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