Plant communities, forage quality, and diet composition on summer ranges of mule deer (2017–2019), Wyoming, USA

Many animals track ephemeral peaks in food abundance and quality that
propagate across landscapes. Migrating ungulates, in particular, track
waves of newly emerging plants from low-elevation winter ranges to
high-elevation summer ranges – known as “green-wave surfing.” Because
plants lose crude protein and gain insoluble fiber with maturation,
ruminants are expected to exploit peaks in forage quality among individual
plants (i.e., Forage Maturation Hypothesis). Although ample evidence
supports the long-standing hypothesis that migratory ungulates surf peaks
in forage quality during migration, the hypothesis that ungulates track
peaks in forage quality at a small scale (i.e., microsurf while on summer
range) remains less known. We studied a partially migratory population of
mule deer in Wyoming, USA, to understand whether temperate ungulates
optimize use of high-quality forage as plants grow and senesce on
disparate summer ranges. Specifically, we evaluated how crude protein,
digestible energy, and relative abundance changed throughout the growing
season and whether deer altered their diet to reflect species-specific
changes in plant phenology. In support of the Forage Maturation
Hypothesis, forage quality declined as large-scale patterns of phenology
progressed away from a remotely sensed metric of peak green-up for most
plant species on the summer ranges of deer that migrated short (<50
km), medium (50–130 km), and long distances (>130 km). Declining
rates in forage quality among plant species were heterogeneous, providing
deer with the phenological diversity required to microsurf. Deer changed
their diet throughout the growing season and prioritized consumption of
some plants, including Rosa woodsii and Purshia tridentata, as
the rank of forage quality increased (P < 0.01). In light of the
complexities common to studies on foraging behavior, our findings suggest
that deer may have some potential to microsurf on summer range when
heterogeneity in resource phenology is prevalent. Moreover, our findings
validate the accuracy of remote sensing in quantifying peak forage quality
for plants within sagebrush shrublands and montane habitats.