During the lastfourdecades, advancingperspectives inbehavioural ecology have generated a surgeofresearch focusing on individual behavioural variation.The resulting literaturehas highlightedthe substantialeffects of thisvariationon ecological and evolutionary processes,whilst alsoleadingto thewidely accepted view that individualvariation may represent the‘end product’ofnatural selection.Despite receiving considerable recent attention,many questions remain unanswered,and thus there is still keen interest inunravellingtheproximate and ultimate causesof behavioural diversity,and in improvingonour currentunderstandingof itsmaintenancewithin natural populations.This has led to a number of theoretical developmentscentring on links between behavioural, physiological, and life-historytraits(the Pace-of-Life Syndrome hypothesis-POLS), which are predicted —along with factors affecting individual state —to underpin behavioural expression among-and within-individuals.Also central tothesetheoretical explanationsaretrade-offs between current reproduction and future survival, where more risk-prone individuals are expectedto benefit from increased resource gains and faster growth, at the expense of higher mortality by predation.In order to examinethese predictions, this thesis aimed to build on existing knowledgeby responding to recentcalls for more stringent empirical testing of key hypotheses, andby addressing important outstanding questions relating to the causes,constraints and consequencesof individual behaviour. Specifically, the work examined individual risk-taking behaviour in the saltmarsh periwinkle (Littoraria irrorata).An important consumer on the intertidal marshes of the Eastern USA, L. irrorataecology—includingtheir characteristic circumtidal migrations onsaltmarsh vegetation—hasbeen studied extensively, but as yet, notat the individual level. Therefore,the work comprises fourempiricalstudies,whichaimedto examine (a) the consistencyof risk-related behaviours, (b)the influence of local environmental conditions on behavioural consistency and flexibility, (c)associations between individual behaviour andfactors affecting individual state, and (d) the possible trade-offs associated with behaviourand individual fitness. Findingsrevealed, for the first time in this species,that risk-taking behaviour(boldness), activity,andlatency to climb plant stems with the incoming tide areconsistent individual attributes (personalities), and that these traits associateto form a risk-related behavioural syndrome. In addition,findings revealedconsiderable behavioural flexibility(plasticity)across environmental contexts, including diel and tidal cycles, and in response to temperature gradients—highlighting the importance of local environmental conditionsin shaping individual behaviour.The workalsoprovidesa rare example of domain general plasticity, where behavioural responsescarryover across contextual gradients,s uggestingunderlying physiology as a common mechanism, and raisingthe possibility of correlational selection on plasticity.Further, evidence presented forcovariation between boldness, resting metabolic rate(RMR)and somatic growth,indicatedclear among-and within-individual correlations,providingcompelling support for the POLS hypothesis. This, along with findings indicatingthat behaviour appears to beinfluenced by intrinsic and extrinsic factors associated withrisk perception,provides evidence that individual behavioural differences may be driven by underlying physiology, as well aswith factors affecting individual information state. However, contrary to theoretical predictions,findingsalsorevealedthat bolder individuals were more likely to surviveexposuretoa key marshpredator,thusraisingquestions relating to the maintenance of behavioural diversity in L. irrorata. In particular,where higher risk-taking propensity does not appear to be involved in fitness trade-offs under the conditions studied. Taken together, thesefindings illustrate the importance of behavioural variation in determining individual performance under varying conditions, whilst alsocontributingto the growing body of literature examining the causes and consequences of individual behaviour. Finally, the workalso provides a solid foundation for future studiesexamining the role of individual behaviour in the ecology ofthis species.