The Hybrid Hypothesis

A new theory of human origins


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Appendix - Infertility in the gorilla

Martin et al. measured dimensions (head length, head width, midpiece length, principal piece length, and total length) of spermatozoa in a variety of primates. Their results indicate that, when compared with other primates, the spermatozoa of humans and gorillas are about ten times more variable in form (334.5,Table 1).

It is now well established that gorillas suffer from infertility far more often than is the case in other mammals. Thus, Martin et al. found that in the gorilla

Spermatozoa are markedly pleomorphic [i.e., variable in form] within any individual semen sample … Other pongid species, and indeed other anthropoids examined are remarkably uniform in size and appearance."1

The two samples examined by Seuánez et al. were both taken from individuals that had fathered offspring. The semen examined may therefore have been of higher quality than that of the typical gorilla. Nevertheless, they found that 27 percent of the human, and 29 percent of the gorilla, spermatozoa were abnormal in some way or another (if not in shape, then at least with respect to other features, such as staining, cytoplasmic features, maturity, etc.).
McKenny et al.(1944) reported that smears from the testes of a 15-year-old captive gorilla showed no spermatozoa (328.7). Brandt (1972) records another case in which a gorilla failed to produce spermatozoa (81.6).
Seuánez et al. found that 16 percent of the spermatozoa in gorilla semen are abnormal in shape (this was in comparison with 18.3 percent in humans).2 This study involved only two gorillas, but Gould and Kling3(1982) examined semen from a much larger sample and found that four of the 15 gorillas examined produced no sperm at all. In the remaining 11 individuals, an even higher proportion of abnormal sperm was evident than that reported by Seuánez and his colleagues (the exact percentages of abnormal spermatozoa reported were: 60, 40, 45, 60, 60, 40, 51, 90, 85, 80, and 30). A wild-caught gorilla examined by Platz et al.4 showed 92.5 percent abnormal sperm. Many of these spermatozoa had abnormally shaped nuclei. Raphael et al. (1989) evaluated four male gorillas and found, in all four cases, that semen volumes were low, that 85 to 95 percent of the spermatozoa were abnormal, and that few or none of the spermatozoa actually present were motile.5

If anything, gorillas are even less fertile than human beings. They commonly suffer from a condition known as testicular atrophy (or hypoplasia), which is seen in many interspecific hybrids. "Indeed, testicular hypoplasia and infertility is found throughout the literature on adult gorillas. Most often an aetiological [i.e., causative] factor for this infertility remains unknown" (Benirschke and Adams6). This condition has been consistently observed, not just in captive specimens, but in free-living individuals as well. Although various workers who have reported testicular atrophy in gorillas have deemed it an abnormal or pathological state, it seems actually to be characteristic of gorillas because all reports describing gorilla testes in any detail concur in the observation of atrophy. As early as 1937, Koch7 observed that the seminiferous tubules of a gorilla, Bobby, then residing at the Berlin Zoo, were atrophied and sclerotic. Wislocki8 (1942) compared the testes of a wild-shot gorilla with those of a chimpanzee, stating they were

from a mountain gorilla conservatively estimated to have weighed 450 lbs and to have been fully adult. The testes were exceedingly small, the pair weighing 36 grams … In the chimpanzee the seminiferous tubules are large and densely filled with maturing stages of the germinal cells, including spermatids and spermatozoa. In the gorilla, on the contrary, the tubules are few and quite small and are widely separated from one another by fields of stroma and interstitial cells. Nevertheless the tubules are active and contain both spermatids and spermatozoa. The presence of these cells indicates that the gorilla's testes in question are functionally active. The testes may not have reached their maximum growth or full activity, however, for in that event one would have anticipated more abundant spermatogenesis than is actually present." [italics added]
In general, Steiner observes, "Small external genitalia appear to be a constant finding in the gorilla, both wild and captive."
In Steiner et al. stated (538.4,5) that "no male [gorilla] in captivity is known to have reached sexual maturity, although the other three species of anthropoid apes have reproduced under similar living conditions." Of course, some captive gorillas have exhibited an ability to breed since this statement was made (1955). But even in 1981, Dixson (140.1) noted that "only three of the thirteen male gorillas in British collections are of proven breeding ability." The infertility of captive gorillas is such a problem that even expensive technical procedures such as in-vitro fertilization and gamete intrafallopian transfer have been deemed worthwhile (291.4; 322.2), and captive populations are still not self-sustaining (186.6; 456.7).

Another case of testicular atrophy, this time in a captive gorilla, was reported four years later by Clarke.9 In 1954, Steiner dissected the corpse of a lowland gorilla. Although the animal was well-nourished and physically mature, Steiner found that "the reproductive tract was not only underdeveloped, but abnormal."10 He observed that the "testes were highly abnormal, showing a severe atrophy and sclerosis of the seminiferous tubules" and that the seminal vesicles contained no spermatozoa.11 In a separate paper on the same individual, Steiner et al. (1955) found that "Microscopically, the testes showed a virtually complete atrophy and sclerosis of the seminiferous tubules and a great relative, if not actual, increase in the number of interstitial cells of Leydig in all areas; an unquestionable increase, represented by large solid sheets, was present in many places."12

In 1961 Inoue et al. examined a wild-caught 40-year-old male and stated that "both the penis and the testicles are remarkably small — the latter are sparrow's-egg-sized."13 In the same year Hosakawa commented on the abundance of interstitial tissue (which produces no sperm) present in the testicles of a 40-year-old male caught in the wild the year before.14 The next year Hall-Craggs15 examined the testicles of two more wild gorillas, an adolescent and an adult. He found that the testicles were very small, that they contained large masses of interstitial cells separating the seminiferous tubules, and that the epididymis contained no spermatozoa. He further states that "The large proportion of testicular interstitial tissue found in captive animals is present in the two free gorillas, and is therefore a normal finding" (italics added). An additional case of testicular atrophy was reported by Antonius et al.16 These authors state that

The testicles described by Antonius et al. were same size as those of another captive gorilla, "Samson," examined by Short (509.15,8).
Each of the testes were about 3.8 cm in length … Microscopic examination revealed that one testis was slightly more atrophic than the other. Interstitial cells occupied about half the bulk of both testes … Although some tubules contained a few spermatogonia [i.e., immature germ cells], in most there was no spermatogenesis. Those free of spermatogonia were lined with Sertoli's cells alone.

Recently, Munson and Montali (1990) reported yet another case of testicular "degeneration" in a gorilla.17 Moreover, in referring to available records at Washington's National Zoological Park, these authors stated that in both males and females "infertility was a major problem in all gorillas."18 Böer (1983) also reported "severe testicular atrophy" in a captive gorilla.19 Foster and Rowley (1982) performed testicular biopsies on six wild-caught gorillas and found that spermatozoa were absent in all six.20 Dixson (1981) examined the testicles of three gorillas, two orangutans, and one chimpanzee. He found that

Although Steiner et al. (538.4) suggested testicular atrophy may be due to malnutrition, Dixson and his colleagues (140.8) countered this supposition by noting that the most normal of the three sets of gorilla testicles they had examined belonged to "Oban" who was extremely malnourished after suffering from a gastrointestinal disorder for several years.
the testes from the orang-utans and chimpanzee were apparently normal whereas those from the gorillas exhibited varying degrees of atrophy. In 'Guy', a silverback which had lived for over thirty years at the London Zoo, the testes were extremely small, measuring 27 X 18 mm (right) and 28 X 19 mm (left). The corresponding weights including the atrophic epididymes were only 6.85 grammes and 4.4 grammes respectively. The entire genital tract of this male was removed, yet it was not possible to identify with certainty either the prostate or seminal vesicles. Sections of the testes exhibited a diffuse nodular appearance, the seminiferous tubules were totally degenerate and fibrosed and none of the stages of spermatogenesis could be identified. Fine structural observations, using the electron microscope, revealed that the Leydig cells, which normally produce testosterone, were non-functional. 'Jojo', a much younger male which had died at Chester Zoo, exhibited a similar testicular picture. In the third gorilla, 'Oban' from the Yerkes Regional Primate Research Center, the testes had a more normal appearance. The seminiferous tubules were small and surrounded by large amounts of interstitial tissue, but this is not an indication of abnormality, since both Wislocki and Hall-Craggs have described these features in material from wild-shot gorillas. However, in 'Oban' sloughing and degeneration of the germinal epithelium was apparent and mature spermatozoa could not be identified in either the seminiferous tubules or epididymis. The causes of testicular atrophy in captive gorillas are not known.21

Descriptions of testicular atrophy in gorillas are peculiarly reminiscent of accounts describing the testicles of the common mule and of other sterile, or partially sterile, hybrids. In comparing the testes of the mule with those of the horse, Makino22 found that in the mule "the seminiferous tubules of the testes show a remarkable decrease in diameter and contain a very small number of germ cells. In the majority of tubules, only one or two layers of such germ cells line their inner wall, and most of the cells found there are spermatogonia [i.e., germ-cell precursors]. The tubular lumina [i.e., the interior of the seminiferous tubules] are empty, or sometimes contain a mass of degenerating cell debris. In striking contrast to the shrinkage of the seminiferous tubules, the interstitial tissue shows a remarkable development between the widely separated tubules." Other researchers paint a similar picture of hybrid testes, in which seminiferous tubules are small, the interstitial tissue overdeveloped, and the germ cells degenerate.23


[1]. (334.5, Legend to Plate 3)

[2]. (507.2,345)

[3]. (206.9)

[4]. (432.7)

[5]. (452.45)

[6]. (67.4,144)

[7]. (271.6)

[8]. (602.65,286-287)

[9]. (104.2)

[10]. (537.3,146)

[11]. (537.3,156-157)

[12]. (538.4,7)

[13]. (249.3,33)

[14]. (242.5)

[15]. (230.2)

[16]. (29.1)

[17]. (393.3,102)

[18]. (393.3,102)

[19]. (73.8)

[20]. (187.6,122)

[21]. (140.1,140-141); See also (140.8).

[22]. (330.5,225)

[23]. (57.6; 64.2; 64.5; 223.45; 242.3; 252.4; 269.98,518; 318.6,500; 330.7; 444.1; 539.1; 539.2,892; 539.22; 540.9; 542.3,360; 596.3)

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