Identifying Determinants of Background Matching and Disruptive Colouration Using Computer Simulations and Humans as Predators

Abstract

In nature, many animals bear markings (or pattern elements) on their body to reduce detection through background matching and disruptive colouration, but studies of these strategies are surprisingly limited. This thesis investigated the role of element size, density and contrast in background matching, through the use of humans as predators in virtual computer simulations. In addition, this thesis also attempted to develop a disruptive index to quantify and predict survivorship of different-patterned morphs. Manipulative studies on how the determinants of colour pattern such as element size affect background matching are scarce. Experiment in Chapter 2 examined the role of element size in background matching using a `high resolution¿ experiment comparing all combinations of eight virtual morphs and eight backgrounds with different element sizes. Using human predator search time as a measure of morph survivorship, a 3D surface graph (morph element size class × background element size class × search time) was produced, giving a detailed understanding of how morph and background element size affected survivorship. While predator search time was longest when the element size classes of morphs and backgrounds were similar, an inexact match still provided some protection. Search time was significantly higher in combinations where the element size of the morph was larger than that of the background and vice versa. This experiment demonstrates, for the first time, a convex tradeoff relationship in a habitat with two visually distinct backgrounds, i.e. generalists were potentially favoured over specialists when the difference between the two backgrounds was small. Experiments in Chapter 3 aimed to improve our current understanding of the importance of element contrast and element density in background matching. Survivorship patterns of two morphs (low and high density or contrast) on 15 backgrounds from low to high density or contrast were obtained, again using human as predators. Element contrast was found to be much more important than element density. However, the lack of element density effect on search times could have been due to differential fragmentation of background elements by the prey morphs placed in different locations, as suggested by the large variation in predator search times. The effects of disrupted edge length, number of marginal elements and variation in area of marginal elements on disruptive colouration were previously untested. Using these factors, Chapter 4 focused on developing a novel index that may quantify the degree of disruptive colouration and predict a morph¿s survivorship based on morph pattern or morph location. Correlation tests and linear models showed that a higher disruptive index led to lower survivorship, the opposite of what was predicted. Instead of disruptive colouration, the index was found to reflect how element geometry affected background matching. Additionally, morph location was shown to be as important as morph pattern in predicting survivorship. These findings demonstrated the complexity of background matching and disruptive colouration and would be important for future studies.