Rapid behavioural gregarization in the desert locust, Schistocerca gregaria entails synchronous changes in both activity and attraction to conspecifics

Stephen M Rogers,Darron A Cullen,Michael L Anstey,Malcolm Burrows,Emma Despland,Tim Dodgson,Tom Matheson,Swidbert R Ott,Katja Stettin,Gregory A Sword,Stephen J Simpson

doi:10.1016/j.jinsphys.2014.04.004

Abstract

Desert Locusts can change reversibly between solitarious and gregarious phases, which differ considerably in behaviour, morphology and physiology. The two phases show many behavioural differences including both overall levels of activity and the degree to which they are attracted or repulsed by conspecifics. Solitarious locusts perform infrequent bouts of locomotion characterised by a slow walking pace, groom infrequently and actively avoid other locusts. Gregarious locusts are highly active with a rapid walking pace, groom frequently and are attracted to conspecifics forming cohesive migratory bands as nymphs and/or flying swarms as adults. The sole factor driving the onset of gregarization is the presence of conspecifics. In several previous studies concerned with the mechanism underlying this transformation we have used an aggregate measure of behavioural phase state, Pgreg, derived from logistic regression analysis, which combines and weights several behavioural variables to characterise solitarious and gregarious behaviour. Using this approach we have analysed the time course of behavioural change, the stimuli that induce gregarization and the key role of serotonin in mediating the transformation. Following a recent critique that suggested that using Pgreg may confound changes in general activity with genuine gregarization we have performed a meta-analysis examining the time course of change in the individual behaviours that we use to generate Pgreg. We show that the forced crowding of solitarious locusts, tactile stimulation of the hind femora, and the short-term application of serotonin each induce concerted changes in not only locomotion-related variables but also grooming frequency and attraction to other locusts towards those characteristic of long-term gregarious locusts. This extensive meta-analysis supports and extends our previous conclusions that solitarious locusts undergo a rapid behavioural gregarization upon receiving appropriate stimulation for a few hours that is mediated by serotonin, at the end of which their behaviour is largely indistinguishable from locusts that have been in the gregarious phase their entire lives.

Highlights

Desert Locusts, Schistocerca gregaria, display a dramatic polyphenism, being able to transform reversibly between two extreme forms or phases that differ considerably in many aspects of their biology including behaviour, physiology and morphology
Phase change is driven by fluctuations in population density: when population density is low, locusts occur in the solitarious phase, but increasing numbers and the concentration of locusts onto scarce resources triggers the transformation to the gregarious phase
From the individual behavioural variables that define the model (Table 2) it is clear that long-term gregarious locusts were more active than long-term solitarious locusts, spending only 61%

Summary

Introduction

Desert Locusts, Schistocerca gregaria, display a dramatic polyphenism, being able to transform reversibly between two extreme forms or phases that differ considerably in many aspects of their biology including behaviour, physiology and morphology, The process of gregarization is initiated and maintained by stimuli from other locusts, but it is a process that occurs over many different time scales, from hours to generations.

Methods

Results

Conclusion