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Title: The effect of avian predation on cryptic polymorphic prey
Author: Cooper, Jonathan Munro
ISNI:       0000 0001 3562 2756
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 1984
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There is good experimental evidence that predators often remove more of common prey types. This apostatic selection could maintain colour polymorphism within prey species. Many colour pattern morphs of species in nature appear to match components of the background. My aims were to gather experimental evidence for background tracking and for apostatic selection on cryptic prey. All the experiments use wild passerine birds as predators and pastry, coloured with food dye, as prey. Selection was infered from the numbers of prey eaten from dimorphic populations. Three different designs of background were used: cylindrical prey closely resembled inedible wooden models (masquerade), disc shaped prey rested on flat boards (simple crypsis), and cylindrical prey rested on hessian sheets among coloured stones (crypsis). A field investigation into the polymorphism and crypsis of the meadow grasshopper, Chorthippus parallelus was also conducted. The masquerade experiment showed that selection by wild birds could be background colour composition dependent but background tracking seemed an unlikely outcome. The simple crypsis experiment was an unsuccessful attempt to develop a design to provide crypsis for artificial prey. The experiment with cylindrical prey and stones provided evidence for background colour composition dependent selection: the predators selected against the morph that resembled the uncommon background colour component. Further support for background tracking came from an experiment where predators generated morph frequency equilibria that were proportional to the background composition frequencies, after a series of 25 simulated generations. An experiment designed to test how distinctness of colour morphs is maintained provided evidence that predators effect a kind of disruptive selection such that a polymorphic population arrives at morph colour pattern optima determined by the colour composition of the background. In an experiment designed to test apostatic selection on prey that match the background predators were presented with a range prey frequencies and selection measured when prey matched the background was compared to two situations where they did not. Selection was apostatic in all three backgrounds but strongest in the matching one. The ecological conclusion is clear: apostatic selection for crypsis by predators would maintain colour polymorphisms in cryptic prey species and would give rise to parallelisms of colour morph frequency and corresponding background colour element frequency. The field investigation provided a possible example of the outcome of such selection on a cryptic polymorphic species. Assuming the selection observed happens in nature, the results may help to explain the widespread occurrence of polymorphism in cryptic prey.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Ecology