Forage intake by black-tailed deer in a natural environment: bout dynamics

Abstract

We examined the within-bout and seasonal dynamics of intake rate for free-ranging tractable black-tailed deer (Odocoileus hemionus sitkensis) throughout a 2-year period. Based on log frequency analyses of 487 complete active bouts, we concluded that active bouts consisted of a series of foraging bouts separated by ~2 min of nonforaging activity. Black-tailed deer spent ~92% of their active time foraging, independent of sex or season. Dry matter intake rates varied significantly with season, from ~0.5 g/min active in winter up to 1.5 g/min in summer, but few differences were attributable to sex. The large leaf size of skunk-cabbage (Lysichiton americanum) enabled deer to attain and sustain intake rates that approached predicted theoretical Rmax values of 10–12 g dry matter/min (L.A. Shipley and D.E. Spalinger. 1992. Can. J. Zool. 70: 1743–1752). Daily dry matter intake varied from 300 g/day in winter to > 1300 g/day in summer. Deer maintained a constant intake rate throughout only one-third of all active periods; intake was reduced at the end of ~40% of-all bouts, while intake rates increased towards the end of 30% of bouts. We discuss the implications of these dynamics for sampling of incomplete bouts, which is frequently employed by observers. Finally, we discuss the importance of selected, not available, biomass in understanding the foraging intake of ungulates.