The effect of dietary fat content on lactation energetics in the European hare (Lepus europaeus)

European hares selectively feed on plants with high fat and hence energy content. We hypothesized that these dietary requirements limit the ability of hares to adjust daily food intake during periods of high energy requirements, namely lactation. Our measurements in captive lactating females show that does kept on a low-fat diet increased food intake compared to does on a high-fat diet but assimilated significantly lower amounts of energy. Further, does fed a low-fat diet showed a prolonged rise of food intake during lactation, reduced milk energy content and lower milk mass production at large litter sizes. We hypothesize that impaired milk production under suboptimal fat supply is due to the inability of females to increase the capacity of nutrient-processing organs rapidly enough to meet the high energy demands of precocial juveniles with high metabolic costs. Thus, in hares, the production of precocial young may be viewed as a constraint, caused by their inability to dig thermally buffered burrows, rather than as an adaptive reproductive strategy. We suggest that the interaction of lactation energetics, dietary requirements, and reduced plant diversity in modern agricultural landscapes has facilitated the decline of hare populations across Europe over the last decades.