The Evolutionary Pathway to Obligate Scavenging in Gyps Vultures

Brian J Dermody,Andrew L Jackson,Colby J Tanner,Robert Desalle

doi:10.1371/journal.pone.0024635

Abstract

The evolutionary pathway to obligate scavenging in Gyps vultures remains unclear. We propose that communal roosting plays a central role in setting up the information transfer network critical for obligate scavengers in ephemeral environments and that the formation of a flotilla-like foraging group is a likely strategy for foraging Gyps vultures. Using a spatial, individual-based, optimisation model we find that the communal roost is critical for establishing the information network that enables information transfer owing to the spatial-concentration of foragers close to the roost. There is also strong selection pressure for grouping behaviour owing to the importance of maintaining network integrity and hence information transfer during foraging. We present a simple mechanism for grouping, common in many animal species, which has the added implication that it negates the requirement for roost-centric information transfer. The formation of a flotilla-like foraging group also improves foraging efficiency through the reduction of overlapping search paths. Finally, we highlight the importance of consideration of information transfer mechanisms in order to maximise the success of vulture reintroduction programmes.

Highlights

Certain species of Gyps vultures represent the only extant vertebrate obligate scavengers on earth, having evolved highly specialised physiologies and behaviour to enable them to exploit spatially rare and temporally ephemeral food resources at the cost of the ability to kill prey [1,2]
Given that information transfer is dependent on visual cues from conspecifics, the density of foraging birds in the sky is critical as birds must remain in visual range of at least one other conspecific to access the information transfer network
We propose that extra-roost mechanisms are sufficient to impose selection pressure for communal roosting; the fitness benefits vultures derive from being spatially concentrated at the beginning of the foraging day

Summary

Introduction

Certain species of Gyps vultures represent the only extant vertebrate obligate scavengers on earth, having evolved highly specialised physiologies and behaviour to enable them to exploit spatially rare and temporally ephemeral food resources at the cost of the ability to kill prey [1,2]. When a vulture discovers a carcass it drops its feet, which increases drag and causes the bird to descend [7]. This action is observed by other vultures and they in turn descend in the direction of the descending bird, creating a chain of descending vultures within visual range. This process is so efficient that it can lead to several hundred birds reaching a carcass within hours of the initial discovery [8]. Given that information transfer is dependent on visual cues from conspecifics, the density of foraging birds in the sky is critical as birds must remain in visual range of at least one other conspecific to access the information transfer network

Methods

Results

Discussion

Conclusion