A Colourful Distribution

Skinks come in a variety of colours and patterns. But why and how are these colour polymorphisms maintained? Genevieve Matthews, a PhD student at Monash University, has been studying skinks for four years. Her research examines the maintenance of genetic variation in the form of colour pattern polymorphism in the delicate skink, and the costs associated with sexual conflict. Here, Genevieve summarises her recent publication in the Journal of Animal Ecology that studied avian predation intensity as a driver of colour polymorphism.

Lizards are one of the most diverse groups across the animal kingdom – and nowhere more diverse than in Australia. There are more than 6,500 species of lizards currently recognised across the world and over 800 native to Australia. Adapted to an incredible range of habitats and ecosystems from deserts to rainforests, lizards are a useful group to study, and more than half of those in Australia are skinks (Scincidae).

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A plain-back skink (Photo: Genevieve Matthews)

The Liopholis skink genus contains 11 species, including both alpine and desert-adapted skinks. Across various habitats, distributions and levels of conservation interest, the Liopholis species belong to a colourful genus. Some species sport leopard-pattern spots on their sides, some have a patterned back, some with both, some with a general all-over pattern and some with no pattern at all. Around half of the Liopholis species have multiple wardrobes; more than one of these colour pattern types, or morphs, is represented within a single species. In White’s skink, Liopholis whitii, three different morphs can be found: patterned sides and back, plain back with patterned sides, and patternless.

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A patterned skink (Photo: Genevieve Matthews)

Colour pattern polymorphisms like this are not uncommon in general, nor in reptiles, but it’s the distribution of colour morphs in White’s skink that makes it particularly interesting. Along eastern Australia, the colour morphs of White’s skink change in frequency according to latitude. The rare patternless morph, strangely, is present in one small cluster at a low to mid-range latitude of the total species distribution. On the other hand, northern populations consist of around 80% patterned variants, which increases gradually southward until populations in Tasmania are composed of only patterned individuals. But why?

Polymorphisms can represent alternate strategies for a species to deal with a selection pressure. That is, different morphs may go about dealing with the same environment in two different ways, both of which are at least partially or temporarily successful, even though it’s usually a difficult business to find one successful strategy. This depends entirely on the current available genetic variation and the state of the environment and its selection pressures at the time.

Without considering genetic inheritance, we expect that the adaptive significance of the White’s skink morphs is highly relevant when explaining their latitudinal distribution. In other words, each colour pattern morph should provide some benefit to the individual based on its latitude. Selection differs from population to population, so morph frequency variation between populations is not unexpected. But such a strong spatial gradient begs a more involved explanation.

Among ectotherms, we naturally predict that climate-related temperature should interact with a species’ thermal limits to produce a large effect on its distribution. We hypothesised that Liopholis whitii morphs might have different thermal physiologies that explain why they are distributed in such a smooth latitudinal cline. Though hotly contested, there is evidence to suggest that melanistic ectotherms have faster heating rates than lighter-coloured ones. The dorsal patterned Liopholis whitii have dark spots and stripes that may allow them to maximise the intake of heat in cooler southern populations, relative to the plain-back populated northern locations.

The stunning Girraween National Park in south-east Queensland harbours both plain-back and patterned skinks, perfect for collecting and testing the thermal physiology of both. Over six weeks, I and several field assistants caught some of each morph, representing both males and females. As we caught them, we assessed their microhabitat for its thermal properties and its structure, as well as its reflectance composition, or background colours. Skinks that were collected underwent tests for their sprint speed, heating and cooling rate and their own colour properties were measured with a spectrometer, before being released back to their burrows.

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Morphology measurements taken in the field (Photo: Genevieve Matthews)

Despite the beautiful surrounds, and the pleasure of handling such a gorgeous species, field collection wasn’t without its downsides. One notable catching attempt ended with a juvenile brown snake in my armpit. Data collection ultimately successful without mishap, what we found was surprising: there was very little difference between the thermal physiology of patterned and plain-back morphs, and only subtle differences in their use of microhabitat. More interesting, however, was the choice of background colour among morphs. By using models of bird visual systems, we could determine how well avian predators could distinguish each morph against the microhabitat it selected. In general, both morphs were discriminable to birds against their background, but to differing degrees. For the most common bird eye type and illumination, plain-back morphs were more conspicuous than patterned morphs.

This prompted a further question: how was predation intensity related to morph frequency across latitude? I collected information from the literature about all potential predators of White’s skink, and how likely they might be to consume the species, to produce a potential predation intensity score across latitude. I further included information about temperature and rainfall and constructed a series of models to see which factors best explained the distribution of colour pattern morphs. The best model suggested that bird predation intensity alone explained the gradient in morph frequency. Paradoxically, the morph most conspicuous to birds (plain-back) occurs where bird predation is highest.

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A White’s skink peeking out of its burrow in Girraween National Park (Photo: Genevieve Matthews)

By looking solely at a single population along this latitudinal cline, we only have one snapshot in space and time with which to compare the broad scale patterns of predation intensity. It is very likely that conspicuousness and background matching interact with behaviour, gene flow, more complex thermal physiology, or other forces of selection to produce the observed patterns. Nonetheless, predation plays a key role in the smooth frequency cline of this species, contrary to our expectations. By combining local and distribution-wide data on a single polymorphic species, we know that colour pattern morphs are important in predator avoidance, however complex the relationship may be across and within populations.

More Info:

Matthews, G., Goulet, C. T., Delhey, K., Atkins, Z. S., While, G. M., Gardner, M. G., & Chapple, D. G. (2018). Avian predation intensity as a driver of clinal variation in colour morph frequency. Journal of Animal Ecology87(6), 1667-1684.

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