Energetic Cost of Lactation in Microtus pinetorum

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Abstract
Captive lactating pine voles (M. pinetorum) were monitored for total metabolizable energy intake (MEI). Metabolic energy requirements increased throughout the 21 days of lactation. Lactating females exhibited positive relationships between MEI and litter size (1-3 young) and MEI and litter weight. The average litter size was 2.2 .+-. 0.1 (SE). The average neonate weighed 2.0 .+-. 0.1 (SE) g at birth. Tissue production by young was estimated at 1.76 kcal/g of live weight gain. The amount of energy deposited as suckling growth was related to both litter size and day of lactation. The average female had an MEI of 418.9 .+-. 21.2 (SE) kcal for the 21 days of lactation, an estimated 134.7 .+-. 9.4 (SE) kcal beyond the maintenance requirement for the nonbreeding female. The average litter incorporated an estimated 35.1 .+-. 1.8 (SE) kcal for energy into tissue production, which was 26.0% of the metabolizable energy above nonbreeding female requirements. The metabolic cost of supporting a litter increased with each day of lactation. The average lactating female and her litter had a 47.5% increase in metabolizable energy requirement above that for the nonbreeding female.