Mechanisms of the group-size effect on vigilance in Columbian ground squirrels: dilution versus detection

Antipredator vigilance decreases with group size in many vertebrate prey species. This pattern might be explained by collective detection (increased probability of observing a predator) or risk dilution (decreased likelihood of being captured with larger group size). If dilution is the main cause of the group-size effect, nearest-neighbour distances and distance to the edge of the group would affect vigilance, and alarm calling and factors that affect hearing alarm calls would not affect vigilance. The opposite outcomes are expected if detection is the primary influence. We used both model fitting and hypothesis testing to determine which mechanism had the greater effect, using 14 populations of Columbian ground squirrels, Spermophilus columbianus. Vigilance decreased significantly with increases in group size (R2 = 0.70, P = 0.0002). Model fitting revealed that the detection model was a statistically good fit to our data and the dilution model was not. We found no significant effect of nearest-neighbour distance on the proportion of time vigilant. Distance of individuals to the edge of the meadow habitat had a marginally significant effect on vigilance in one analysis, but no significant effect in three other analyses. Alarm calls occurred in 56.1% of observations and had a significant effect on vigilance. Wind speed did not show a significant effect on proportion of time vigilant during foraging bouts (mixed models: F = 2.41, P = 0.12), but squirrels were observed to forage less in high wind. All of these results support the detection hypothesis. Group type and means of information transfer about predators may be indicators of which mechanism, detection or dilution, has the greatest influence on the group-size effect on vigilance.