IRSCHICK, D. J.; LAILVAUX, S. P.; RAMOS, M.; VANHOOYDONCK, B.; HERREL, A.; LOSOS, J. B.; Tulane University; Tulane University; Tulane University; University of Antwerp; University of Antwerp; Washington University in St. Louis; ; : Life and death in the big city: Sexual signals, parasitism, and selection

Elaborate sexual signals, such an deer horns or colorful elongated tail patches on birds, are considered hallmarks of the sexual selection process, and are thought to play a key role in influencing male reproductive success. However, while the size or color of such sexual signals can change across seasons in some animal species (e.g., birds), these changes are typically caused by endogenous hormonal rhythms. Male green anole lizards (Anolis carolinensis) possess large colorful throatfans that play a key role in territorial defense, and ultimately, reproductive fitness. We provide evidence that selection, driven in part by the presence of internal parasites, likely alters the relative size of the dewlap across seasons in these lizards. We show that the average male dewlap in spring is about 33% larger compared to similarly sized males in winter, and that dewlap size returns to its previous size the following spring. Mark-recapture studies show a marked turnover of about 90% of the adult males between Spring and Winter. This high mortality is driven in part by the presence of damaging internal parasites (larvae from a botfly) that infects male lizards at a much higher rate compared to females and juveniles. We propose that intense selection eliminates males with large dewlaps in the fall and winter, with younger large-dewlapped overwintering males replacing these eliminated males the following spring. Based on this dramatic morphological shift over such a short time span (< 8 months), we calculate that seasonal changes in dewlap size represent by far the most rapid case of natural selection ever documented.