Gestation in white-tailed deer (Odocoileus virginianus) of northern regions occurs throughout winter, when foragequantity and quality are limited. Our objective was to measure the energy cost of gestation during winter and spring in order todetermine its impact on energy balance of deer. We used indirect respiration calorimetry to measure the metabolism of 21pregnant deer every 2–4 weeks during gestation (January–May). Fasting metabolic rates (FMR) were used to develop apredictive equation to evaluate temporal energy costs. A measurable increase in metabolism occurred on day 91 of gestation.FMR (kJ/kg body mass (BM)0.75 per day) of pregnant deer rose curvilinearly (FMR = 0.02(days)2 – 3.261(days) + 465.2), with92.2% of the increase occurring in the third trimester; costs were 45% greater in the last trimester for pregnant than fornonpregnant deer. Peak FMR of pregnant deer at 200 days gestation was 617 kJ/kg BM 0.75 per day, 84% above that ofnonpregnant deer (335 kJ/kg BM 0.75 per day). The total energy cost of gestation, in terms of FMR, was 78 004 kJ/kg BM 0.75 per200 days, a 16.4% increase above that of nonpregnant deer. The temporal increase in energy costs was correlated with springgreen-up, indicating important relationships between energy demands, food quality and availability, spring weather, andphysiological adaptations in deer.

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