Foraging efficiency: energy expenditure versus energy gain in free-ranging black-tailed deer
Foraging efficiency (metabolizable energy intake/energy expenditure when foraging) was determined over a 2-year period in nine free-ranging Sitka black-tailed deer (Odocoileus hemionus sitkensis) in Alaska, and related to foraging-bout duration, distances travelled, and average speeds of travel. We calculated the energy-intake component from seasonal dry matter and energy content, dry matter digestibility, and a metabolizable energy coefficient for each plant species ingested. We estimated energy expenditures when foraging as the sum of energy costs of standing, horizontal and vertical locomotion, sinking depths in snow, and supplementary expenditures associated with temperatures outside thermoneutrality. Energy intake per minute averaged 4.0 times more in summer than winter; energy expenditure was 1.2 times greater in summer. Animals obtained higher amounts of metabolizable energy with higher amounts of energy invested. Energy intake during foraging bouts in summer was 2.5 times the energy invested; in contrast, energy intake during winter was only 0.7 times the energy expended. Changes in body mass of deer throughout the year increased asymptotically with foraging efficiency, driven primarily by the rate of metabolizable energy intake. Within a season, summer intake rates and winter rates of energy expediture had the greatest effects on the relation between foraging efficiency and mass status. Seasonal changes in foraging efficiency result in seasonal cycles in body mass and condition in black-tailed deer. Body reserves accumulated during summer, however, are essential for over-winter survival of north-temperate ungulates because energy demands cannot be met by foraging alone.