INTRODUCTION
Colour is one of the most striking and varied components of visual
signals throughout the natural world. The cost of producing vivid
colours can be critical for maintaining signal honesty. Colours produced
by carotenoid pigments are a text-book example of honest signals of
individual quality because carotenoids cannot be synthesized by animals
(i.e. must be ingested), and have a range of essential physiological
functions (Olson & Owens 1998; Svensson & Wong 2011). Consequently,
supply may limit carotenoid intake and there may be trade-offs in
allocation of pigments to ornamentation versus other physiological roles
(Koch & Hill 2018). The literature on carotenoid-based coloration is
dominated by studies of birds; however, the majority of vertebrates
(ectothermic vertebrates – fish, reptiles, amphibians) can produce
yellow to red skin colours using carotenoids and/or a biochemically
distinct class of pigments called pteridines (Bagnara & Matsumoto
2006). Pteridines can be used instead of, or together with carotenoids
to produce yellow-red colours and the two pigment classes can frequently
be found together in coloured integument (Bagnara & Hadley 1973;
Bagnara & Matsumoto 2006). Pteridines are synthesised de novowithin pigment cells from abundant purine molecules (Bracher et
al. 1998; Ziegler 2003; Braasch et al. 2007) and have limited
antioxidant function (McGraw 2005). This suggests that, in contrast with
carotenoids, production costs of pteridine-based colours may be minimal
and it is unlikely that supply is limited or that there are trade-offs
in allocation. However, few studies have examined pteridine pigments in
the context of colour signalling and the physiological costs and
evolutionary drivers of variation in pteridine pigments remain largely
unknown.
What explains the use of
carotenoid or pteridine pigments when both can produce spectrally
similar colours? One possibility is that pteridines compensate directly
or indirectly for variation in the environmental availability of
carotenoids (Grether et al. 1999). Under direct compensation,
pteridines directly replace carotenoids with a similar hue
(carotenoid-mimicry hypothesis). This is expected to result in a
negative correlation between the concentrations of similarly coloured
carotenoid and pteridine pigments. Under indirect compensation,
pteridines compensate for geographic variation in carotenoids,
irrespective of hue. Different species or populations may have different
colours, depending on local selective pressures, but use a higher
proportion of pteridines when carotenoid availability is low. In this
case, we expect a positive association between combined carotenoid
concentration and environmental carotenoid availability, and a
corresponding negative association with combined pteridine
concentration, but we do not necessarily expect correlations between
specific carotenoid and pteridine pigments. Whether direct or indirect
compensation for environmental carotenoid availability explains the
prevalence of pteridine pigments among species has not been tested in
any taxonomic group, to our knowledge, due to insufficient data on
pigment concentrations.
Within the broad classes of carotenoids and pteridines, specific
pigments have different hues, are acquired or metabolised in different
ways and therefore have different costs and roles in colour production.
Carotenoids are produced by plants and the most dominant carotenoids in
angiosperms are yellow xanthophylls such as lutein (Heath et al.2013). Insect herbivores generally sequester carotenoids in proportion
to the concentration found in the diet (Heath et al. 2013). Red
ketocarotenoids, such as astaxanthin and canthaxanthin are comparatively
rare in terrestrial ecosystems (primarily produced by microalgae and
yeast), but some animals, including birds and turtles, can metabolically
convert dietary yellow carotenoids to red ketocarotenoids (Lopeset al. 2016; Mundy et al. 2016; Twyman et al.2016). Due to the cost of metabolic conversion, or low dietary
availability for terrestrial animals, ketocarotenoids are more strongly
associated with measures of individual quality and sexual selection than
dietary yellow carotenoids, particularly in birds (Weaver et al.2018). Pteridines similarly vary in colour from yellow (e.g.
sepiapterin, xanthopterin) to red (e.g. drosopterin, erythropterin,
riboflavin) but other pteridines (e.g. pterin, biopterin,
isoxanthopterin) found in skin pigment cells (chromatophores) are
assumed to be colourless. Colourless pteridines can be found in large
quantities within chromatophores, though it is unclear whether or how
they may contribute to integument coloration (Bagnara & Matsumoto
2006). The different costs and roles in colour production for different
types of carotenoids and pteridines influence expected associations with
environmental factors and sexual selection. To understand evolutionary
drivers of pigment variation, it is therefore essential to quantify
specific metabolites in integument tissue; however, this has only been
attempted for carotenoids, and only in some groups of birds (Prumet al. 2012; Friedman et al. 2014b, a; Ligon et al.2016).
Here, using an extensive dataset of concentrations of 11 pigments, we
test whether pteridine pigments compensate, directly or indirectly, for
environmental availability of carotenoids among 27 species of Australian
agamid lizards (186 skin samples, 79 individuals, 28 populations with
distinct coloration). Specifically, we use highly accurate liquid
chromatography-mass spectrometry to quantify pigment concentrations in
skin tissues of agamid lizards (McLean et al. 2017; McLeanet al. 2019). We tested whether pigment concentrations are
associated with environmental gradients indicative of carotenoid
availability. Since this relationship may depend on the strength of
sexual selection, we simultaneously tested for relationships between
pigment concentrations and proxies for the strength of sexual selection
(sexual dichromatism and sexual size dimorphism). To distinguish directversus indirect compensation, we examined correlations between
carotenoid and pteridine pigments with similar hue. Lastly, we evaluated
how carotenoid or pteridine concentrations covary with skin colour (hue,
saturation, luminance).