Doctor of Philosophy
School of Biological Sciences - Faculty of Science
Healey, Mo, The evolution of colour polymorphism in the painted dragon, Ctenophorus pictus, PhD thesis, School of Biological Sciences, University of Wollongong, 2008. http://ro.uow.edu.au/theses/71
Many studies of colour polymorphism across a range of taxa have shown that morphs proliferate by operating different reproductive tactics. To investigate whether this is the case with our model species, the Australian painted dragon lizard Ctenophorus pictus, we characterised a number of morph-specific traits. We demonstrate that red-headed males beat yellow-headed males in staged contests for females and that this is linked to what appears to be a convention of red dominance in male-male interactions set very early in ontogeny, long before colouration has developed. Red dominance was also observed in the field, where red males have territories with higher perch density and begin their territory defense three weeks before yellow males in some years. Red males also have higher testosterone levels than yellow males, which may provide a potential proximate link to the behavioural differences between them. In female choice experiments on single males of different colours, females did not preferentially associate with a particular morph. However, when females were allowed to choose between pairs of males of the same versus different colours, they preferred to associate with male pairs that were polymorphic. We suggest that this may be the result of selection arising from polyandrous mating benefits and show experimentally that polyandry results in increased hatching success. Sperm competition trials in the lab showed that although yellow males copulated for a shorter period of time, they sired three times as many offspring as red males. Our mating experiments also showed that yellow males had significantly larger testes in relation to body condition than red males and that sperm storage played a highly significant role in male reproductive success. These morph-specific differences are indicative of different reproductive strategies, with red males being more coercive or better mate guarders than yellow males, whilst yellow males may employ a sneaker type strategy to gain matings. However, we could not verify the distinct differences in morph-specific reproductive tactics in the wild that we would have predicted from our trials in captivity. For example, inherent red dominance could result in yellow males being marginalised to poorer quality territories in terms of access to females, food, perch sites or shade. However, with a territory acquisition experiment in the wild, we found no significant effect of colour category per se, although on average red males remained closer to the release sites (our proxy for territory acquisition ability) than yellow males. We would also predict ongoing disruptive selection on space use, with red males defending well-defined territories and yellow males having larger and more loosely defined territories. Selection analysis showed that selection on space use in a natural population was not disruptive in either of the two years studied. Thus, divergence of male reproductive strategies in our model species does not seem to be related to differences in space use or territoriality. If yellow males employ a sneaker strategy, we would expect them to sire more offspring per copulation and have a greater proportion of offspring from clutches with mixed paternity. However, we show that the frequency of mixed paternity in the wild is low (< 20 %), and that all morphs on average have the same number of offspring sired per year. A contributing factor to this difference in results could be habitat constraints on the evolution and expression of morph-specific behaviour and their evolutionary divergence in painted dragons. Habitat heterogeneity in our population (along a man-made fire trail) may be too low to allow each morph to exploit their optimal strategies, and we show that polyandry is significantly more common on territories where vigilance opportunities are relatively poorer. Painted dragons are also polymorphic in bib colour. Red males with a bib are in better body condition than red males that lack a bib, which contrasts sharply to yellow males, in which males with a bib are in poorer condition than yellow males that lack a bib. In a field experiment we found that males without a bib lost within-clutch paternity significantly more often to rivals than bibbed males. We increased the work load of males and compared ‘loaded’ males against controls with respect to how they maintained body mass during the mating season. Unexpectedly, bibbed males consistently lost more body weight across all treatments and controls. Thus, higher quality signalers may be more efficient at converting signals into fitness, in spite of higher marginal costs. During the last three months we have managed to stimulate cryptic females to express their underlying colouration using testosterone implants, and we are currently collecting data that could allow us to elucidate the genetic inheritance of this fascinating colour polymorphism.
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