Red River Hog
Potamochoerus porcus porcus

Order: Artiodactyla
Family: Suidae

1) General zoological data of species

Red river hogs are often referred to as African "bush pigs", but two distinct forms (species) exist: the Potamochoerus porcus [porcus] here discussed, and P. larvatus. Other authors (Kingdon, 1979) have considered these animals to be of subspecific relationship (P. porcus porcus, P. p. larvatus). In the past, the genus was named Koiropotamus (Leister, 1939). Grubb (1993) stated that too many subspecies of Potomochoerus have already been proposed but that no good evidence has been forthcoming to show that P. porcus and P. larvatus hybridize nor that there are intermediate forms. Other authors describe intermediates between the much darker, more Southern P. p. larvatus and the red river hog. Grubb considered them to be "good" species and separated the "red river hog" from the "bushpig". Vercammen et al. (1993) discussed aspects of conservation and distribution of these species in the same volume. We have recently obtained specimens of P. p. larvatus and find them to be much darker than red river hogs, and the female to be much larger. Moreover, they have much denser and longer hair (Please see photograph below).

The red river hog is widely distributed in sub-Saharan Africa and occurs in large "sounders" (groups of pigs). They are highly prized as zoo exhibits because of their beautiful decoration and unusual ears ("Pinselohrschwein"). Males are larger and heavier than females (54-115 kg). Newborns weigh between 650-900 g (Puschmann, 1989). Males have "warts" ("Apophysen") as well as bony protrusions in front of their eyes (Nowak & Paradiso, 1983). Life span is around 20 years, and the age of female maturity is 3 years.

Leister (1939) considered bushpigs to have derived relatively recently from European Sus stock, a conclusion also supported by Thenius (1970) who reviewed the entire phylogeny of suids.

Because of the similarity of placentation to other Suidae (especially the domestic pig), this is the only species of Suidae discussed in this book.

3) Implantation

Initial implantation is accomplished primarily by yolk sac epithelium, which is then the dominant membrane, but it lasts only a short time. There is much uterine "milk" with absorption through this epithelium. By day 24 of the domestic pig pregnancy, the allantois attaches all around the periphery and the yolk sac shrinks (Ramsey, 1982). Additional details of early phases of pig reproduction are available from Geisert (1998).

4) General characteristic of placenta

I have examined a twin placenta of term red river hogs. The placentas were separate and were of surviving neonates. One placenta weighed 275 g, the other was 300 g. They measured 83 and 85 cm in lengths, and were between 5 and 13 cm in width when laid open. The cords had inserted mid-marginally but were absent from the specimens. There is a very large allantoic sac. In concordance with other suid placentas, the ends of the placenta showed marked degenerative changes. Mossman (1987) suggested that the reason for this is the early fusion of allantois and chorion, making the distal tips more vulnerable to crowding. A study of a wide variety of wild suids was published by MacDonald & Bosma (1985), but it did not contain the bushpig.

The placenta is large and diffuse, with an uniform thickness of maximally 2 mm. It is of the so-called "folded" villous type (in contrast to the villous or labyrinthine types). That is to say, there are folds of the chorionic trophoblastic surface that tend to enlarge the surface areas. These folds interdigitate with the maternal endometrial surface. The space seen in most preparations that separates these epithelia is artifactual and due to tissue shrinkage in fixation. It contains a minimal amount of uterine secretions, primarily though in areas that are referred to as the areolae. These are specialized regions, interspersed with the "folds", where the two epithelia reach maximal contact. This has been demonstrated in a comparative study by Björkman (1973) that also provided electronmicrographs of the "barrier". These EMs demonstrated convincingly the extensive microvillous interdigitations between the two epithelia. There is also a thick basement membrane to support the trophoblast. Additional fine-structural detail has been published by MacDonald (1976). The most complete study of the pig placenta was published by Heuser (1927) and should be read if more details are needed.

7) Uteroplacental circulation

This has not been documented to our knowledge.

8) Extraplacental membranes

The fetal placental surface of my specimens had a slimy, glassy, edematous appearance, which Mossman (1987) described as being typical for suids. There is a large allantoic sac surrounding the amnion and lying between amnion and chorion.

10) Endometrium

There is no decidualization during pregnancy in this bicornuate uterus but endometrial ridge develop.

11) Various features

None relevant to this species.

12) Endocrinology

The reproductive cycle is similar to that of Sus scrofa (Eckstein & Zuckerman, 1956). Cycles are about every 21 days, ovulation is spontaneous (not induced), and estrus lasts 2-3 days. These authors also refer to the estrogens and progestins in pregnancy. Progestins are low initially but rise sharply with a peak at 11-15 days and then drop to near zero. Estrogens rise towards term. The duration of pregnancy in the domestic pig is between 112 and 115 days, longer in wild ancestors. Additional information is available from Geisert (1998).

Berger et al. (2002) used fecal steroid metabolites to study the reproductive physiologyy of warthogs, red river hogs and the babirusa. They were able to define the length of cycles (35-37 days), identify non-cycling sows, and monitor pregnancy.

13) Genetics

The karyology of suids is still incomplete. The domestic pig has 38 chromosomes, but Sus scrofa has two morphotypes (2n=36 or 38), and lower numbers are found in giant forest hog (2n=32), warthog (2n=34). The "bushpig" (red river hog) has 34 chromosomes (Melander & Hansen-Melander, 1980). The sex chromosomes, as described by these authors, are unusual in their morphology and differ from those of other suids. Specifically, the X-chromosomes of the female specimen that these investigators studied differed in size and appearance. Numerous karyotypes of different animals of red river hogs in our laboratory have all shown 2n=34 and the X-chromosomes have been large metacentric elements without any structural differences. Bosma et al. (1991) had come to the same conclusion earlier. They studied a "bushpig from the Duisburg Zoo (Germany) but did not indicate whether it was a "red river hog" or another subspecies. Upon inquiry, however, it was learned that the animal they studied was in fact a red river hog. They found 34 chromosomes as well. Hybrids have not been described. Marczynska & Pigon (1973) described an "African native pig" karyotype (2n=38) but did not exactly specify the species. It is doubtful that they had a bushpig for study.

Watanabe et al. (1985) have studied the mtDNA of European and Asiatic pigs.

14) Immunology

No relevant studies are known to us.

15) Pathological features

There are a large number of diseases affecting swine, for instance hog cholera and the African swine disease, a virus infection thought to have originated from the carrier warthog. These and other aspects of pathological states can be reviewed in Smith et al. (1972). Embryonic loss is common in pigs, perhaps due to crowding. Geisert (1998) suggested that it may be as frequent as 10-20% of embryos that are lost. Conjoined twin fetuses have been observed, testifying to the occurrence of occasional monozygotic twinning in domestic pigs.

16) Physiological data

No physiologic studies have been carried out in this species; nevertheless, numerous veterinary studies on domestic pigs have been done and can be read in the relevant textbooks and in Geisert (1998). Fetal development and pig anatomy are detailed by Patten (1947).

17) Other resources

Cell strains are available through CRES of the San Diego Zoo www.sandiegozoo.org (CRES).

18) Other features of interest

Domestic pigs have first been cloned in 2000. It would be of interest whether the "resorbed" fetuses have chromosomal errors.

References

Berger, E.M., Leus, K. and Schwarzenberger, F.: Faecal steroid metabolites for non-invasive assessment of reproduction in common warthogs (Phacochoerus africanus), red river hogs (Potamochoerus porcus) and babirusa (Babyrousa babyrussa celebensis). Pp. 411-412, In, Proceed. Europ. Assoc. Zoo- and Wildlife Veterinarians, Heidelberg, May 8-12, 2002.

Björkman, N.: Fine structure of the fetal-maternal area of exchange in the epitheliochorial and endotheliochorial types of placentation. Acta anat. 86:1-22, 1973.

Bosma, AA., deHaan, N.A. and MacDonald, A.A.: The current status of cytogenetics of the Suidae: A review. Bongo 18:258-272, 1991.

Cell strains of many specimens are available from CRES at the San Diego Zoo: www.sandiegozoo.org.

Eckstein, P. and Zuckerman, S.: The oestrus cycle in the mammalia. Chapter 4 (pp. 226-397), in Marshall's Physiology of Reproduction, 3rd ed. A.S. Parkes, Ed. Longmans, Green and Co., London, 1956.

Geisert, R.D.: Pigs. Pp. 792-799 in Vol. III of, Encyclopedia of Reproduction, E. Knobil and J.D. Neill, eds., Academic Press, San Diego, 1998.

Grubb, P.: The Afrotropical Suids Phaecochorus, Hylochchoerus, and Potamochoerus. Chapter 4, pp. 66-75, In, Pigs, Peccaries and Hippos. IUCN, Gland, Switzerland, 1993.

Heuser, C.H.: A study of the implantation of the ovum of the pig from the stage of the bilaminar blastocyst to the completion of the fetal membranes. Contrib. Embyol. Carnegie Inst. 19:229-243, 1927.

Kingdon, J.: East African Mammals. Vol. III, part B (Large Mammals). Academic Press, N.Y. 1979.

Leister, C.W.: The wild pigs of the world. Bull. N.Y. Zool. Soc. 42:121-130, 1939.

MacDonald, A.A.: Uterine vasculature of the pregnant pig: A scanning electron microscope study. Anat. Rec. 184:689-698, 1976.

MacDonald, A.A. and Bosma, A.A.: Notes on placentation of Suina. Placenta 6:83-92, 1985).

Marczynska, B. and Pigon, H.: Somatic chromosomes of the native African pig. Cytologia 38:111-116, 1973.

Melander, Y. and Hansen-Melander, E.: Chromosome studies in African wild pigs (Suidae, Mammalia). Hereditas 92:283-289, 1980.

Naaktgeboren, C. and Zwillingberg, H.H.L.: Untersuchungen über die Auswüchse am Amnion und an der Nabelschnur bei Walen, und Huftieren, mit besonderer Berücksichtigung des europäischen Hausrindes. Acta Morphol. Neerl.-Scand. 4:31-60- 1961.

Nowak, R.M. and Paradiso, J.L.: Walker's Mammals of the World. 4th ed. Vol. II. Johns Hopkins University Press, 1983.

Patten, B.M.: The Embryology of the Pig. 2nd ed. The Blakiston Co. Philadelphia, 1931 (reprinted 1947).

Puschmann, W.: Zootierhaltung.Vol. 2 Säugetiere. Deutscher Landwirtschaftsverlag, Berlin, 1989.

Ramsey, E. M.: The Placenta. Human and Animal. Praeger, N.Y., 1982.

Smith, H.A., Jones, T.C. and Hunt, R.D.: Veterinary Pathology. Lea and Febiger, Philadelphia, 1972.

Thenius, E.: Zur Evolution und Verbreitungsgeschichte der Suidae (Artiodactyla, Mammalia). Z. Säugetierk. 35:321-342, 1970.

Vercammen, P., Seydack, A.H.W. and Oliver, W.L.R.: The bush pigs (Potamochoerus porcus and P. larvatus). Pp. 93-101, In, Pigs, Peccaries and Hippos. IUCN, Gland, Switzerland, 1993.

Watanabe, T., Hayashi, Y., Ogasawara, N. and Tomoita, T.: Polymorphism of mitochondrial DNA in pigs based on restriction endonuclease cleavage patterns. Biochem. Genet. 23:105-113, 1985.