Signal design, viewing geometry and courtship behaviour in the iridescent guppy

Iridescent colour signals produce intense brightness and hues that vary with the relative positions of the signaller, receiver, and light source. Iridescent signals are generally understudied when compared to pigmented signals, and many paradigms that have been investigated for pigmented colour research are yet to be replicated or applied to iridescence. The angle specific nature of iridescence suggests several evolutionary advantages over pigmented signals. Iridescence can generate signals that are brighter than pigmented signals and generate signals that can also flash on and off or shift dramatically through the colour hues according to the viewing angle. The often-narrow viewing angle means that the signaller and receiver must coordinate in some capacity for effective signal transmission. Consequently, iridescent signals can be biased towards conspecifics while avoiding attracting predators. However, are animals making use of these theoretical benefits consistently in nature? The aim of this thesis is to extend our real-world understanding of iridescent signalling, using the freshwater guppy. Specifically, I ask if and how male guppies maximise the efficacy of their iridescent signal in different light environments through their display behaviour. I address these questions in four data chapters that investigate: 1) the timing of male display behaviour; 2) the visibility of the iridescent signal from different viewing points and the display positioning of guppies; 3) the impact of the light environment on behaviour and iridescent signalling; and 4) population differences in iridescence signalling.

Iridescence is challenging to measure rigorously. Pigment colouration can be easily measured using spectrometry or photographs, as there is minimal variation due to either lighting or viewing direction. Meanwhile, methods for effectively measuring iridescent signals are still being perfected. For flat surfaced signals, such as the butterfly wing, spectrometry can be applied to measure across different angles of light, signal, receiver positions, generating large data sets. What do we do with more complicated animals? Guppies are round and can move freely in space along all three dimensions, thus the signal can be viewed from a very large range of angles. Additionally, although they appeared to have some general display preferences, males can be seen displaying at different distances and relative positions to females. To understand what a female may see we must understand the signal broadly.

To begin to unpick this complexity, Chapter Two investigates the timing of male display efforts, and whether individual males differ significantly in allocating their displays over the day. While I found that at a population level, males followed an overall temporal trend in displaying, individual males do not significantly differ from each other. This helped to both become familiar with male displays and to inform the methodology of the future chapters.

Moving onto solving the challenge of understanding iridescence in a fish, Chapter Three trialled a new method of measuring iridescence. By simultaneously using five lower resolution cameras, we traded off the detail of a spectrometry approach with a rapid measuring of colour on anaesthetised individuals. The colour data were then compared to three-dimensional coordinate data recorded during the displays of these males towards female guppies. Males show a strategy of displaying more under arrangements that presented more iridescence to the female.

How do signallers react to changing environments that alter their iridescence-behaviour relationship? Chapter Four investigates how display behaviour and iridescent signal interact under three different lighting environments. We found that males adjust their behaviour in line with environmental interactions on their signal to maintain the strategy of displaying more where more iridescence would be perceived. Different populations of guppies might vary in the size of the iridescence signal due to unique prevailing selective pressures or, in the case of my research, due to artificial selection and domestication. Chapter Five shows that while significantly different in iridescence and behaviour, males from two populations showed a remarkably consistent relationship between display behaviour and visible iridescence, confirming similar observations in previous chapters.

In conclusion, the largely overlooked iridescence of male guppies offers an intriguing and complex visual signal that has evolved to be biased towards the female. This adaptive strategy is robust to both differences in environment and population.