4.2 The effect of both EE and HR on FCE
In this study, we employed two indicators to assess the FCE of grazers.
Our results demonstrate that FCE increases with harvest rate but
decreases with energy costs (Fig. 4). Notably, we defined foraging
efficiency as the ratio of harvest rate to energy costs, which
significantly enhanced the FCE (Fig. 4). Both harvest and energy
expenditure are predictive of FCE. Of these, physical activity can cause
the greatest variation in the rate of energy expenditure (Wilson et al.,
2018). For instance, foraging has been shown to elevate energy
consumption in wapiti and moose by 33% and 28-39%, respectively
(Karasov, 1992), while an improved harvest rate has been linked to
increased body weight gain (Van der Graaf et al., 2005). In addition,
our findings reveal that estimating energy costs provides a more
comprehensive prediction of FCE across both groups, as opposed to the
harvest rate, which only predicts FCE within each group individually
(Fig. 4). This suggests that the harvest rate is not directly
proportional to energy consumption, pointing to underlying physiological
differences in energy expenditure. Our analysis further clarifies that
FCE is more influenced by harvest rate in the lamb group compared to the
dry ewe group (Fig. 5), in the case of larger sheep, harvest rate
contributes to FCE but is offset by the energy expenditure required,
leading to a contrasting conclusion in the lamb group. The interaction
effects in lambs neutralize the impact, although they do not eliminate
the negative effect (Fig. 5). Energy expenditure during foraging
includes step-level feeding strategies (Supplementary Fig. 1) and unique
fluctuation patterns (gut throughput rate: foraging-ruminating), akin to
those observed such as the tail-beat oscillations of sharks during
swaying, the wing-beat cycle of birds, and the push-off by limbs in
terrestrial animals (Gleiss et al., 2011).
Although our controlled experiments enabled us to assess the
relationship between harvest and energy expenditure across
different-sized animals and their impact on FCE, they did not fully
capture the optimization of behavioral strategies during the diffusion
process (Benoit et al., 2020; KlarevasâIrby et al., 2021). The grazers
may seek more efficient resources to improve their harvest rate or adopt
more efficient diffusion strategies, both of which can influence
foraging efficiency. Therefore, we hypothesize that grazers in a
free-ranging environment may exhibit altered relationships between
harvest rate and energy costs, subsequently affecting FCE.
Conclusions
Our study introduces an ecological research framework that links
observable foraging behaviors of herbivores to their proficiency in
converting resources into energy.
The rapid utilization of
resources during acquisition is pivotal for optimizing energy conversion
efficiency in animals. However, the energy expended in this process
inherently restricts the efficiency of resource acquisition. A
significant finding of our study is that animals of varying sizes
demonstrate distinct differences in energy expenditure and resource
acquisition rates. Typically, smaller animals, such as lambs, are more
heavily influenced in terms of resource acquisition efficiency, whereas
larger animals, like adult sheep, are predominantly affected by energy
consumption. This distinction provides a novel approach for inferring
the internal physiological states of animals, including foraging
behavior and energy transformation, by observing their behavioral
patterns. Conclusively, comprehending and forecasting the energy
conversion efficiency of terrestrial herbivores is crucial for
evaluating their survival and development in ecological habitats.
Furthermore, insights into the growth stages of these animals enhance
the precision of growth rate assessments.