Implications of Cropping Rate Variability on the Costs of Vigilance




Kurpiers, Elizabeth M.

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Large grazing ungulates must forage for much of their active time to meet the energy demands of their massive bodies because they depend on nutrient-poor grasses and forbs. As such, maximizing efficiency of foraging bouts is selectively advantageous. Vigilance is a social and antipredator behavior that can result in reduced predation risk; however, vigilance directly competes with foraging and imposes an inherent foraging efficiency cost. As more time is allocated to vigilance, less time is available to forage. Using 199 focal observations of Roosevelt elk (Cervus elaphus roosevelti) collected in the Redwood National and State Parks of Northern California, I explored if increased elk cropping rate resulted in increased short-term forage intake and subsequently compensation for foraging time lost to vigilance. Notably, compensation via increased cropping rate is likely only feasible when food searching time is minimal and bite sizes are small, conditions that were satisfied in our study area. I developed and compared two novel models that describe possible mechanisms through which cropping rate compensation could occur. The Strategic Compensation Model illustrates a scenario where cropping rate compensation occurs due to an adaptative strategy by foraging elk to increase their cropping rate during periods of increased vigilance. In contrast, the Incidental Cropping Variability Model describes a situation where cropping rate variability is not strategic, but rather occurs due to the influences of social and environmental factors on cropping rate. I compared the proportion of time elk spent vigilant to their cropping rate (bites۰observation length-1). Four linear mixed-effect model were estimated and compared using Bayesian Information Criterion model selection analysis. Three of the four models were identified as competing models, and each of the competing models contained the predictors: the proportion of the foraging bout spent vigilant, proportion of the foraging bout spent within one body length of a conspecific, time of day, and length of the focal observation. The influence of these predictors on cropping rate demonstrates that external factors influence cropping rate and supports the idea that cropping rate variability is incidental, occurs sporadically, and can compensate for vigilance in certain circumstances.



Vigilance, Cervus elaphus, Forage intake rate


Kurpiers, E. M. (2022). <i>Implications of cropping rate variability on the costs of vigilance</i> (Unpublished thesis). Texas State University, San Marcos, Texas.


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