THE DERMAL CHROMATOPHORE UNIT

Color change serves many antipredator functions and may allow animals to better match environments or disrupt outlines to prevent detection. Rapid color change could potentially provide camouflage to animals that frequently move among microhabitats. Determining the adaptiveness of whole-animal rapid color changes in natural habitats with respect to predator visual systems would greatly broaden our fundamental understanding of the evolution of rapid color change. We tested whether whole-body color change provides water anoles (Anolis aquaticus) with camouflage against avian predators and whether these rapid changes allow them to shift between environment matching and edge disruption. We manipulated A. aquaticus placement in natural microhabitats and used digital image analysis to quantify color matching, pattern matching, and edge disruption produced by microhabitat-induced color change. Color change reduced lizard detectability to predators in microhabitat-specific ways. Environment matching was favored when lizards were in solid-colored microhabitats, regardless of exposure to predators. Edge disruption was instead induced by high exposure and varied by body region. We provide the first evidence that rapid color change permits a tetrapod to flexibly employ the most optimal camouflaging strategy by form (e.g., color matching vs. edge disruption) to minimize detection in the eyes of its predators.

Rapid skin colour changes in amphibians and other colour changing animals are possible due to different distributions of pigment cells (chromatophores) and the movement of pigment within them. Amphibians possess three types of chromatophore: xanthophores, iridophores and melanophores which are collectively referred to as the dermal chromatophore unit. Male stony creek frogs (Litoria wilcoxii) are capable of undergoing rapid colour change from brown to yellow during amplexus. Based on previous studies, it is expected that this is achieved through a change in chromatophore distribution or pigment movement within chromatophores. We examined brown and yellow dorsal skin samples from male L. wilcoxii using light microscopy which allowed us to determine differences in chromatophore and pigment distribution between each colour phase. Additionally, we compared thigh skin sections, which are comprised of permanently yellow and black patches. We found that in dorsal skin sections of yellow frogs, melanophore pigment granules had aggregated to the centre of the melanophore underneath the yellow xanthophore, whilst pigment was dispersed throughout the melanophores, partially covering the xanthophores in brown frogs. Black thigh sections consisted of elongated melanophores, other cell types appeared to be absent. In contrast, yellow thigh sections contained only xanthophores. This study demonstrates that the process of colour change in L. wilcoxii is through pigment aggregation and dispersion within melanophores. In addition, we show that there is significant variation in pigment cell distribution between colour changing and non-colour changing integument.

Colour-changing signals are independent of social interactions, but may signal body condition in an <i>Anolis</i> lizard

Engineering color, pattern, and texture: From nature to materials

In the tadpole of the tree frog Hyla arborea, the color of the dorsal skin was dark brown. Dermal melanophores, xanthophores, and iridophores were scattered randomly under the subepidermal collagen layer (SCL). After metamorphosis, the dorsal color of the animal changed to green and the animal acquired the ability of dramatic color change, demonstrating that the dermal chromatophore unit (DCU) was formed at metamorphosis. Fibroblasts invaded the SCL and divided it into two parts: the stratum spongiosum (SS) and the stratum compactum (SC). The activity of collagenase increased at metamorphosis. The fibroblasts appeared to dissolve the collagen matrix as they invaded the SCL. Then, three types of chromatophores migrated through the SCL and the DCU was formed in the SS. The mechanism how the three types of chromatophores were organized into a DCU is uncertain, but different migration rates of the three chromatophore types may be a factor that determines the position of the chromatophores in the DCU. Almost an equal number of each chromatophore type is necessary to form the DCUs. However, the number of dermal melanophores in the tadpoles was less than the number of xanthophores and iridophores. It was suggested that epidermal melanophores migrated to the dermis at metamorphosis and developed into dermal melanophores. This change may account for smaller number of dermal melanophores available to form the DCUs.

Acentrogobius virgatulus (Jordan &amp; Snyder, 1901) is a small coastal species of goby found along the Western Pacific. It is commonly found in Maizuru bay along the muddy sediment between the intertidal zone and depths of 10 m. In June and July of 2022, two independent agonistic interactions between male A. virgatulus were observed and recorded during its spawning season. One interaction, lasting over 4 minutes, included certain aggressive behaviors such as jaw locking, mouth gaping, fin extensions, rapid color changes, and fast strikes to the head and body. Another interaction exhibited similar mouth gaping, fin extension, and rapid color changes but did not lead to further escalation. These behaviors coincide with those found in similar species and provides in situ evidence of these uncommon interactions. This is the first record of agonistic behavior by an Acentrogobius species. Accumulating findings such as these can contextualize intraspecific interactions, reveal differences across multiple species, and guide future experiments.

We propose an explanation for the rapid color changes on the color–magnitude diagrams observed by Gahm et al. in the T Tauri star RY Lup during its deep minima. Our calculations have shown that the hot accretion spot on the stellar surface in combination with the inhomogeneous structure of the gas–dust clouds obscuring the star can be responsible for these changes. The observed rate of of the color changes allows one to estimate the velocity of the screen across the stellar disk =100 km s-1. This velocity is close to the typical gas velocities near T Tauri stars.

Animals capable of rapid (i.e., physiological) body color change may use color to respond quickly to changing social or physical environments. Because males and females often differ in their environments, the sexes may use changes in body color differently, reflecting sexual dimorphism in ecological, behavioral, or morphological traits. Green anole lizards, Anolis carolinensis, frequently switch their dorsal body color between bright green and dark brown, a change that requires only seconds, but little is known regarding sexual dimorphism in their color change. We tested three hypotheses for the function of body color (thermoregulation, camouflage via background-matching, and social communication) to determine the ecological role(s) of physiological color change in anoles. First, we examined instantaneous body color to determine relationships between body color and body temperature, substrate color and type, and whether these varied between the sexes. Next, we examined the association between color change and behavioral displays. Altogether, we found that males were more likely to be green than females, and larger lizards were more often green than smaller ones, but there was no evidence that anole body color was associated with body temperature or background color during the summer breeding season. Instead, our results show that although the sexes change their color at approximately the same rates, males changed color more frequently during social displays, while females remained green when displaying. In sum, social communication appears to be the primary function of anole color change, although the functions of body color may differ in the nonbreeding season.Significance statementMany animals can change their body color in response to their environments, and in many species, males and females experience different environments. In this study, we examined whether the sexes of green anole lizards use the ability to rapidly change their body color between green and brown for different functions. We found that, when a lizard was first sighted, its body color did not appear to match its background color in either sex (suggesting that color change does not contribute to avoidance of detection by potential predators), and body color was not associated with temperature for either sex (i.e., color was unlikely to influence body temperature). Yet, males changed color more often when performing social displays to other lizards, while females remained green during social displays. Thus, rapid color change plays an important role in social communication in both sexes, highlighting how males and females may use the same behavior to convey different messages.

Many chameleons, and panther chameleons in particular, have the remarkable ability to exhibit complex and rapid colour changes during social interactions such as male contests or courtship. It is generally interpreted that these changes are due to dispersion/aggregation of pigment-containing organelles within dermal chromatophores. Here, combining microscopy, photometric videography and photonic band-gap modelling, we show that chameleons shift colour through active tuning of a lattice of guanine nanocrystals within a superficial thick layer of dermal iridophores. In addition, we show that a deeper population of iridophores with larger crystals reflects a substantial proportion of sunlight especially in the near-infrared range. The organization of iridophores into two superposed layers constitutes an evolutionary novelty for chameleons, which allows some species to combine efficient camouflage with spectacular display, while potentially providing passive thermal protection.

The fundamental unit of rapid, physiological color change in vertebrates is the dermal chromatophore unit. This unit, comprised of cellular associations between different chromatophore types, is relatively conserved across the fish, amphibian, and reptilian species capable of physiological color change and numerous attempts have been made to understand the nature of the four major chromatophore types (melanophores, erythrophores, xanthophores, and iridophores) and their biochemical regulation. In this review, we attempt to describe the current state of knowledge regarding what classifies a pigment cell as a dynamic chromatophore, the unique characteristics of each chromatophore type, and how different hormones, neurotransmitters, or other signals direct pigment reorganization in a variety of vertebrate taxa.

Animals employ various mechanisms for camouflage, including color change, that may facilitate habitat use. However, the extent to which these mechanisms operate under nocturnal conditions is unclear. To investigate this, we combined a background-induced color change experiment with visual modeling to test whether altering backgrounds for a tropical tree frog (Pithecopus hypochondrialis) could induce short-term color change under nocturnal conditions to match the viewing background, as perceived by three predator classes: snakes, mammals, and birds. We demonstrated that frogs can change color multiple times from green to brown and back across grass and leaf litter backgrounds in dim conditions. Frog visual contrast varied by predator and background. Brown frogs matched against leaf litter across all predators, whereas green frogs were more variable and comparatively less well matched against grass. Notably, frogs achieved near-optimal color matching against both backgrounds for avian predators, with green frogs matching into grass and brown frogs matching into leaf litter. Our study provides evidence that P. hypochondrialis undergoes rapid background-induced color changes at night maintaining effective camouflage, at least against avian predators. We emphasize the need to assess rapid color change against visually guided predators in natural conditions and the importance of understanding viewing conditions for illuminating the ecology and evolution of camouflage.

Amphibian tadpoles are capable of avoiding threats (predators, uv radiation, etc.) through changes in coloration, behavior, and shape. In this paper, we tested how quickly European tree frog (Hyla arborea) tadpoles can change body pigmentation to achieve crypsis and whether color change is reversible. Additionally, we tested how different environmental background colorations affect the body length, shape, and ontogenetic trajectories of tadpoles. We also analyzed if tadpoles can relate to their coloration and choose the appropriate background to enhance crypsis. For this purpose, we reared tadpoles on white and black backgrounds for 36 days. Halfway through the experiment, half of the tadpoles from each treatment were placed on the alternative background. Our results suggest that H. arborea tadpoles are capable of rapidly responding to color changes in their environment, however, color-matching with the white background is poor. These quick color changes are reversible. Rearing in different background coloration and rapid color changes do not affect tadpoles’ length variation but affect tadpoles’ shape. Tadpoles introduced to the white background at the start of the experiment developed deeper tail fins and more pronounced snouts. We also found that H. arborea tadpoles actively choose an appropriate background to achieve maximum crypsis. This study represents the basis for the future analysis of adaptive coloration in tadpoles as it has a very complex function in anurans.

Hyperolius viridiflavus nitidulus inhabits parts of the seasonally very hot and dry West African savanna. During the long lasting dry season, the small frog is sitting unhidden on mostly dry plants and has to deal with high solar radiation load (SRL), evaporative water loss (EWL) and small energy reserves. It seems to be very badly equipped to survive such harsh climatic conditions (unfavorable surface to volume ratio, very limited capacity to store energy and water). Therefore, it must have developed extraordinary efficient mechanisms to solve the mentioned problems. Some of these mechanisms are to be looked for within the skin of the animal (e.g. protection against fast desiccation, deleterious effects of UV radiation and overheating). The morphology of the wet season skin is, in most aspects, that of a "normal" anuran skin. It differs in the organization of the processes of the melanophores and in the arrangement of the chromatophores in the stratum spongiosum, forming no "Dermal Chromatophore Unit". During the adaptation to dry season conditions the number of iridophores in dorsal and ventral skin is increased 4-6 times compared to wet season skin. This increase is accompanied by a very conspicuous change of the wet season color pattern. Now, at air temperatures below 35° C the color becomes brownish white or grey and changes to a brilliant white at air temperatures near and over 40° C. Thus, in dry season state the frog retains its ability for rapid color change. In wet season state the platelets of the iridophores are irregularly distributed. In dry season state many platelets become arranged almost parallel to the surface. These purine crystals probably act as quarter-wave-length interference reflectors, reducing SRL by reflecting a considerable amount of the radiated energy input.EWL is as low as that of much larger xeric reptilians. The impermeability of the skin seems to be the result of several mechanisms (ground substance, iridophores, lipids, mucus) supplementing each other.The light red skin at the pelvic region and inner sides of the limbs is specialized for rapid uptake of water allowing the frog to replenish the unavoidable EWL by using single drops of dew or rain, available for only very short periods.

Bright colouration appearing in one sex only can be driven by components of sexual selection including female choice, male competition or mate recognition. Male Litoria wilcoxii undergo rapid colour change from brown to yellow during amplexus, however, the function, if any, is unknown. We tested possible behavioural functions by observing breeding aggregations and behavioural responses (colour change, movement, call and amplexus duration) to varying stimuli (including model male and female frogs). We also examined whether colour change was a by-product of hormone release by comparing spermatic urine of frogs injected with epinephrine (colour change hormone) and hCG (triggers spermiation). Finally, the predation cost of being bright yellow was examined by placing frog models (yellow and brown) in the field and measuring predator attack rate. The behavioural responses of males to model females, brown/brown models (female with amplexing brown male), and brown/yellow models (female with amplexing yellow male), were similar to reactions towards real females, with the important exception that males did not attempt amplexus with brown/yellow models. Epinephrine injections triggered colour change but not sperm release in male frogs, while hCG induced sperm release but not colour change. Attack rates were low in predation trials with no difference in attack rates between yellow and brown models observed. Our study presents a novel function for rapid dynamic colour change as an intrasexual signal during amplexus that could avert sperm competition and displacement by other males. Colour displays during breeding are believed to have evolved through mechanisms of sexual selection (female choice, male–male competition or sexual recognition). Stony creek frogs (Litoria wilcoxii) have been observed to rapidly change colour from brown (similar to female colouration) to bright yellow during amplexus, which is unusual as the colour change occurs after mate selection. Behavioural experiments were used to test hypotheses on the evolutionary function of colour change in this species. In L.wilcoxii, colour functions as an intrasexual signal during amplexus, we hypothesise that this could avert sperm competition and/or displacement by other males during amplexus. The function presented here is novel among amphibians, however as data on dynamic colour change in amphibians is lacking, this trait may be more common.

Thermochromic gemstones exhibit fascinating color-changing properties in response to variations in temperature and light exposure. However, limitations in current experimental techniques for the analysis of thermochromic and photochromic minerals mean that the underlying defects and mechanisms responsible for the effects remain largely unknown. To address this gap, we present an in situ absorption spectroscopy system that integrates precise temperature control with external illumination for real-time absorption monitoring. This system can detect subtle and rapid color changes in both faceted and rough gemstones caused by thermochromic and photochromic responses. In this study, we examine the renowned color-changing chameleon diamond and its cousin, the yellow 480 nm band diamond, to demonstrate the system's capabilities. Our findings provide valuable insight into the mechanisms that drive thermochromic color changes. In addition, this system offers an alternative approach to studying the optical responses of materials under different excitation energies and temperatures, facilitating a deeper understanding of associated optical defects.