Ocular microscopy of Bradypus variegatus

Abstract

Brazil is classified as one of the richest countries in biodiversity worldwide (Mittermeier et al., 2005). The Xenarthra superorder is one of the oldest groups within the Brazilian fauna (Rezende et al., 2010). The group is divided into two orders, namely Cingulata, consisting of armadillos, and Pilosa, consisting of the sloths and anteaters. Pilosa xenarthrans have peculiar anatomical and physiological characteristics marked by low metabolism, low body temperature, and morphological adaptations to the most diverse types of niches (Cubas et al., 2007).Bradypus variegatus is a species popularly known as common sloth or brown-throated sloth, with a Brazilian geographical distribution (Mares & Genoways, 1982). It has long, thick and wavy hair, differentiated in the region of the face, the colour of which varies from pale brown to grey with whitish spots (Eisenberg et al., 2001).Information about the anatomy of these animals remains scarce, and multiple aspects remain unknown about their particularities,

The organ of vision morphology of the southern two‐toed sloth (Choloepus didactylus Linnaeus, 1758; Pilosa, Choloepodidae)

“…Although examined group of animals in our study were small, the obtained results of the eyeball macroscopic measurements were higher or similar to the results of the brown-throated sloth where there is a special microenvironment for their renewal and proliferation (Sanchez & Daniels, 2016). Macroscopically, the tapetum lucidum in the southern two-toed sloth was not identified, as was the case with Bradypus variegatus (Braz et al, 2020;Sena et al, 2021), while our histological studies showed the presence of choroidal tapetum lucidum cellulosum, the cells of this latter being characterized by folding of the cell membrane and irregular layering. Such structure of tapetum lucidum proves its degeneration, which was similarly demonstrated in the Asiatic black bear (Ursus thibetanus; Paszta et al, 2022) and ranch mink (Mustela vision; Braekevelt, 1989;Ollivier et al, 2004).…”

“…Macroscopic and morphometric studies in the upper and lower eyelids of the southern two‐toed sloth showed thick eyelashes in the anterior palpebral margin in both fleshy eyelids, similar to the brown‐throated sloth ( Bradypus variegatus ) (Braz et al, 2020) where they are covered outside by a short hair layer and inside by the bulbar conjunctiva. Histologically, both eyelids in our southern two‐toed sloth proved the lack of tarsal glands as in the brown‐throated sloth (Sena et al, 2021). In the palpebral conjunctiva, we found structures of the CALT in the form of lymphoid follicles, diffuse lymphocytes and specialized high endothelial venules allowing the lymphocyte migration and exchange between eyeball tissue and other organs of the MALT (Knop & Knop, 2002).…”

“…In our southern two‐toed sloth, the pupil post‐mortem was round like in Bradypus variegatus (Hayssen, 2009, 2011). In our histological studies, the retina was not captured in the field of view, but according to other authors in the photoreceptor layer, there are many slender rods with few ganglion cells and cones are rare or absent (Britton, 1941; Gilmore et al, 2000; Goffart, 1971; Hayssen, 2011), while De Sena et al (2021) suggest that cones and rods could not be discriminated.…”

“…There could be also unique features of an individual animal. According to Sena et al (2021), a lacrimal gland in the brown-throated sloth was composed of a considerable number of serous acini, while in southern two-toed sloth had mucous character. Our research in the southern two-toed sloth also showed the presence of a deep gland of the third eyelid, which was a simple branched alveolar multilobar gland producing a serous secretion, which was not shown by Sena et al (2021).…”

“…This plays a key role in the sloth's slowness (Britton, 1941; Piggins & Muntz, 1985; Wislocki, 1928). Hitherto, the study of the eyesight of the sloth was performed by a few authors (Adam, 1999; Andrade‐da‐Costa et al, 1987, 1989; Beebe, 1926; Britton, 1941; Collin, 2008; Hayssen, 2009, 2011; Krieg, 1961; Menegaux, 1908; Meritt Jr., 1985; Piggins & Muntz, 1985; Prince, 1956; Rochon‐Duvigneaud, 1943; Sena et al, 2021; Smith, 1898; Tirler, 1966; Walls, 1942; Wislocki, 1928). Additionally, the geometry and tonometry of the eyeball were described in the sloth (Braz et al, 2020).…”