Equus asinus somalicus
(Equus africanus somaliensis)
1) General Zoological Data
The Somali wild ass was once widely distributed over Northern Africa but is now either extinct in Africa or is very rare. Two subspecies of African wild asses (Somali and Nubian) are commonly recognized, the Nubian wild ass being somewhat smaller. Some 70+ of these wild asses are kept in many zoological gardens. Pitra et al. (1995) indicated that the captive population ultimately had derived from 3.6 founders and that it is thus significantly inbred. They also did some initial studies to ascertain the genetic diversity in the captive stock. In contrast to the domestic burro (the presumed derivative of the Nubian wild ass by domestication 6,000 years ago), Somali Wild Asses are pale, yellow. The typical Somali Wild Ass may have a faint stripe across the shoulder and also possesses delicate stripes on the legs; it has a short, standing mane. More details of these animals are found in Krumbiegel (1958), and Groves (1986) presented an excellent, broad overview of the African asses. He also reviewed the disagreement that exists concerning the notion that the domestic animal comes from the Nubian wild ass. The real facts are still outstanding. No doubt, much interbreeding of these wild asses with domestic donkeys, both in the wild and in captivity, further complicates the picture. Maximum life span is over 22 years in captivity.
General Gestational Data
One placenta of this species was obtained from a mare that died with sepsis as a result of colonic obstruction by a huge, stone-hard fecalith. The female fetus was slightly autolyzed and weighed 15,700 g; it had a CR length of 80 cm. It was normally developed. On opening the uterus, the placenta was found to have been mostly detached from the uterine wall, but without there being any intervening blood. The brown placental surface had a nearly uniform cover of microvilli.
Another placenta came from a term gestation and weighed 2,180 g; it had a 32 cm long umbilical cord with three blood vessels. No macroscopic picture is available of that specimen. A third placenta in August, 2003 weighed 1,925 g, measured 52 cm in greatest diameter and had a 49 cm umbilical cord with four vessels.
Singletons are the norm and are born after an approximately 380 day gestation period (11 months). They weigh around 25-30 kg. Adults weigh around 250 kg.
There are no data on early placentation in the Somali Wild Ass, while this has been described extensively in the domestic horse (see that chapter).
General Characterization of the Placenta
Details of fetal/maternal barrier
This is a rather diffuse epithelio-chorial placenta with simple villi that abut the endometrial surface. There is no trophoblastic invasion of the uterus, except perhaps at the chorionic girdle. That region was not identified in this term organ, as one may see it in the delivered horse placenta (see chapter on domestic horse). It is likely, however, that a girdle with invasion and eCG production exists in this species. If younger specimens become available, that aspect needs verification.
The trophoblast is cuboidal and uninuclear, binucleate cells were not found. Under the chorioallantoic plate, the trophoblast is more cylindrical, especially in between the villi (the "areolar" regions). As in the domestic horse and many ungulate placentas, this cylindrical trophoblast often has yellow pigment granules within the cytoplasm. Iron stains (Prussian blue) are negative. The nature of this pigment is uncertain. It is not associated with hemorrhages, as is suggested by the term "haemophagous organ" that is extensively discussed in sheep placentation. I do not believe this pigment to derive from hemoglobin breakdown. Alternatively, I suggest that it may represent melanin.
The umbilical cord inserted at the cranial part of the uterus, had three blood vessels and a large allantoic duct. The amnionic portion measured 28 cm, the allantoic portion was 10 cm long. Another cord, from a term delivered placenta, measured 32 cm in length and was otherwise identical. The cord is partitioned into a somewhat longer amnionic portion, where its vessels are contained in Wharton's jelly and associated with smaller vessels and the allantoic duct, and the allantoic section. The latter commences at the site of insertion of the allanto-amnion and corresponds to the opening of the allantoic duct into the allantois. Here, the cord vessels are separated and only loosely associated and then diverge over the allantoic surface. In the amnionic portion of the cord it is covered with plaques of sqamous metaplasia. The vascular split of the allantoic vessels, as is usual in equines, is well shown in the gross photographs above. Aside from the two large arteries and one vein, numerous small blood vessels are found in the cord. Some, but only a minority, associate with the allantoic duct.
Two additional placentas at term with 32 and 49 cm long umbilical cords had three and four blood vessels, respectively.
The vasculature of the horse placenta has been beautifully described by Steven (1982), and subsequently by Wooding and Flint (1994). The reader is referred to the chapter on the domestic horse for a much more detailed discussion of this aspect.
Trophoblast external to barrier
There is no trophoblastic infiltration of the endometrium, except in the small "girdle" during early gestation. This has not been seen in the wild ass but is best studied in the domestic horse and burro. That infiltration is temporary and is the site of eCG production.
No other specific features are worth mentioning that have not been referred to in other portions of this chapter.
All donkeys, including the wild asses, have normally 62 chromosomes (Ryder et al., 1978). As a consequence, hybrids between African donkeys and domestic donkeys are fertile, as are hybrids between the Nubian (E. asinus africanus) and the Somali Wild Asses (E. asinus somalicus). Gray (1972) lists a hybrid with a kulan at Rome, but does not discuss its fertility.
A much more extensive study of the chromosomes, including banding patterns, was done by Houck et al. (1998). It involves a pedigree of Somali Wild Asses at the San Diego Zoo and revealed novel findings. Animals with 2n=62, 63, and 64 chromosomes were identified. In addition, some animals had a heritable pericentric inversion of chromosome # 2, and animals with 63 chromosomes were fertile. It was presumed that fission in a heterochromatic region was the mechanism of this chromosomal variety.
To my knowledge there have been no immunological studies on Somali Wild Asses.
Butt, W.R., Chard, T. and Iles, R.K.: Hormones of the placenta: hCG and hPL. Pp. 461-534 of Volume 3, Part one, in Marshall's Physiology of Reproduction, fourth edition, G.E. Lamming, ed. Chapman & Hall, London, 1994
Gray, A.P.: Mammalian Hybrids. A Check-list with Bibliography. 2nd edition. Commonwealth Agricultural Bureaux Farnham Royal, Slough, England, 1972.
Griner, L.A.: Pathology of Zoo Animals. Zoological Society of San Diego, San Diego, California, 1983.
Groves, C.P.: The taxonomy, distribution, and adaptations of recent equids. Pp. 11-65, in Equids in the Ancient World. Meadow, R.H. and Uerpmann, H.-P. eds. Dr. Ludwig Reichert Verlag, Wiesbaden, 1986.,
Houck, M.L., Kumamoto, A.T., Cabrera, R.M. and Benirschke, K.: Chromosomal rearrangements in a Somali wild ass pedigree, Equus africanus somaliensis (Perissodactyla, Equidae). Cytogenet. Cell Genet. 80:117-122, 1998.
Krumbiegel, I.: Einhufer. A. Ziemsen Verlag, Wittenberg, 1958.
Pitra, C., Streich, W.J., Reinsch, A., Fickel, J. and Mann, W.: Die Population des Somali-Wildesels (Equus africanus somalicus Sclater) in menschlicher Obhut: Demographische und genetische Aspekte. Zool. Garten 65:245-257, 1995.
Ryder, O.A., Epel, N.C. and Benirschke, K.: Chromosome banding studies of the equidae. Cytogenet. Cell Genet. 20:323-350, 1978.
D.H.: Placentation in the mare. J. Reprod. Fertil. Suppl. 31:41-55, 1982.
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