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Last updated:
May 21, 2004.
Pongo pygmaeus & Pongo abelii

Order: Primates
Family: Pongidae

1) General zoological data of species

There are two species of orangutans (formerly considered to be subspecies); they differ in chromosome structure: the Bornean orangutan (Pongo pygmaeus) and the Sumatran orangutan (Pongo abeli[i]). Their separation is estimated to have occurred at 1.1 MYA and much mtDNA variation was found in Bornean and also in Sumatran orangs, indicating significant variation of both species (Warren et al., 2001; Muir et al., 2000). The question of species vs. subspecies status is variably handled, with Muir et al. (1998) considering that there is sufficient evidence for separation. In general though, many authorities now also favor species ranks.

Orangutans are severely endangered primates with difficult attempts being made to reintroduce orphaned and confiscated animals back into the wild. Rijksen (1986) gave a status report on their conservation. Another general consideration on primate conservation includes extensive discussion on orangutans (Cowlishaw & Dunbar, 2000). General considerations of orangutan evolution, etc. may also be found in Jones et al. (1995). Both species are held in numerous zoos with effective breeding colonies having been established. Since the recognition of chromosomal differences between the two forms, the hybrids have generally been prevented from breeding. Lindburg et al. (1984) provided an excellent summary of the captive reproduction (416 born in 10 years) in zoo collections and considered many primate species' breeding records.

Many studies that trace the evolutionary lineage of apes - indeed primates in general - have been undertaken. They are superbly summarized by Gagneux & Varki (2001). These authors also found a major mutational change of sialic hydroxylase that distinguishes man from apes and may be seminal for human evolution. This review also includes reference to the retrovirus envelope protein gene specifying "syncytin", the protein that causes the fusion of trophoblastic cells.

Schwartz (1984) reviewed the great similarities of orangutan morphology to those of man, much greater than those when compared with chimpanzee or gorilla. The general evolution of orangutan was reviewed by Smith & Pilbeam (1980) who believed that a terrestrial form of ancestor must have existed in the Pliocene.

2) General gestational data

The length of their gestation averages 265 days, with singleton births being the norm; occasional twins are recorded (see below). Newborns average in weight between 1,500-2,300 g (Puschmann, 1989; own observations). Sexual maturity occurs at 6-7 years. Life expectancy is 50-57 years. The maternal weight (not pregnant and at term) is around 70 kg. Males are heavier; Sumatran orangutans are smaller.

Since the recognition of chromosomal differences between the two forms, the hybrids have generally been prevented from breeding. Lindburg (1984) provided an excellent summary of the captive reproduction (416 born in 10 years) in zoo collections and considered many primate species' breeding records.
  Bornean orangutan (Pongo pygmaeus) at San Diego Zoo.
  Sumatran orangutan at San Diego Zoo.
  Hybrid orangutan female at San Diego Zoo.
3) Implantation

The menstrual cycle of orangutans is 28 - 32 days, with a small amount of menstrual bleeding occurring for a few days. The length of gestation averages 265 days (Puschmann, 1989). Birth interval is up to 4 years. Some animals eat the placenta after delivery, but this is irregular.

Attachment of the blastocyst is usually antimesometrial and the implantation is interstitial (Mossman, 1987). Although specific details are yet unknown, because of the similarities of all other reproductive features of orangutans and humans, one can expect that implantation occurs around day 6-7 after fertilization (day 20-21 of the menstrual cycle). Also, the early expansion of the blastocyst has not been witnessed other than by Strahl (1903). He had five uteri from a collection by Selenka that were extremely similar to human gestations. The embryos ranged from tiny (about second week of gestation) to 11 cm in length. The earliest implanted blastocyst was rather larger, bulging on the endometrial surface and was studded with villi. The only other significant difference he described was in the decidua basalis. It had pronounced radial folds that differed from the human specimens described to that date. Thus, the paucity (absence) of atrophied villi in the chorion laeve remains enigmatic. They are regular features of man and chimpanzee (see chapter on Bonobo). Moreover, since villi were seen by Strahl on the protruding membranes of early embryos one might expect them to have become atrophied and incorporated into the membranes.

4) General characteristics of placenta

The first detailed description of an orangutan placenta comes from Graham-Jones & Hill (1962), after Strahl (1903) had earlier seen five specimens in utero. It weighed 285 g, measured 17.5 x 15.5 x 2.65 cm and had an umbilical cord of 20 cm length, with eccentric insertion and few spirals. Soma (1983) tabulated the result of five placental examinations. One was meconium-stained, two had infarcts and chorioamnionitis was also observed. The weights and sizes provided by him are similar to ours. He showed electronmicroscopic observations of the villous surface and of the amnion. These structures were identical to human placental findings.
The most recently placenta of a term gestation weighed 190 g, measured 14.5 x 12 x 1.5 cm and had numerous infarcts (+/- 30%). The infarcts were of varying ages and probably did not contribute to the neonatal demise (1,410 g) which followed a traumatic delivery in this 43 year-old Sumatran orang utan.

The placenta of both orangutan species is a nearly round disk with membrane insertion at the edge. At times, a marginal ring (circummargination) is observed. The placenta of one species cannot be distinguished from that of the other species. The placenta may be located anterior or posterior in the uterus. The size of 14 placentas that I have examined varied between 15 and 20 cm in diameter and 2 cm in thickness. The average weight was 300 g, but there were wide variations (266-430 g). The maternal surface shows the cotyledons that irregularly subdivide the major villous districts of the fetal circulation. These cotyledons are supplied by maternal arterioles that bring blood to the intervillous space and, from the fetal side, by major stem blood vessels. Generally speaking, except for its smaller size, the orangutan placenta is very similar to the human placenta.

The amnion covers the fetal surface. It often detaches when the placenta is handled, as it is only passively pressed against the chorionic membrane beneath. It lacks all blood vessels. The chorion carries the major fetal blood vessels, of which the arteries cross over the veins. From the underside of the chorionic membrane arise the villi of the placenta. The orangutan has a hemochorial placentation with about 15-20 cotyledons in the single disk. The villi are of the "folded" type. Aside from the villous trophoblast that covers all villi seamlessly, there is extravillous trophoblast. This is similar to the cells seen in human placentas and have there been designated as "X-cells" (Benirschke & Kaufmann, 2000). Cysts form often in the center of these cellular deposits. Fibrinoid also accumulates in these cells over time and consists, in part of the "major basic protein" (MBP). This protein is similar to that in the granules of eosinophilic leukocytes, appears in large quantity within the circulation of pregnant women, but has no known function. Toward term, this fibrinoid may calcify. Calcification is generally minimal in orangutan placentas and is of no importance anyway.

Twins are not uncommon (see below). I have seen two twin placentas. The ones shown here were separate disks, the other was from a prematurely delivered set of twins and the two placentas were fused. Nevertheless, the "dividing membranes" (between the two sacs) were composed of two amnions and two chorions. This arrangement does not permit one to make the diagnosis of zygosity. Indeed, I am not aware of any zygosity ascertainment of orangutan twins.
  Mature twin placentas of hybrid orangutan neonates. There are two separate disks, fetal side (left), and maternal side (below left).
  Mature twin placentas of hybrid orangutan neonates. Note the subdivision of the villous tissue into lobes ("cotyledons").
5) Details of barrier structure

The architecture of the villous surface is exactly like that in other apes and in human placentas. The syncytiotrophoblast is outermost, and the cytotrophoblast lies beneath the syncytium. It provides the new cell divisions and their incorporation into the syncytium is a continuous process. This is a typical villous hemochorial placenta that is virtually indistinguishable from human or pongid placentas of other species.
  Structure of terminal (tertiary) villi of orangutan placenta. F.V.=fetal vessels. A few cytotrophoblastic cells are seen beneath the syncytium in the large villus at right.
6) Umbilical cord

The umbilical cord of orangutans, like that of the gorilla, is unusually long (Naaktgeboren & Wagtendonk, 1966), certainly longer that the normal human cord. Spatz (1968) attempted to find a correlation of the length of the umbilical cord with body length and weight of the newborn. He speculated that the length may have evolved to allow nursing in arboreal species. I believe that cord length better correlates with fetal movements in utero. There are two arteries and one vein in the umbilical cord of orangutans. Occasionally, the remnant of an obliterated allantoic duct is found in the center between the two umbilical arteries, within Wharton's jelly. The arteries do not possess elastic fibers. Graham-Jones & Hill (1962) injected the cord vessels of the orangutan placenta which they described and showed that, as in humans, there was a communication between the two arteries close to the placental insertion. This has been referred to as "Hyrtl anastomosis". It occurs as a general rule in most human placentas. The cord inserts anywhere on the disk, commonly near the center. It measures between 55 and 72 cm in length in the mature placentas I have examined, and it has few twists in either direction. There is no vitelline duct at term. The cord's surface is a flat squamous amnionic epithelium. Normally, there are no remnants of omphalomesenteric (vitelline) duct or vessels.

Heinrichs and Dillingham (1970) described the cords of fraternal twins (who survived) as being 40 and 45 cm long; they were growth-discordant, with the smaller female twin having the shorter cord. This twin also had a true, tight knot.

The umbilical cord of the most recent specimen obtained was 54.5 cm long, had few spirals, was nearly marginally inserted and contained three vessels. It is shown next.

Cross section of umbilical cord with thee blood vessels.

  Portion of orangutan umbilical cord.
  The fetal surface vessels in oranges are similar to those of humans. They have no elastica and are difficult to differentiate. Their muscular walls are also eccentric, as in this artery. This is due to the release of amnionic pressure at delivery with "bulging" (herniation) of the upper portion into the former cavity.
7) Uteroplacental circulation

The circulation presumably is essentially the same as that found in the human placental circulation. Maternal spiral arterioles inject blood into the intervillous space. Here it is dissipated through the villous meshwork and returned, after deflection under the chorionic surface, to leave the placenta via maternal veins in the decidual floor. Although the intervillous space in which maternal blood circulates may appear to be wide and spacious, this is the result of fixation shrinkage artifact of the villi. Electronmicroscopic observations in human placentas have shown clearly that this space is more or less sinusoidal and that the villi approach each other closely. The syncytiotrophoblastic surface has a dense microvillous border for optimal exchange. Electronmicrographic observations on such primate villous surfaces have been produced, i.a. by Panigel (1968). This author has also attempted dual perfusion of primate cotyledons.

The circulation of primate placentas has best been studied by Ramsey and her colleagues (1963, 1966). Nevertheless, most studies were done on rhesus monkeys, and only some on human placentas; none have been performed in orangutans. Therefore, little is as yet known about the endometrial spiral arterioles of orangutans, whose modification in human pregnancy plays an important physiological role for maximal perfusion of the intervillous space.

The fetal heart perfuses the villous stems from the umbilical arteries and accomplishes the fetal circulation of the entire villous ramifications, the tertiary villi being the principal exchange structures.
  Fetal blood vessels in various-sized villi. The latter are surrounded by maternal blood in the intervillous space (I.V.S.)
8) Extraplacental membranes

The membranes insert at the edge of the placental disk and are composed of an inner amnion that applies loosely to the chorion; beneath this is a layer of extravillous trophoblast. This is connected to a thick layer of decidua capsularis. The free membranes are usually referred to as "chorion laeve". In contrast to human and chimpanzee placentas, atrophic villi are not found in the free membranes. Some degree of decidual necrosis is frequent and there may also be some inflammation. There are no remnants of allantoic or vitelline structures.
  A thick decidua capsularis is present on the "chorion leave". It contains maternal blood vessels, but no atrophied villi.
9) Trophoblast external to barrier

Extravillous trophoblast ("X-cells") infiltrates the decidua basalis. Perhaps these cells reach the myometrium, as is the case in human gestations, but that has not been verified for orangutans. Also, the maternal arterioles that are densely infiltrated by X-cells and whose walls are modified in human gestation, show few changes in the orangutan placentas that I have examined. This modification of spiral arterioles is held to be most important for human placental perfusion and it is defective in pre-eclampsia ("toxemia of pregnancy"). The X-cells often fuse in the decidual floor and make some attempt at giant cell formation. Although Strahl (1903) had some implantation placental material available from five young gestations, he merely described intermingling of trophoblast with the decidua and did not refer to infiltration into the myometrium. He expressed surprise that there was no fibrinoid at the site of implantation.
  Thrombosis of maternal vessels with adjacent decidual necrosis are frequent features and of no major importance. The decidua basalis of the "floor" is to the left.
  Cystic "degeneration" in extravillous trophoblast and fibrinoid in placental floor (left).
  This is the maternal "floor" of the orangutan placenta. The pink band represents Nitabuch's and Rohr's fibrinoid layers, products of the "X-cells". This extravillous trophoblast invades the decidua basalis and originates from the anchoring villi of young placentas. (I.V.S. = intervillous space).
10) Endometrium

There is typical decidualization of the endometrium during pregnancy, similar to that in other apes and women.

11) Various features

There is no subplacenta and metrial glands are not present. There are, however, remnants of the maternal endometrial glands found in the decidua basalis that may contain inspissated secretion.

12) Endocrinology

Large amounts of estrogens are secreted during pregnancy. The estrogens come most likely from the placental aromatization of fetal adrenal androgen precursors (Czekala et al., 1983; Pepe and Albrecht, 1995). Chorionic gonadotropin is present in pregnancy and can be used for pregnancy diagnosis. It is produced by the syncytiotrophoblast.
  Cross-section of a portion of a fetal adrenal gland. As in human gestations, there is a wide "fetal cortex" and very narrow "definitive zone". The fetal zone produces the androgen precursors for placental aromatization to estrogens.
  Table from Czekala et al. (1983) showing the total estrogen excretion of various apes in pregnancy. That of the orangutan is similar to the human gestation.
Orangutan infants display little sexual dimorphism and sex identification may be difficult. Testes descend only after dentition is completed. Among adult males, some animals exist that seem to be "subadult" and are described as "arrested males". They have lower T, DHT, LH levels than those displaying full secondary sexual characteristics (Maggioncalda et al., 1999). The have normal FSH levels and may be fertile.
Corticotropin-releasing hormone (CRH) and circulating corticotropin-releasing hormone-binding protein (CRH-BP) are found in orangutans, as it is in all apes (Bowman et al., 2001). The decrease of CRH-BP in late gestation has been linked to parturition.

13) Genetics

Orangutans have 48 chromosomes; they have been intensively studied by many investigators (Dutrillaux et al., 1975; Hamerton et al., 1963; Turleau et al., 1972; others). There are specific differences between several chromosomes of Bornean vs. Sumatran orangutans (Seuánez, 1976, 1978, 1982, 1986, de Boer& Seuánez, 1982). It is especially the pericentric inversion of segments of chromosomal arms in chromosome 2 that has allowed the rapid determination of hybrids (of which there are many in captivity). Their identification by cytogenetic study has allowed zoos to preclude their future reproduction. Ryder & Chemnick (1993) thus studied 144 captive animals in zoos; they not only identified the hybrids but also found differences in the mtDNA of these species. Hybrids with other apes have not been recorded.

Xu & Arnason (1996) studied the mitochondrial DNA of both species and found them to be remarkably different, thus further justifying species separation. Other mtDNA studies have been referred to in section #1 above. Additional mtDNA studies come from the laboratories of Kenyon & Moraes (1997).

Seuánez (1986) reported on initial studies of genes and evolutionary relations, topics that are beyond the scope of this book.

Trisomy of chromosome 22 (equivalent to human 21) that resulted in Down's syndrome was reported in a 7 year-old female Sumatran orangutan by Andrle et al. (1979). The parents had been 12 and 13 years old at conception, and she had numerous normal siblings.

14) Immunology

No immunologic studies on orangutans are known to me.

15) Pathological features

Infarcts of the placenta are frequent and I have seen a placenta with 70% infarction and a stillborn baby. The fetus associated with this placenta also had its cord entangled around the neck. Furthermore, there was abruptio placentae (retroplacental - maternal - bleeding). The mother had much proteinuria during pregnancy and may have had a form of pre-eclampsia from which she succumbed. A fatal case of placenta previa was published by Kingsley & Martin (1979). The female bled around the time of expected delivery, stopped bleeding but was found dead 11 days later, having succumbed from extensive hemorrhage. The placenta was a marginal placenta previa and the fetus was in breech position. The most recently observed organ-utan placenta had 30-40% of its villous tissue infracted. The infarcts were of different ages and there was, again, no maternal decidual vascular disease to explain the infarcts.

Chorioamnionitis due to ascending infection from the endocervix was the cause of the next-shown baby's premature delivery (half-way through gestation) and its neonatal demise. In this condition, maternal leukocytes migrate from the decidua capsularis (of the membranes) and from the intervillous space under the fetal placental surface towards the amnionic cavity. This is a frequent cause of premature delivery in human pregnancies. Eventually, as in this case, the fetal circulation participates in the inflammatory process (surface vessels and cord vessels - "funisitis") (Benirschke, 1983).

The hybrid female shown above had a large atrial cardiac septal defect that was surgically repaired (Greenberg et al., 1999). Other anomalies are rarely reported.

  Deciduitis and chorioamnionitis in the membranes of a prematurely delivered orangutan fetus.
  Deciduitis and chorioamnionitis in the membranes of a prematurely delivered orangutan fetus.
Infarcts of the placenta may be massive and cause fetal demise. Whether they are produced in a manner similar to those in human pregnancy is unknown. In human gestations, such massive infarcts as shown next are usually the result of "toxemia of pregnancy" (Pre-eclampsia).
  Excessive amount of infarction in the placenta led to fetal demise of this oranutan. Indicated by "I" are the numerous infarcts, comprising at least 60% of the villous tissue.
  This shows the necrotic, infarcted villous tissue at the placental floor.
A variety of infectious diseases (pneumonia, nematodes) has been reported (Scott, 1992). Diverticulitis, with fatal peritonitis, was recorded by Murray et al. (2000) to have occurred in a 30 year-old Sumatran orangutan. Miyagi et al. (1999) described death of a 37 year-old animal due to Coxsackie B4 myocarditis. Griner (1983) described the post partum death of an orangutan due to "toxemia of pregnancy" (renal necrosis, proteinuria - same case as referred to above). Lowenstine (1986) referred to a mast cell tumor of the eyelid, an ovarian granulosa cell tumor (with endometriosis), uterine leiomyomas in orangutans, and an esophageal squamous cell carcinoma in a 35 year-old animal. Generally speaking though, neoplasms are uncommon.

16) Physiological data

No relevant studies have been reported on orangutans.

17) Other resources

Cell strains of numerous animals of both species and their hybrids are available from CRES at San Diego Zoo.

18) Other relevant features

Twins are occasionally observed, and they may be delivered at term to grow up, as did those associated with the twin placentas shown above. Geissmann (1990) found a 1.1% twinning frequency (7 in 626 births) in orangutans, not much different from other apes and man. No specific studies as to zygosity have been reported. Fraternal twins (M/F) with a fused, diamnionic dichorionic twin placenta were described by Heinrichs & Dillingham (1970). The male weighed approximately 1,500 g, the female was only about 900 g; they did well being hand-reared. The umbilical cords were 40 and 45 cm long, with the former having a tight knot, that which belonged to the smaller, female infant.

Schroeder et al. (1978) reported on the nature of fetal hemoglobin chains of orangutans.


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