Gross anatomy, histological, and histochemical analysis of the eyelids and orbital glands of the neonate pygmy hippopotamus (Suina: Choeropsis liberiensis or Hexaprotodon liberiensis, Morton 1849) with reference to its habitat

Abstract

The pygmy hippopotamus is phylogenetically related to members of both the Suidae and Cetacea. However, differences in their habitats may have resulted in variation in the anatomy and physiology of the ocular adnexa between these species. Therefore, this study focuses on the identification of accessory organs of the eye, which are typical for the pygmy hippopotamus and are comparable to organs present in mammals related to it. Moreover, the secretions produced by the superficial gland of the third eyelid, the deep gland of the third eyelid and the lacrimal gland were examined, as they ensure eyeball protection. In the upper and lower eyelids, numerous serous glands where identified, which were typical for the pygmy hippopotamus and similar as in the Cetacea. This study enabled to identify additional folds in the eyelids of the pygmy hippopotamus. Lymphoid follicles and diffuse lymphocytes were not found in the lymphoid region in the upper or lower eyelids and the third eyelid, which was most likely caused by the age of the studied hippopotamuses. An accurate histochemical analysis revealed that the secretions of the pygmy hippopotamus are very similar to the Sus scrofa. The structural differences between the pygmy hippopotamus and representatives of Cetacea are most likely caused by the fact that most of Cetacea live in saltwater and are exposed to more frequent fluctuations in water temperature compared to the pygmy hippopotamus, which lives in fresh water and does not lead a migratory lifestyle like the Cetacea.

Genomic and anatomical comparisons of skin support independent adaptation to life in water by cetaceans and hippos

“…We also extended these findings to a second hippopotamid species, Choeropsis liberiensis. Meibomian glands, which are modified sebaceous glands in the eyelid skin, are also absent in H. amphibius, as has also been reported for C. liberiensis [39]. Further, examined vibrissa follicles in newborn Tursiops truncatus and adult Eschrichtius robustus also lack sebaceous glands.…”

“…Hippos have numerous bristle-like whiskers on their muzzle and tail, but unlike cetaceans they also have pelage hairs that are sparsely distributed across most of the body [15]. Sebaceous glands have not been reported in hippos, but previous histological studies have only examined limited regions of the body [15,39]. Hippo skin contains anatomically complex sweat glands [37,38] that secrete a distinct red-orange pigmented sweat that is thought to have sunscreen and/or antimicrobial properties [40,41].…”

“…Hippos spend their days in the water where they remain cool, but emerge at dusk to feed on grasses and other vegetation. H. amphibius and C. liberiensis have superficially similar-looking skin, with histological studies primarily on the former [15,37,38] with one recent exception [39]. Similar to cetaceans, adult H. amphibius has a thick epidermis and its stratum basale forms distinct projections into the papillary dermis [15].…”

Genomic and anatomical comparisons of skin support independent adaptation to life in water by cetaceans and hippos

“…Therefore, we thoroughly investigated several regions of hair-bearing skin and provide more definitive evidence for the absence of sebaceous glands in both extant hippo species. Meibomian glands, which are modified sebaceous glands in the eyelids, are also absent in both hippos (also see [39]). Further, vibrissa follicles in newborn dolphin and adult gray whale lack sebaceous glands.…”

“…Hippos have bristle-like whiskers on their muzzle, and pelage hairs that are sparsely distributed across most of the body [15]. Sebaceous glands have not been reported, but previous histological studies have only examined limited regions of the body [15,[37][38][39]. Hippo skin contains anatomically complex sweat glands [37,38] that secrete a red-orange pigmented sweat that may have sunscreen and/or antimicrobial properties [40,41].…”