Investigating Differences in Vigilance Tactic Use within and between the Sexes in Eastern Grey Kangaroos

Guillaume Rieucau,François-René Favreau,Pierrick Blanchard,Anne W Goldizen,Olivier Pays,Julien G A Martin,Jane M Waterman

doi:10.1371/journal.pone.0044801

Abstract

Aggregation is thought to enhance an animal’s security through effective predator detection and the dilution of risk. A decline in individual vigilance as group size increases is commonly reported in the literature and called the group size effect. However, to date, most of the research has only been directed toward examining whether this effect occurs at the population level. Few studies have explored the specific contributions of predator detection and risk dilution and the basis of individual differences in the use of vigilance tactics. We tested whether male and female (non-reproductive or with young) eastern grey kangaroos (Macropus giganteus) adopted different vigilance tactics when in mixed-sex groups and varied in their reliance on predator detection and/or risk dilution as group size changed. This species exhibits pronounced sexual dimorphism with females being much smaller than males, making them differentially vulnerable toward predators. We combined field observations with vigilance models describing the effects of detection and dilution on scanning rates as group size increased. We found that females with and without juveniles relied on predator detection and risk dilution, but the latter adjusted their vigilance to the proportion of females with juveniles within their group. Two models appeared to equally support the data for males suggesting that males, similarly to females, relied on predator detection and risk dilution but may also have adjusted their vigilance according to the proportion of mothers within their group. Differential vulnerability may cause sex differences in vigilance tactic use in this species. The presence of males within a group that do not, or only partially, contribute to predator detection and are less at risk may cause additional security costs to females. Our results call for reexamination of the classical view of the safety advantages of grouping to provide a more detailed functional interpretation of gregariousness.

Highlights

Aggregation is commonly thought to provide security benefits to animals [1,2] through a greater power of detection of predators (‘‘the many eyes hypothesis’’ [3]) and the dilution of risk where the nearby presence of companions decreases the likelihood of any given individual being the victim of an attack [4,5]
Two candidate models were found to explain the vigilance tactics used by males well: the Low males investment (LMI) security model accounting for mothers with juveniles and the security model
We discuss this point in view of previous results published on this topic and potential differences between the sexes that might have influenced the evolution of their vigilance tactics

Summary

Introduction

Aggregation is commonly thought to provide security benefits to animals [1,2] through a greater power of detection of predators (‘‘the many eyes hypothesis’’ [3]) and the dilution of risk where the nearby presence of companions decreases the likelihood of any given individual being the victim of an attack [4,5]. Through the combination of improved collective vigilance and risk dilution, animals in large groups benefit from an enhanced safety that allows them to reallocate time saved in vigilance to other fitness-improving activities [8]. This reallocation might be crucial for prey species (including large mammalian herbivores) that are highly constrained by their food acquisition [9,10]. Vigilance levels exhibited by group-living animals reflect a trade-off between minimizing their risk of predation and gathering information about their social environment

Methods

Results

Conclusion