The evolution of integrated defence systems

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INTRODUCTION: Go into most Canadian backyards and you are likely to see numerous examples of protective colour patterns: moths that resemble their backgrounds, warningly-coloured ladybugs, and harmless hover flies that gain protection by resembling stinging wasps or bees. Predation has provided some of the most celebrated examples of the power of natural selection, and it has shaped the way both academics and the general public think about evolutionary processes. However, it is increasingly recognized that most prey species have not evolved a single line of defence, but a portfolio of defences that are deployed sequentially when earlier strategies fail. For example, many camouflaged moths attempt to startle potential predators when discovered, and many warningly-coloured ladybugs engage in reflex bleeding on contact. Likewise, to deal with multiple threats, many prey species have evolved a Swiss-army knife of predator-specific defences, with certain defences targeted at particular predators such as birds or parasitic wasps. What explains these back-up defences, why do some species have them and not others, and why do some species put all of their resources into one line of defence when other species invest in a range of deterrents? OBJECTIVES: Our broad goal is to understand how different anti-predator traits combine to protect organisms from being killed, and to understand why species have evolved different systems to protect themselves. To do this we will investigate how specific pairs of defences combine. For example, we will ask if and why only unpalatable species have evolved an ability to survive being grasped, why certain species engage in mimicry while flying but resort to hiding when at rest and why some mimics evolve to look like their noxious models in appearance but other mimics behave like them too. METHODS: To address the above aims we will use a variety of methods including evaluation of the critical wing tear forces of butterflies in the field, physical inspection of museum specimens, presentations of artificial prey to wild birds and an analysis of the flight characteristics of grasshoppers and butterflies in wind tunnels. To complement this work we will gather data on the distribution of defences across whole family trees and test whether certain defensive traits, such as evasive ability and toxin level, are associated. We will then develop mathematical models to help understand why these associations arise. The work provides excellent training opportunities for skills that are in short supply in Canada, including insect identification and mathematical modelling. SIGNIFICANCE: Although multi-level defences are widely investigated by military theorists, our understanding of how defensive systems are integrated in the natural world is still in its infancy. This research is not about adding footnotes, but testing "big picture" hypotheses aimed at understanding the diversity of defensive traits we see in the natural world.