4.1 The contrasting relationship between HR and EE
Our findings confirm the hypothesis grazers increased energy investment in foraging results in a lower harvest rate from available resources (Fig. 3), when herbivores optimize their harvest rate, akin to time-minimizing grazers that prioritize high energy gain within shorter handling times (Bergman et al., 2001a; Hazen et al., 2015), they also demonstrate a preference for energy efficiency. This is exemplified by the foraging behaviors of wood bison, which balance residence time and patch quality for efficient energy utilization (Bergman et al., 2001). The novelty of our study lies in illustrating that grazers’ foraging strategies encompass not only a preference for harvest rate efficiency but also for optimizing energy expenditure.
The observed as the harvest rate increase a more pronounced decrease in energy expenditure in larger herbivores compared to smaller ones (Fig. 3). Despite the lack of significant size-related differences in harvest rates among grazers, larger sheep demonstrate notably higher energy consumption during foraging than their smaller counterparts, ODBA does not directly measure variation in physiological performance “capacity” but rather the combination of intrinsic physiological constraints and behavioral decisions made in a dynamic environment (Payne et al., 2016), likely due to their overall posture and activity patterns (Chimienti et al., 2020; Halsey et al., 2011). Furthermore, the impact of energy expenditure on harvest rate is markedly greater in larger sheep, implying that smaller sheep exhibit higher foraging efficiency (Fig. 4). This conclusion is drawn from observable differences in harvest rate and energy costs, highlighting behavioral distinctions in foraging efficiency. Although, our study did not delve into physiological aspects such as digestive efficiency in grazers, where higher basal metabolic rates might lead to increased energy consumption and thus reduced foraging efficiency (Nagy, 2005; White et al., 2009). Regarding the differences of biting behaviors in response to resources conditions larger herbivore due to their capacity for larger bite sizes, generally consume more food (Spalinger and Hobbs, 1992; Wilmshurst et al., 2000). However, smaller grazers exhibit a higher relative consumption compared to their larger counterparts in low-biomass grasslands. Smaller animals have shown greater proficiency in maintaining high food intake in environments where resources are limited or dispersed (Fortin, 2006). This efficiency is particularly notable in low productivity or patchy environments, where smaller herbivores can sustain higher food intake levels more effectively than larger herbivores (Fortin, 2006). This finding underscores the adaptive foraging strategies of smaller herbivores in resource-scarce ecosystems.