Predator vision and defensive coloration: from mechanism to function

Few things are more important to an animal than avoiding predation. The various strategies involved in reducing the risk of predation include camouflage to hinder detection or recognition, startle displays and eyespots to scare predators, and 'dazzle' markings making it difficult for predators to judge how fast the prey is moving. As soon as Darwin and Wallace presented their theory of evolution by natural selection, around 150 years ago, the numerous ways of avoiding predation provided crucial examples for illustrating and defending natural selection and adaptation. The study of animal coloration has also influenced many areas of human applications and culture. For example, camouflage is of great importance in military applications, and has influenced peaceful areas of culture such as art and fashion. Conspicuous displays, like startle responses and eyespots, can also influence applications, such as producing non-harmful deterrents for avian crop pests. However, as the Darwinian anniversary approaches (200 years since Darwin's birth, and 150 years since the Origin of Species was published), researchers still know surprisingly little about many areas of protective coloration. This project has three main aims. The first is to investigate how various forms of defensive coloration reduce the risk of predation in terms of predator vision. Birds are the principle predators used in this project, and represent the main visual predation threat for many animals. Considering their visual abilities is crucial because, unlike humans, birds can perceive ultraviolet light and have a more sophisticated colour vision system than humans. By understanding what specific processes of predator vision are exploited, we can explain the form, function, and evolution of different defensive strategies in nature. The second aim is to investigate how different defensive strategies can be combined on the same animal (such as camouflage and conspicuous signals), and how they relate to each other. In most previous work, different defensive strategies have been considered in isolation, despite the fact that most animal markings result from a range of selection pressures and fulfil multiple functions. The third aim is to extrapolate the principles learned from the study of defensive coloration to other areas of behaviour and signalling. The model system used here is brood parasitism, where a parasitic bird 'tricks' other species into caring for its young. This has, over co-evolution, often led to hosts having increasing ability to recognise and reject foreign eggs, and counter-adaptations in parasites to evade host defences, including egg mimicry. First, I will create simulations of evolution, where computer generated prey on artificial backgrounds are subjected to selection pressure (to be hidden against the background or to prevent capture when moving) by an artificial 'predator', based on features of real predator visual systems. Prey which survive, then reproduce, with some offspring incurring mutations for new colour patterns. Over time, those markings most successful in preventing predation will spread, further evolve, and optimise their defensive capability. In this way, I will predict what types of prey coloration evolve under different selection pressures and how they exploit specific aspects of predator vision. Second, artificial prey, designed to a bird's visual system, will be presented to bird predators in aviaries and the field. These are not intended to resemble real species, but to derive general principles about what types of marking are most effective in preventing predation, including what makes effective camouflage markings and startle displays. Finally, in extending the principles of defensive signals, I will ask how closely the egg of a brood parasite must match those of its host to prevent being detected as an intruder, and how cuckoo chick markings both increase host provisioning and decrease the risk of predation.

Avoiding predation is a crucial aspect of many animals fitness, with defensive strategies including camouflage, startle displays, and 'motion dazzle' markings. This project aims to: (1) base explanations of animal defensive coloration in formal mechanistic terms of how the visual signals are perceived by a predator, (2) use the principles derived to understand the form of defensive signals in nature, and (3) apply these principles to other areas of behaviour and signalling, using parental care and brood parasitism as a model system. Birds are the principle receivers used in this project. By understanding what mechanisms of the predator's vision are exploited, we can explain the form, function, and evolution of different defensive strategies in nature. In addition, defensive strategies have generally been considered in isolation, despite the fact that most animal markings typically result from multiple selection pressures and fulfil several functions. Understanding how defensive signals work can allow the derivation of general principles to be applied to other areas of behaviour and signalling. Simulations of prey evolution will involve computer generated prey, with 'genes' coding for their appearance, being subjected to selection pressure based on features of real predator visual systems. These will predict what types of prey coloration evolve under different selection pressures, and the specific aspects of predator vision they exploit. Artificial prey will be presented to avian predators in field sites and aviaries to derive general principles about what markings are most effective in preventing predation, including camouflage and startle displays. Finally, experiments will investigate how closely a cuckoo egg must match those of its host to prevent detection, and how the markings of cuckoo chicks can increase host provisioning. The project links behaviour, evolution and visual signalling, to theories from visual psychology and techniques from computer science.