TESTING ENERGY-EXPENDITURE HYPOTHESES - REALLOCATION VERSUS INCREASEDDEMAND IN MICROTUS-PENNSYLVANICUS

I tested the ''reallocation'' and ''increased demand'' hypotheses of s easonal allocation of energy using female meadow voles Microtus pennsy lvanicus (Ord, 1815). I quantified the daily energy expenditure of adu lt females by the doubly labelled water method, both in the summer (re productive) and winter (non-reproductive) seasons. Females were studie d in field enclosures and were neither pregnant nor lactating, which m ade it possible to quantify the effect of season on non-reproductive c osts. 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 sh owed 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% tha t of non-reproductive ones. According to these results, females seem t o 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 co sts measured in captive voles can be transposed to field conditions, a nd propose an index of reallocation of energy that describes the strat egy used by individuals. This index should permit the placement of eac h new species studied along the continuum of strategies that probably exists in mammals.