Body size modulates the extent of seasonal diet switching by large mammalian herbivores in Yellowstone National Park

Prevailing theories about animal foraging behaviours and the food webs
they occupy offer divergent predictions about whether seasonally limited
food availability promotes dietary diversification or specialisation.
Emphasis on how animals compete for food predominates in work on the
foraging ecology of large mammalian herbivores, whereas emphasis on how
the diversity of available foods generally constrains dietary opportunity
predominates work on entire food webs. Reconciling predictions about what
promotes dietary diversification is challenging because species’ different
body sizes and mobilities modulate how they seek and compete for
resources—the mechanistic bases of common predictions may not pertain to
all species equally. We evaluated predictions about five large-herbivore
species that differ in body size and mobility in Yellowstone National Park
using GPS-tracking and dietary DNA. The data illuminated remarkably strong
and significant correlations between body size and five key indicators of
diet seasonality (R2 = 0.71-0.80). Compared to smaller species, bison and
elk showed muted diet seasonality and maintained access to more unique
foods when winter conditions constrained food availability. Evidence from
GPS collars revealed size-based differences in species’ seasonal movements
and habitat-use patterns, suggesting that better accounting for the
allometry of foraging behaviours may help reconcile disparate ideas about
the ecological drivers of seasonal diet switching.