Peak energy turnover in lactating European hares: the role of fat reserves

European hares (Lepus europaeus) in central Europe have high energetic costs of reproduction, mainly due to precocial, rapidly growing young that rely largely on energy-rich milk. Thus, hares in this climate build up large fat stores during winter that are then gradually depleted during the spring-to-autumn breeding season. We hypothesized that the diminishing fat stores of females over the breeding season might affect resource allocation, peak energy assimilation during lactation or the total investment in offspring. Therefore, we measured energy intake, milk quality and milk production throughout lactation in spring, summer and autumn in females raising (size-manipulated) litters with three young each, under a natural photoperiod but at buffered ambient temperatures inside our facility. Over the course of the breeding season, the amount of milk production remained constant, but the fat content of the milk decreased. Hence, total energy transfer to young decreased significantly in autumn. By using undecanoic acid as a tracer of body fat mobilization, we were able to show that milk fat partially originated from maternal fat stores, particularly in spring. The peak sustained energy assimilation rates of lactating females were significantly higher in autumn, due to increased rates of food intake. We conclude that fat stores allow female hares to downregulate energy intake and expenditure early in the breeding season, whereas late breeding forces them to reach peak energy intake levels. Accordingly, we suggest that in hares, peak energy turnover during lactation varies with the availability of fat reserves. Limits to the sustained metabolic rate serve as variable constraints on reproductive investment. Thus, there might be a trade-off in energetic costs to mothers rearing early versus late litters in the year.