Okapi: A giraffe-zebra hybrid?

imageOn their rumps and legs, okapis have striping like that of a zebra.

imageThe 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. Experimentation does not occur because theory says it would be useless to try.

imageOkapi tongue

imageGiraffe tongue

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) × 2 = 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. As those authors state, “okapi semen collected by electroejaculation routinely contain high numbers of non-motile and plasma membrane-damaged spermatozoa, apparently unrelated to season or the length of time since the male was housed with a breeding female.”

imageGiraffe and zebra drinking together Giraffe and zebra drinking together at Kruger Park.

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.”