Author(s): Braulio A. Assis 1,*, Benjamin J. M. Jarrett 2, Gabe Koscky 3, Tracy Langkilde 1, Julian D. Avery 4
Animal coloration is integral to visual communication channels and selective processes, and is an important subject of investigation in studies on social selection . Coloration of conspicuous ornamental traits can predict the outcome of intrasexual competition and mate preference in many taxa [2-6], and for this reason is expected to be under strong selection. Notably, animal coloration can be plastic, as rapid changes in hue may occur in response to environmental conditions such as temperature, hydration, and background coloration, particularly among ectotherms [7-13]. Such dynamic changes make these color traits an important target of investigation on their roles in social interactions and signaling potential , but this same complexity brings challenges to researchers: to carefully consider optimal color quantification methods that account for plasticity, and to accurately estimate signal strength when color states are variable. One alternative is to standardize the abiotic conditions that are most representative of natural settings in which color traits may influence animal interactions. However, this is not always possible (e.g., when measuring traits under field conditions) or ecologically realistic (e.g., when thermoregulation influences fitness via multiple routes). Another option would be to develop a metric that is completely independent from abiotic factors, or at least conserves ranks of color of individuals across that environmental gradient. The latter would be sufficient to answer several important biological questions related to other qualitative fitness correlates such as mate choice [5,15] and social hierarchy [16,17] and may thus be a valid effort.
Unlike pigment-based coloration, structural colors typically arise from the reflection of light on nanostructures located inside iridophores [18-20] or on matrices of keratin  and chitin . The conformation of such structures may respond to changes in temperature and osmolarity , altering the wavelengths reflected and consequently the hue perceived by the receiver. Due to these characteristics, color traits that are structural in nature may demand more attention when determining methods and environments for their quantification. To fully capture the color properties of such complex traits, researchers often employ spectrophotometry, as it can generate raw spectral data that is not biased towards the human visual system . However, this approach alone does not inform us about how a given signal is perceived by potential receivers. Models that account for visual sensitivity of signal receivers are being more commonly employed [24,25] but are not the norm. A detailed assessment of how different color metrics and visual models behave across environmental treatments could provide guidelines of best practices for working with organisms that exhibit color plasticity and how these changes may be perceived by target receivers.
Eastern fence lizards, Sceloporus undulatus , display conspicuous color patches on their ventral gular and abdominal regions. These are present in sexually mature males and seem to be relevant in visual displays during social interactions . During encounters with rivals and potential mates, male fence lizards often perform push-up displays, elevating their bodies...