Honest signals and sexual conflict: Female lizards carry undesirable indicators of quality

Effects of Urbanization on Ventral Patch Size and Phenotypic Correlates of Patch Expression in Male Western Fence Lizards (Sceloporus occidentalis)

In some animals, males use colorful ornaments or badges to visually communicate with conspecifics. These traits can be condition-dependent, suggesting that environmental changes could impact the intensity of male sexual signals. Drastic habitat changes caused by urbanization can act as physiological stressors, potentially affecting male signaling traits through changes to condition or immune function. Here, we quantified the effects of urbanization on ventral patch size and correlates of patch expression, namely body size, body condition, corticosterone concentrations, and ectoparasites in male Western Fence Lizards (Sceloporus occidentalis). We compared three aspects of male ventral color patches between urban and natural populations: area of the throat patch, total area of the paired belly patches, and total area of the black borders of the belly patches. All three area measurements across both habitat types were positively related to body size, and total belly patch area was positively related to body condition, indicating that these traits may signal male competitive ability and/or quality. Males from urban populations had larger throat patches than those from natural populations after controlling for body size. This difference in patch size was associated with a difference in probability of ectoparasite infection, but not with differences in corticosterone concentrations or body condition between urban and natural populations. Our results may indicate an effect of urbanization on immune function affecting male patch expression, although this idea remains untested. Overall, we show that urbanization can impact male sexual traits, which may have repercussions for visual communication in urban environments.

Evolutionary Loss of Male-Specific Coloration Is Associated with the Loss of Androgen Receptor Expression in Skin of Sceloporus Lizards

AbstractHormones can induce trait development in one species yet have no effect on the same trait in a closely related species, but the mechanisms underlying these differences are unclear. Here, we compare two closely related lizard species to explore the cellular mechanisms associated with the evolutionary loss of hormonally mediated ventral coloration. The eastern fence lizard (Sceloporus undulatus) has sexually dimorphic blue and black ventral coloration that develops when maturational increases in androgens induce melanin synthesis in males. The closely related striped plateau lizard (Sceloporus virgatus) has sexually monomorphic white ventral skin that does not produce melanin in response to the same signal. We used immunohistochemistry to localize the androgen receptor (AR) in the skin of both species and to test whether the loss of ventral coloration in S. virgatus corresponds to the loss of AR in the skin. We found that the ventral skin of S. virgatus displays little or no AR staining in the pigment cell layer, potentially explaining the loss of androgen sensitivity in this tissue, relative to the robust AR staining in the same layer of S. undulatus. Based on the location of three markers for melanophores (microphthalmia-associated transcription factor, dopachrome tautomerase, and tyrosinase), AR appears to be present in melanophores in S. undulatus. However, we could not detect these melanophore markers in the skin of S. virgatus. Therefore, the evolutionary loss of ventral coloration may have occurred via the loss of the AR-producing melanophore in mature ventral skin, preventing the development of a male-typical trait and sexual dimorphism in this tissue.

Natural parasites in conjunction with behavioral and color traits explain male agonistic behaviors in a lizard

Male competition conforms to a cost-benefit model, because while aggression may increase reproductive prospects, it can also increase the risk of injury. We hypothesize that an additional cost in aggressive males would be an increase in parasite load associated with a high energy investment into sexual competition. Some of these infections, in turn, may downmodulate the level of host aggression via energetic trade-offs. We staged dyadic male contests in the lab to investigate the relationships of multiple parasites with the agonistic behavior of lizard hosts, Sceloporus occidentalis. We also included both color and behavioral traits from opponents in the analyses because (1) color patches of lizards may serve as intraspecific signals used by conspecifics to assess the quality of opponents, and (2) contests between male lizards fit classical models of escalated aggression, where lizards increase aggression displays in response to an opponent's behavior. The results conform to our hypothesis because male lizards displayed more pushups when they had more ticks. Moreover, some parasites may modulate the levels of aggression because lizards infected by hematic coccidians performed fewer pushups. Interestingly, lizards also displayed fewer pushups when both the chroma and size of the opponent's blue patch were greater. The results thus also supported the role of the blue patch of S. occidentalis as a sexual armament, because it contributed to the deterrence of aggression from opponent lizards. We revealed that natural parasitic infections in lizard hosts can contribute to their agonistic behavior. We encourage future studies to account for parasites in behavioral tests with lizards.

Signalling on islands: the case of Lilford’s wall lizard (Podarcis lilfordi gigliolii) from Dragonera

Studies of the effects of insularity on animal signals are scarce, particularly in lizards. Here, we use Lilford’s wall lizard from Dragonera (Podarcis lilfordi gigliolii) to ask how island conditions have affected its repertoire of social signals, focusing on two visual signals shared by many Podarcis species: ultraviolet (UV)–blue-reflecting ventrolateral colour patches and visual displays. We examined whether the number or spectral characteristics of the UV–blue patches are associated with traits related to individual quality. We also used visual models to assess visual conspicuousness and to measure sexual dichromatism. We did not observe foot shakes or any other visual displays usually found in continental Podarcis. We found that none of the UV–blue patch variables covaried with morphometric variables indicative of fighting ability or body condition in males, suggesting that this coloration does not signal individual quality. We also found very little sexual dichromatism. In particular, the UV–blue patches of females seem over-expressed and more similar to those of males than those of continental Podarcis. Ancestral state reconstruction reveals that the lack of sexual dimorphism in the UV–blue patches is a derived condition for P. lilfordi gigliolii and other Podarcis living on small islands. Our results thus show a pattern of reduced social signalling in P. lilfordi gigliolii relative to mainland Podarcis, with some signals being lost or under-expressed (visual displays) and others losing their signalling function (UV–blue patches). We hypothesize that these changes are attributable to the high population density of P. lilfordi gigliolii, which discourages territorial behaviour and promotes extreme social tolerance, making most social signals unnecessary. More work will be needed to determine whether this is a common pattern in lizards inhabiting small and densely populated islands.

Shared and unique responses in the microbiome of allopatric lizards reared in a standardized environment

The gut microbiome can influence host fitness and, consequently, the ecology and evolution of natural populations. Microbiome composition can be driven by environmental exposure but also by the host's genetic background and phenotype. To contrast environmental and genetic effects on the microbiome we leverage preserved specimens of eastern fence lizards from allopatric lineages east and west of the Mississippi River but reared in standardized conditions. Bacterial composition was indistinguishable between lineages but responded significantly to host age-a proxy for environmental exposure. This was accompanied by a continuous decrease in bacterial diversity in both lineages, partially driven by decreasing evenness seen only in western lizards. These findings indicate that longer exposure to a homogeneous habitat may have a depreciating effect on microbiome diversity in eastern fence lizards, a response shared by both lineages. We highlight the importance of such effects when extrapolating patterns from laboratory experiments to the natural world.

Masculinized Sexual Ornaments in Female Lizards Correlate with Ornament-Enhancing Thermoregulatory Behavior

The adaptive significance of colorful or exaggerated traits (i.e., ornaments) expressed in females is often unclear. Competing hypotheses suggest that expression of female ornaments arises from maladaptive (or neutral) genetic inheritance from males along with incomplete epigenetic regulation, or from positive selection for ornaments in females under social competition. Whether costly or advantageous, the visibility of such traits can sometimes be behaviorally modulated in order to maximize fitness. Female eastern fence lizards express blue badges that are variable in size and color saturation. These are rudimentary compared to those seen in males and carry important costs such as reduced mating opportunities. Body temperature is a well-established enhancer of badge color, and thus thermoregulation may be one way these animals modulate badge visibility. We quantified realized body temperatures of female lizards paired in laboratory trials and observed that females with larger badges attained higher body temperatures when freely allowed to thermoregulate, sometimes beyond physiological optima. In this association between phenotype and behavior, females with larger badges exhibited thermoregulatory patterns that increase their badges’ visibility. This signal-enhancing behavior is difficult to reconcile with the widely held view that female ornaments are maladaptive, suggesting they may carry context-dependent social benefits.

Fitness Costs of Maternal Ornaments and Prenatal Corticosterone Manifest as Reduced Offspring Survival and Sexual Ornament Expression

Colorful traits (i.e., ornaments) that signal quality have well-established relationships with individual condition and physiology. Furthermore, ornaments expressed in females may have indirect fitness effects in offspring via the prenatal physiology associated with, and social consequences of, these signaling traits. Here we examine the influence of prenatal maternal physiology and phenotype on condition-dependent signals of their offspring in adulthood. Specifically, we explore how prenatal maternal testosterone, corticosterone, and ornament color and size correlate with female and male offspring survival to adulthood and ornament quality in the lizard Sceloporus undulatus. Offspring of females with more saturated badges and high prenatal corticosterone were less likely to survive to maturity. Badge saturation and area were negatively correlated between mothers and their male offspring, and uncorrelated to those in female offspring. Maternal prenatal corticosterone was correlated negatively with badge saturation of male offspring in adulthood. Our results indicate that maternal ornamentation and prenatal concentrations of a stress-relevant hormone can lead to compounding fitness costs by reducing offspring survival to maturity and impairing expression of a signal of quality in surviving males. This mechanism may occur in concert with social costs of ornamentation in mothers. Intergenerational effects of female ornamentation and prenatal stress may be interdependent drivers of balancing selection and intralocus sexual conflict over signaling traits.