When can cropping rate compensate for increased vigilance?

Abstract

Herbivores use vigilance to reduce predation risk and interact socially, yet it imposes a foraging efficiency cost. As individuals spend more time with their head up being vigilant, time available to search for and ingest food decreases. We explored whether ungulates can strategically modify behaviours to compensate for vigilance costs via increased cropping rate when food searching time was near-zero and bite sizes were small. We compared the proportion of time individuals had their head up to their cropping rate (bites/observation length) in 271 observations of Roosevelt elk (Cervus elaphus roosevelti). Using a linear mixed-effect model, we estimated the head up–cropping rate relationship and found that elk cropping rate was constant across varying lengths of time spent with their head up, indicating no vigilance compensation occurred via increased cropping rate. We discuss settings when cropping rate compensation is expected and other behaviours that might mitigate vigilance costs.