Ophthalmology of Xenarthra: Armadillos, Anteaters, and Sloths

Abstract

Xenarthra (from Ancient Greek, meaning xénos, “foreign, alien” +árthron, “joint”) is a superorder of placental mammals that originated in South America during the Paleocene era, roughly 59 million years ago. Members of this group are thought to be one of the most ancient groups of mammals and include armadillos, anteaters, and sloths. Although its visual system has historically been understudied, the role of this group as an animal model for several human diseases of rod photoreceptors such as retinitis pigmentosa (Nakamura et al. 2016) and Leber congenital amaurosis (van der Spuy et al. 2005) may prove pivotal: they are considered completely colorblind (rod monochromats), an otherwise non-existent retinal adaptation among vertebrates that are not living underground or deep within the sea (Douglas et al. 1995; Meredith et al. 2013; Emerling and Springer 2014; Mohun et al. 2010). This claim is supported by behavioral (Newman 1913; Mendel et al. 1985; de Sampaio et al. 2016), anatomical (Wislocki 1928; Walls 1942; Watillon and Goffart 1969; Piggins and Muntz 1985), and genomic and phylogenetic (Emerling and Springer 2014) evidence. Despite support for rod monochromacy, many species within Xenarthra are diurnal and occupy niches receiving direct or indirect sunlight. Although rod monochromacy does not provide high visual acuity and can even result in total blindness in high luminance conditions, there is debate on how much Xenarthrans rely on vision and whether or not they predominantly use other senses, particularly in photopic conditions (Emerling and Springer 2014). Limited information exists on the visual capabilities, ophthalmic anatomy, and naturally occurring ophthalmic disease processes that affect Xenarthran eyes. In addition, detailed reports of clinical examination findings and comprehensive results of basic ocular diagnostic tests are lacking. Furthering our knowledge of the visual systems and ophthalmic pathologies in this group of animals may aid in conservation efforts (e.g., prevention of vehicular trauma of which Xenarthrans are frequent victims), rehabilitation, or welfare in captivity.