A spectrum of speculums: Testing adaptive hypotheses for male speculum coloration in male Brown-Throated sloths (Bradypus variegatus).

Sloths exhibit specialized morphological and behavioral adaptations for crypsis, including slow movement and camouflaged pelage that reduces detection by predators. However, adult males of three three-fingered sloth species (Bradypus spp.) possess a distinctive dorsal patch called a speculum, characterized by bright pigmentation and oily secretions, presenting an apparent evolutionary paradox. The function of the speculum is unknown, especially due to the logistical constraints of conducting research on wild sloths. Here, we present the first quantitative analysis of speculum morphology in Brown-Throated Sloths (Bradypus variegatus) to evaluate potential functional hypotheses. Morphometric measurements revealed a positive correlation between speculum area with body size, while color analysis revealed weak correlations between body length and pigmentation patterns. These findings are consistent with sexual selection hypotheses and support the speculum as a sexually selected trait, yet its persistence is puzzling given the species' rod monochromacy and vulnerability to visually acute predators. The speculum's sexually dimorphic expression and secretory properties suggest potential multimodal signaling functions, possibly integrating visual and olfactory components. This study provides the first test of the relationship of the speculum's relationship to body size in sloths, which informs hypotheses about its functional and evolutionary significance.