Group-living animals may need to spend less time being vigilant, consequently, having more time for other important activities such as foraging (i.e., group size effect). Przewalski’s gazelle (Procapra przewalskii) is a group-living social animal, and a study was conducted in Qinghai Province of China during June–August 2006 by using a continuous focal sampling method to investigate the influences of group size, sex, within-group spatial position, and nearest-neighbor distance on individual vigilance level (defined as scanning frequency per minute). Male gazelles were more vigilant than females. The gazelle’s vigilance level decreased with group size (group size effect), but only for females. The individuals at the central positions within a group were less vigilant than those at the peripheral positions, but the nearest-neighbor distance did not have any significant influence on the individual vigilance level. Our results support the hypotheses of group size effect and edge effects, but the sexual difference in vigilance level and in the response to group size effect on vigilance suggests that there may be sexual difference in the function and targets of vigilance behavior of Przewalski’s gazelles, which warrants more investigation, with incorporation of within-group spatial position, to better understand the mechanism underlying the group size effect and edge effect.
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