation, contrary to the expectation of enhanced urea retention and recycling. High urea levels may reflect the constraint of the energy demand of such recycling (e.g., cost of urea transfer or subsequent protein synthesis with recycled nitrogen) when food intake is still low. Guanacos maintained high and steady levels of glucose from early spring to early autumn, probably reflecting continuous hepatic gluconeogenesis from amino acid precursors. Low levels of nonesterified fatty acids in early spring suggest an apparent lack of seasonal mobilization of lipid stores, which is consistent with previous reports of very low body fat content for related species. Urea and creatinine were higher during pregnancy for females, potentially reflecting seasonal variations in food resources rather than the use of body proteins as an energy source, although future research is needed to address this issue. Our findings suggest that guanacos have a distinctive phenotype of macronutrient utilization compared to typical ruminants, indicating that seasonality in their nutritional state may differ from the general pattern in other large-bodied herbivores. Guanacos show seasonal nutrient and energy use patterns that differ from typical ruminants. Guanacos seem to rely on amino acids for glucose synthesis, to vary their urea recycling, and to rely less on fat storage. Their unique metabolism may be adaptations to arid, low-productivity environments. En entornos estacionales, los mam & iacute;feros herb & iacute;voros emplean estrategias fisiol & oacute;gicas, morfol & oacute;gicas y comportamentales para satisfacer sus necesidades nutricionales a pesar de la variaci & oacute;n de las condiciones ambientales. Los par & aacute;metros bioqu & iacute;micos que representan indicadores precisos del estado nutricional y la funci & oacute;n fisiol & oacute;gica pueden proporcionar un marco para comprender las interacciones animal-h & aacute;bitat a nivel poblacional. En este trabajo evaluamos la variaci & oacute;n de la fisiolog & iacute;a nutricional del guanaco (Lama guanicoe) en una regi & oacute;n templada y semi & aacute;rida (Parque Nacional Monte Le & oacute;n, en el sur de la Patagonia, Argentina), poniendo a prueba la hip & oacute;tesis de que su estado nutricional responde a la variaci & oacute;n estacional de los recursos y las demandas energ & eacute;ticas de su ciclo anual. Capturamos 36 guanacos adultos silvestres entre principios de primavera y principios de oto & ntilde;o austral, y medimos variables bioqu & iacute;micas en el suero sangu & iacute;neo que son indicadores de la ingesta nutricional y la movilizaci & oacute;n de la energ & iacute;a almacenada (glucosa, urea, creatinina, urea:creatinina, alb & uacute;mina, prote & iacute;na total, fosfatasa alcalina, & aacute;cidos grasos no esterificados). La urea fue m & aacute;s alta a principios de primavera, luego de la escasez de alimento en invierno, contrariamente a lo esperado de una mayor retenci & oacute;n y reciclado de urea. Los altos niveles de urea podr & iacute;an reflejar la limitaci & oacute;n de la demanda energ & eacute;tica de dicho reciclado (por ejemplo, el costo de la transferencia de urea o la s & iacute;ntesis de prote & iacute;nas con el nitr & oacute;geno reciclado) cuando la ingesta del forraje es todav & iacute;a baja. Los guanacos mantuvieron niveles elevados y constantes de glucosa desde principios de primavera hasta principios de oto & ntilde;o, lo que probablemente refleja una gluconeog & eacute;nesis hep & aacute;tica continua a partir de precursores de amino & aacute;cidos. Los bajos niveles de & aacute;cidos grasos no esterificados a principios de la primavera sugieren una aparente falta de movilizaci & oacute;n estacional de las reservas de l & iacute;pidos, en consonancia con estudios previos referidos al bajo contenido de grasa corporal en especies emparentadas. La urea y la creatinina fueron m & aacute;s elevadas durante la gestaci & oacute;n en las hembras, lo que podr & iacute;a reflejar variaciones estacionales en los recursos alimenticios m & aacute;s que el uso de prote & iacute;nas end & oacute;genas como fuente de energ & iacute;a, aunque se necesitan m & aacute;s estudios para abordar este mecanismo. Nuestros resultados sugieren que los guanacos tienen un fenotipo diferente de utilizaci & oacute;n de macronutrientes en comparaci & oacute;n con los rumiantes t & iacute;picos, indicando que la estacionalidad en su estado nutricional puede diferir del patr & oacute;n general en otros herb & iacute;voros de gran tama & ntilde;o.