ABSTRACT
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.
É muitas vezes incerta a significância adaptativa de caracteres vívidos e coloridos em fêmeas. Hipóteses para esse fenômeno sugerem uma herança maladaptativa (ou neutra) de caracteres selecionados em machos aliada à regulação epigenética incompleta em fêmeas, ou ainda seleção positiva em fêmeas imposta por competição social. Vantajosos ou não, a visibilidade de ornamentos muitas vezes é modulada por vias comportamentais do portador de modo a balancear seus custos e benefícios. Fêmeas Sceloporus undulatus possuem um par de marcas verde-azuis na parte ventral do pescoço que são variáveis em área e saturação entre indivíduos. Esses ornamentos são rudimentares em relação aos vistos em machos, mas ainda assim estão associados a custos reprodutivos importantes. Nessa espécie, temperatura corporal aumenta a visibilidade desses ornamentos significativamente. Portanto, a termorregulação é um comportamento que fêmeas poderiam empregar para modular a visibilidade de ornamentos. Nós quantificamos temperaturas corporais obtidas por pares de lagartos fêmeas em testes comportamentais e observamos que fêmeas com os maiores ornamentos também obtiveram temperaturas corporais mais altas, às vezes além do ótimo fisiológico. Nesta associação entre fenótipo e comportamento, fêmeas com os maiores ornamentos exibiram padrões de termoregulação que aumentaram sua visibilidade. Este padrão é difícil de conciliar com a perspectiva de que ornamentos são maladaptativos em fêmeas, sugerindo benefícios que são dependentes do contexto social.
ABSTRACT
The evolutionary significance of widespread hypo-allometric scaling of genital traits in combination with rapid interspecific genital trait divergence has been of key interest to evolutionary biologists for many years and remains poorly understood. Here, we provide a detailed assessment of quantitative genital trait variation in males and females of the sexually highly dimorphic and cannibalistic orb-weaving spider Argiope aurantia. We then test how this trait variation relates to sperm transfer success. In particular, we test specific predictions of the one-size-fits-all and lock-and-key hypotheses for the evolution of genital characters. We use video-taped staged matings in a controlled environment with subsequent morphological microdissections and sperm count analyses. We find little support for the prediction of the one-size-fits-all hypothesis for the evolution of hypo-allometric scaling of genital traits, namely that intermediate trait dimensions confer highest sperm transfer success. Likewise, our findings do not support the prediction of the lock-and-key hypothesis that a tight fit of male and female genital traits mediates highest sperm transfer success. We do, however, detect directional effects of a number of male and female genital characters on sperm transfer, suggesting that genital trait dimensions are commonly under selection in nature. Importantly, even though females are much larger than males, spermatheca size limits the number of sperm transferred, contradicting a previous hypothesis about the evolutionary consequences of genital size dimorphism in extremely size-dimorphic taxa. We also find strong positive effects of male body size and copulation duration on the probability of sperm transfer and the number of sperm transferred, with implications for the evolution of extreme sexual size dimorphism and sexual cannibalism in orb weavers.
