Abstract
ABSTRACTNumerous studies have addressed antipredatory benefits of mixed-species flocks of foragers, but studies on individual's vigilance as a function of group size are limited. In the Cheolwon area of the Korean Demilitarized Zone, vigilance of the subordinate White-naped cranes (Grus vipio) in 11 groups composed of conspecifics and the dominant Red-crowned cranes (Grus japonensis) was examined. Vigilance correlated negatively with group size due to negative correlation with the number of conspecifics, but not the dominant heterospecifics. This is consistent with the hypothesis that a decrease in vigilance in larger groups is due to antipredatory benefits from increased predator detection in larger groups (associated with the presence of a larger number of conspecifics). This suggested that the mechanism leads to canceling out of the otherwise expected antipredatory benefits to the subordinate species from the increased predator detection by larger group size (associated with larger number of dominants). This is one of only a few behavioral studies of these endangered crane species in the relatively inaccessible wintering area of international importance in the areas of high conservation value.
Highlights
The question of why some species form interspecific groups has been one of the central issues in behavioral ecology since Hamilton’s (1971) and Pulliam’s (1973) theoretical papers and early empirical studies of gregariousness (e.g. Caraco 1979; Lazarus 1979)
Increased predator detection probability by a group due to an increased group size associated with the increased number of individuals of the dominant species should lead to a decrease in vigilance of the subordinate species
Our previous report suggested foraging benefits to the Red-crowned cranes in the presence of the Whitenaped cranes. We argued that this dominant-subordinate relationship may lead to a lack of negative correlation, or a presence of a positive one, between the number of Red-crowned cranes and the vigilance of White-naped cranes in a two-species group, while such a correlation between vigilance of the subordinate species and the number of conspecifics should be negative in accordance with the antipredatory hypothesis
Summary
The question of why some species form interspecific groups has been one of the central issues in behavioral ecology since Hamilton’s (1971) and Pulliam’s (1973) theoretical papers and early empirical studies of gregariousness (e.g. Caraco 1979; Lazarus 1979). Theoretical models (Pulliam et al 1982; Hart and Lendrem 1984; Beauchamp 2013) predict that vigilance should decrease with an increasing group size because other group members may alert an individual about approaching danger or because the risk of predation in a larger group is lower (antipredatory vigilance hypothesis). In such situations, increase of vigilance in response to an increase in group’s size has been observed in monospecific groups (Knight and Knight 1986). Increased predator detection probability by a group due to an increased group size associated with the increased number of individuals of the dominant species should lead to a decrease in vigilance of the subordinate species
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