Group-size effect on vigilance in mammals

Group-size effects, whereby antipredator vigilance decreases as group size
increases, are widely reported in mammals and birds but a meta-analysis
has only been conducted in birds. We systematically reviewed the
literature on mammalian group-size effects, estimated the effect sizes in
each study, and conducted a phylogenetic meta-analysis. We obtained 296
effect sizes from 97 species belonging to 10 Orders and 26 Families.
Overall, effect sizes indicated a moderate negative effect of group size
(r = - 0.44), but 43% of the effect sizes were compatible with a null
effect of group size. There was significant heterogeneity in effect sizes.
Weaker effect sizes occurred when vigilance was measured as a frequency or
a duration rather than as a percentage of time spent vigilant, when
measured in closed habitats, during the reproductive season, and in
mixed-sex groups or during times when juveniles were absent. We infer a
‘file drawer problem’ because there were relatively few studies with
smaller sample sizes reporting small group-size effects. The results
confirm the importance of group size in explaining variation in mammalian
vigilance, but also suggests that a substantial amount of variation
remains unexplained. We suggest that future studies should aim to study
mammalian group-size effects by quantifying the percentage of time
allocated to vigilance rather than apparently lower-power methods such as
frequency or duration of vigilance.