Group‐Size Effect on Vigilance and Foraging in a Predator‐Free Population of Feral Goats (Capra hircus) on the Isle of Rum, NW Scotland

Abstract

Many previous studies have found that as group size increases, individual vigilance levels decrease and forage intake increases (group‐size effect), but few such studies have considered the impact of within‐group interactions and other confounding factors on the direction of group‐size effects. A free‐ranging population of feral goats (Capra hircus), with little predation threat, was studied on the Isle of Rum (northwest Scotland), from Jun. to Nov. 2000, to investigate the effects of group size on individual vigilance levels and foraging efficiency after taking into account the effect of confounding factors (e.g. sex, season, time of day, habitat, predation risk) and within‐group interactions (via changes in movement rates while feeding). Our results show that, while group size exerted a negative influence on individual vigilance levels and a positive effect on movement rate, foraging efficiency never increased with group size and even decreased at certain times of day. There was no sex difference in individual vigilance in feral goats, but foraging efficiency was higher in females than in males. Goats were more vigilant in fall than in summer. The results imply that the benefits for foraging obtained from the reduced vigilance level in larger groups may be constrained or offset by increased interaction (or competition) within larger groups even in a population that faces negligible predation risk.

Influences of sex, group size, and spatial position on vigilance behavior of Przewalski’s gazelles

“…Group size and scanning frequency were log-transformed to normalize their distributions (Shi et al 2010). We used a general linear model to identify the effects of group size, sex, within-group spatial positions, and nearest-neighbor distance on individual vigilance level.…”

“…Vigilance level was therefore expressed as the scanning frequency per minute because the frequency of vigilance has been shown to be more sensitive to variation in group size than vigilance bout duration (e.g., Beauchamp 2008;Shi et al 2010), and the scanning frequency is the pattern of vigilance, rather than simply the time allocated to vigilance that determines the probability of predator detection (Cresswell et al 2003;Randler 2005).…”

“…Individual scanning frequency data were collected through continuous focal sampling methods (Altmann 1974;Shi et al 2010) during June-August 2006. When a group of gazelles was found, animals were randomly chosen from the periphery and the center of the group.…”

Time allocation of bison in meadow patches driven by potential energy gains and group size dynamics

“…We also found evidence that space use patterns of bison were not entirely consistent with energy maximization principles. When group size increases, foraging in company of others can reduce intake rate, either through exploitative or interference competition (Goss‐Custard 1980, Cresswell 1997, Krause and Ruxton 2002, Courant and Giraldeau 2008, Shi et al 2010). When competition reduces encounter rates with highly profitable food items, optimality theory can predict an increase in diet breadth (Stephens and Krebs 1986).…”

Time as a constraint on the distribution of feral goats at high latitudes

“…2). Time spent ruminating was positively also related to daylength, being highest in the summer and lowest in winter, suggesting that winter forage may have been of too poor quality to be worth devoting more time to rumination (see also Shi 2002). Nonlinear equations do not provide better fits to the data.…”