I tested the "reallocation" and "increased demand" hypotheses of seasonal allocation of energy using female meadow voles Microtus pennsylvanicus (Ord, 1815). I quantified the daily energy expenditure of adult females by the doubly labelled water method, both in the summer (reproductive) and winter (non-reproductive) seasons. Females were studied in field enclosures and were neither pregnant nor lactating, which made it possible to quantify the effect of season on non-reproductive costs. These costs were 20.6% higher in winter than in summer (3.40 vs 2,82 kj x g-1 x day"1). Laboratory data found in the literature showed that the cost of milk synthesis averages 3.53 kJ x g_1 x day"1 Assuming that these costs can be translated to natural conditions, daily energy expenditure of lactating females in the field is 225% that of non-reproductive ones. According to these results, females seem to follow a mixed strategy of both reallocation and increased demand of energy, although the observed pattern of allocation is much closer to an increased demand model. I discuss the assumption that lactation costs measured in captive voles can be transposed to field conditions, and propose an index of reallocation of energy that describes the strategy used by individuals. This index should permit the placement of each new species studied along the continuum of strategies that probably exists in mammals.
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