INTRODUCTION
Colour is one of the most striking and varied components of visual
signals throughout the natural world. Many colours are produced by
pigments, which can be directly or indirectly obtained from the diet, or
alternatively, synthesised by the body (McGraw 2005). Carotenoids are
the main class of dietary pigments and are the primary class of pigments
producing yellow to red coloration in birds; however, the majority of
vertebrates (ectothermic vertebrates – fish, reptiles, amphibians) can
also produce yellow to red coloration using a biochemically distinct
class of pigments called pteridines (Bagnara & Matsumoto 2006).
Pteridines are synthesised de novo within pigment cells from
abundant purine molecules (Bracher et al. 1998; Ziegler 2003;
Braasch et al. 2007). 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 within xanthophore
pigment cells (Bagnara & Hadley 1973; Bagnara & Matsumoto 2006).
However, in sharp contrast to carotenoids, the evolutionary drivers of
variation among species in pteridine pigments remain largely unknown.
What explains the use of carotenoid or pteridine pigments when both can
produce yellow to red colours? One compelling but unsubstantiated
possibility is that pteridines replace carotenoids in environments where
carotenoid availability is limited. Carotenoid limitation is expected to
alter the relative cost of acquiring and sequestering carotenoids
compared to synthesising pteridines. Specifically, when carotenoids are
rare, it may be metabolically cheaper to synthesise pteridines; whereas
when carotenoids are abundant, it may be metabolically cheaper to
acquire, transport and sequester carotenoids (Grether et al.2001; Grether et al. 2005).
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).
Other pteridines (e.g. pterin, isoxanthopterin) are often assumed to be
colourless but may take crystalline form, and by virtue of their
crystallinity, contribute to integument coloration through reflection
and scattering of light, rather than absorption (Oliphant & Hudon 1993;
Palmer et al. 2018; Palmer et al. 2020). These other
pteridines can be found in large quantities within xanthophores (Bagnara
& Matsumoto 2006; McLean et al. 2017; McLean et al. 2019;
Twomey et al. 2020b), suggesting that they may contribute to
integument coloration. The different costs and roles in colour
production for different types of carotenoids and pteridines influence
expected associations with environmental factors and sexual selection.
However, the ecological and evolutionary drivers of pigment variation
remain unknown, with the exception of carotenoids in some groups of
birds (Prum et al. 2012; Friedman et al. 2014b, a; Ligonet al. 2016).
Here, using an extensive dataset of concentrations of
5 carotenoid and 6 pteridine
pigments, we test whether pigment concentrations are associated with
environmental gradients indicative of carotenoid availability 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; McLean et al. 2019). In addition to testing for
associations with environmental gradients, we tested whether pteridines
replace carotenoids with a similar hue (carotenoid-mimicry hypothesis)
(Grether et al. 1999), resulting in a negative correlation
between the concentrations of similarly coloured carotenoid and
pteridine pigments. Since concentrations of different pigment types 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). Additionally, we evaluated how carotenoid or pteridine
concentrations covary with skin colour (hue, saturation, luminance). We
show that environmental gradients can predict the use of biochemically
different classes of pigment for integument coloration in vertebrates.