Corresponding author
* To whom correspondence should be addressed.Junyi Liang (Department
of Grassland Resource and Ecology, College of Grassland Science and
Technology, China Agricultural University, Beijing 100193, China. +86 10
62733381; liangjunyi@cau.edu.cn)
Abstract
Herbivores adopt foraging strategies to maximize efficiency in diverse,
resource-constrained environments. However, the effectiveness of these
strategies may be more constrained by their capacity for energy cost
rather than their ability to acquire resources. The swift utilization of
resources during acquisition is crucial for optimizing energy conversion
efficiency in animals. Nonetheless, the energy expended in this process
inherently limits food conversion efficiency (FCE), an aspect that
remains insufficiently explored in current research. In this study, we
introduced a concept framework that integrates harvest rate (HR) and
energy expenditures (EE) into evaluating herbivore FCE. Utilizing
high-resolution tri-axial accelerometry within a grazing treatments
platform, we analyzed the behaviors of herbivores (Ovis aries ) to
determine the energy costs and time allocation for both lamb and dry ewe
groups. Our analysis demonstrated an inverse correlation between HR and
EE, exhibiting both positive and negative influences on FCE. Notably,
the impact of EE was more pronounced in larger-sized grazers (dry ewes),
while HR significantly influenced smaller-sized grazers (lambs).
However, the interaction effects between these variables tended to
neutralize the variations in FCE observed across both groups. Our
research highlights how the behavioral patterns of grazers, in terms of
resource acquisition and relative energy costs, are pivotal in
determining resource utilization efficiency. Additionally, it reveals
the trade-offs in these behaviors, which transition from being
beneficial to restrictive as the body grows. This finding substantiates
the theory that the behavior of herbivores is a reliable predictor of
their efficiency in resource utilization.
Keywords: harvest rate, energy expenditure, food conversion
efficiency, tri-axial accelerometry, foraging strategy
Introduction
Understanding the mechanisms
underlying food conversion efficiency (FCE) is crucial in determining
how herbivores convert ingested resources into energy for growth,
reproduction, and maintenance (Belovsky, 1986; Illius and Gordon, 1992;
Oonincx et al., 2015; Roehe et al., 2016). A key aspect of this process
is how resource availability supplements the energy harvested by
herbivores (Sollenberger et al., 2012; Vallentine, 2000; Venter et al.,
2019). The uneven distribution of resources has prompted territorial
herbivores to develop time-minimizing strategies to balance
physiological constraints from handling and digestion (Bergman et al.,
2001a; Zubieta et al., 2021). Thus, for field-grazing or wild
herbivores, the time spent foraging becomes a limiting factor in
resource acquisition rates, subsequently impacting the rate at which
food can be converted into energy consumption. However, herbivores are
not only limited by their ability to rate of acquiring energy but also
by their capacity to expend it efficiently (Hudson, 2018; Speakman et
al., 2021). Thus, how both harvest rate (HR) and energy expenditure (EE)
collectively influence FCE in herbivores remains unclear (Fig. 1).
The dynamics of foraging behavior and dietary intake in herbivores are
intricately linked to the availability and constraints of resources
(Illius and Gordon, 1992; Pyke, 1984). This relationship plays a pivotal
role in shaping their energy acquisition strategies. In environments
abundant with resources, herbivores primarily focus on minimizing the
time spent foraging, and optimizing their energy intake within the
shortest possible duration (Gross et al., 1995; Norberg, 2021).
Conversely, scarcity of resources prompts a shift in strategy. In this
case, herbivores may extend their foraging time, thereby increasing
their energy intake to meet the necessary demands, albeit at the cost of
reducing time for other vital activities (Bayliss and Choquenot, 2002;
Bergman et al., 2001a). Crucially, the adaptability of herbivores in
altering their time strategy in response to fluctuating resource
availability is modulated by their energy consumption needs (Fig.1a). In
each feeding bout, grazers exhibit muscle contractions to sustain motion
conservation, markedly affecting energy costs (Gleiss et al., 2011). The
step-level foraging active (consuming energy cost) did not vary
proportionally to the time in foraging, as the intense and strength
differed from thousands of biting activities each day in response to the
resources conditions (Fig. 1a), which accrued significant energetic cost
over prolonged period-ranging corrected to the interaction to the
resources conditions (Shipley, 2007). Thus, the energy cost not only
consumes the physiological energy of herbivores but also reflects the
strategies selection in response to resource conditions which is linked
to the how rate of grazers consume in the given time. However, how the
resource conditions varied in changing the relationship between EE and
HR and further interaction on FCE remains unclear.
The advent of bio-logging technology, specifically herbivores-borne
bio-loggers, allows for the precise measurement of movement and dynamic
body acceleration on a fine temporal scale (Gleiss et al., 2011).
Consequently, this enables researchers to develop indices of EE that are
directly tied to body movement (Gregorini et al., 2008), as well as to
calculate absolute values of EE that correlate with specific animal
behaviors (Williams et al., 2014).
Our research was conducted on a controlled grazing platform, tailored to
minimize dispersal effects and focus on the intrinsic dynamics of
grazing (Bonte et al., 2012; Van Dyck and Baguette, 2005). We first
examine the relationship between HR and EE. It is hypothesized that a
negative correlation exists between these two variables; we anticipate
that herbivores expend less energy and time harvesting when resources
are abundant. Conversely, we expect a rise in EE with diminished
resource availability, reflecting increased foraging effort (Fig. 1a).
Our second objective was to investigate the influence of both HR and EE
on FCE, considering the physiological states of our two subject groups:
lambs and dry ewes. We hypothesized that FCE in the dry ewe group is
more significantly impacted by EE with the assumption that energy costs
scale linearly with the metabolic body weight of herbivore ungulates
(Hudson et al., 2013; Nagy, 2005). Despite facing similar resource
conditions in the grazing plots, we proposed that HR presents a greater
challenge for the lamb group, given the comparative inexperience and
lower resource-harvesting efficiency of these younger animals (Fig. 1b).
Materials and Methods