“The rule is perfect: In all matters of opinion, our adversaries are insane.”
|On their rumps and legs, okapis have striping like that of a zebra.|
|The okapi has a giraffe-like head.|
The okapi (Okapia johnstoni), also known as the forest giraffe or zebra giraffe, is a good example of the way theory affects both perception and research activities. Okapis mix traits otherwise seen only separately either in the giraffe or in the zebra, and under ordinary circumstances this fact would constitute strong evidence that okapis are giraffe-zebra hybrids. Moreover, old news reports say that peoples native to the region where the okapi occurs believe it to be a giraffe-zebra hybrid.
And yet it seems no scientist has investigated this possibility. Why? Well, in trees representing accepted notions of evolutionary descent, giraffes and zebras are placed on widely separate branches. So any biologist worth his or her salt will tell you: The two are simply too far apart for giraffe-zebra hybrids to be possible! Thus, it is not surprising that there are no reports of attempts to produce giraffe-zebra hybrids in captivity, or to produce them via artificial insemination. Experimentation does not occur because theory says it would be useless to try.
The okapi and the giraffe were assigned to the same order (Artiodactyla) because they both have cloven hooves, and to the same family (Giraffidae) because they share certain distinctive features: Both have large eyes and ears, thin lips and a long, extensible tongue that allows them to lick their entire face (even the ears); their backs slope upward from rump to withers; they also share the same dental formula: ( i 0/3, c 0/1, pm 3/3, m 3/3) × 3 = 32. Both, unlike any other mammal, have molars with rugose enamel and bony horns that remain covered with skin throughout life (Nowak 1999, vol. 2, p. 1085).
Yet the rump and legs of an okapi are covered with black-and-white stripes exactly like those of a zebra. Perhaps, then, if okapis had solid hooves instead of cloven ones, they would be classified as perissodactyls (Order Perissodactyla) and would be considered more closely related to zebras than to giraffes. An okapi is about the same size as a Burchell’s zebra.
The chromosome count of an okapi is also like that of a zebra, to which it is not supposed to be related, and unlike that of a giraffe. Giraffes have 30 chromosomes (Taylor et al. 1967; Hösli and Lang 1970; Koulisher et al. 1971), whereas okapis have a variable chromosome number of 44-46, depending on the animal in question; most seem to have 2n = 45 (Ulbrich and Schmitt 1969; Hösli and Lang 1970; Koulisher 1978). The chromosome number of Grevy’s zebra is 2n = 46 and plains zebras have 2n = 44 (Benirschke and Malouf 1967). Variation in chromosome count is itself unusual among mammals, but common in hybrids.
Okapis also produce high levels of abnormal sperm, which is consistent with the idea that they are the products of a distant hybrid cross. Thus, Penfold (2007) reports that 52% percent of the spermatozoa produced by these animals are morphologically abnormal.
It is, of course, well known that giraffes and zebras exist in mixed herds in various parts of Africa, and therefore are in potential breeding contact (these regions include those where okapis occur).
However, zebras are much smaller than giraffes, which might lead one to suppose that they would be physically unable to mate. And yet, hybrids sometimes occur between animals where the disparity in size is even greater. Male Steller sea-lions (Eumetopias jubatus) often mate, and sometimes even successfully hybridize with female California sea-lions. And yet the former average around 1100 lbs while the latter weigh only around 200 lbs., a ratio of 5.5:1 (the female often dies in such encounters) Such cases are nothing unusual in the literature on hybridization. Florio (1983) reports a case of a lion father who weighed 550 pounds (250 kg), while the leopard mother weighed a mere 84 pounds (38 kg), a ratio of 6.54:1.
In the case of a male giraffe 2,628 pounds (1,192 kg) with a female zebra, 770 pounds (350 kg),the weight ratio is only 3.4:1, that is, the difference is less disparate than in either of the two crosses just mentioned.
And this difference would be even smaller with the cross reversed, that is, with a female giraffe and a male zebra. Giraffe females weigh nearly a thousand pounds less than males, while zebra males weigh a bit more than females, which would yield a ratio closer to 2:1, not at all unusual in a hybrid cross. Moreover, giraffes do sometimes lie down, and a male zebra would, of course, have much better access to a recumbent female giraffe.
A final fact consistent with the idea that okapis might be giraffe-zebra hybrids is their rarity at the present day and their absence from the fossil record. Hamilton (1977) says that while giraffes are well-known as fossils, paleontologists have seen no trace of okapis. Zebras, too, are known from fossils (Eisenmann 1992). The IUCN rates the okapi as endangered, although it also states that “there is no reliable estimate of current population size.”)
Comments on the Internet are frequent to the effect that okapis look like a giraffe-zebra cross. And people have several times written me saying that okapis are obvious giraffe-zebra hybrids. This speculation is due to okapis combining, as I have mentioned, the traits of giraffes and zebras. And this belief has been widespread ever since the okapi first became known. Writing in the first years of the twentieth century, only four years after the discovery of the okapi, the British naturalist Richard Lydekker (1904, p. 474), commented that “the absurd idea that the okapi
So Lydekker is implying that if the okapi were a hybrid, multiple specimens would not be known. But he was writing at time when very little was known about natural hybrids and it was widely imagined that natural hybrids never occurred. Today a wide variety of natural hybrids, from many thousands of different crosses, are known.
Sir E. Ray Lankester (1905, pp. 163-165), Director of the Natural History Departments of the British Museum, concurred with Lydekker’s opinion that the okapi could not be a giraffe-zebra hybrid: “As they live in these immense dark gloomy and damp forests they are very difficult to shoot or to catch, and moreover
Today, Lankester’s assertion that “It is a fact that mules or hybrids never are produced by animals living in their natural conditions” seems laughably inaccurate. It is now well known that thousands of crosses occur naturally among birds, mammals and a wide variety of other animals.
And as to his allegation that “no one has ever produced, even in captivity, a hybrid between any creatures so unlike each other as a double-hoofed and a single-hoofed mammal,” there is nothing in the structure itself of hooves that prevents interbreeding. After all, there are breeds of pig (“mulefoot pigs”) that have a solid hoof, like that of a horse or zebra , and yet they can mate freely with ordinary cloven-hoofed pigs.
However, in the minds of many people who raise objections on this difference of single and double hoof, the key issue seems to be that this is the separating factor between Order Perissodactyla (odd-toed ungulates) and Order Artiodactyla (even-toed ungulates), two major divisions of Class Mammalia. Zebras belong to the former of these two orders and giraffes to the latter. True, this is an old distinction in taxonomy, but it is unclear why two animals’ differing in this way should necessarily ban them from producing hybrids. If the okapi were such a hybrid, then obviously hybrids between different orders would in fact be possible. But only experimentation can determine the veracity of such a conclusion, not arm-chair speculation.
And it is not as if there were no evidence of interordinal crosses among animals occurring. Actually, there’s a great deal. It’s just that not everyone accepts that evidence. Among birds, there are numerous reports alleging the occurrence of such crosses:
to name a few. Among mammals we have:
Using artificial insemination, Newman (1915, 1917 and 1918) obtained interordinal hybrids from various fish crosses. And there is even seemingly good reason to suspect that the echidnas and the platypus are derived from crosses between mammals and birds. Margulis et al. (2011, pp. 188-194) list various probable interphylum crosses, which would be even more distant than any of the crosses just mentioned. And what do we have against all this evidence? Seemingly only a lot of preconceptions about what’s possible and what’s not.
And as has already been mentioned, even today on the internet, one still encounters allegations that the okapi must be a giraffe-zebra hybrid. And not everyone agreed with Lankester even in his own day. As he later wrote (Lankester 1920, pp. 132-133), “A very eminent person whom I was conducting some years ago round the
For my own part, I’m suspicious of the dogmatic thinking I’ve encountered on this topic. It’s my hope that the matter will be investigated further and that we will eventually be able to determine the true nature of the okapi’s origins. For example, it would be interesting to see the results of repeated artificial insemination of female giraffes with zebra sperm, and vice versa. Genomic analysis would also be of interest.
By the same author: Handbook of Avian Hybrids of the World, Oxford University Press (2006).
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On the Origins of New Forms of Life
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Prothero: A Rebuttal
Branches of Biology