The Quagga,
Equus quagga quagga, got some
attention during the last decade because of a selective breeding project in
South Africa that tries to “rebreed” the appearance of this Plains zebra
subspecies. The Quagga project is different from those concerning the aurochs or wild
horse, particularly because there are no living descendants of the Quagga to
work on. The identity and evolutional history of the Quagga has been a matter
of debate for a considerable time, but let’s have a look at the history of the equines
referred to as zebras as a whole, because we have to look at the complete
picture in order to understand it better.
Surprisingly,
a study by Orlando et al. 2009 [1] found zebras to be a polyphyletic
arrangement, with the Mountain Zebra E. zebra being more closely related to the
African ass E. asinus, the Grevy’s
zebra E. grevi within hemionine asses
and the Plains zebra as an outgroup to these two clades. According to this
study, zebras are an umbrella term for heavily striped wild asses, if you
will. But the paper itself states these conclusions are to be taken with
caution, and mtDNA analyses might be necessary for better resolving the
non-caballine Equus tree. Another
2009 study finds zebras to be monophyletic, but oddly the onager outside E. hemionus [2].
The
phylogeny of zebras is essential for understanding the evolutional cause of their striped coats, in order to deduce which coat pattern is a plesiomorphy
and which is an apomorphy. If the plesiomorphic state of the clade formed by
zebras and their most recent common ancestor is a bay dun coat colour, it means
that the striped zebra coat pattern evolved several times independently within
Africa (E. simplicidens from the Pleistocene of America is often claimed to be
a zebra-like animal, but I think this is highly speculative based on skeletal
morphology alone). If the plesiomorphic state is the striped coat, it must have
been lost several times again and re-developed into a bay dun coat of the asses
nested within that group according to Orlando et al. 2009. Several hypotheses
try to explain why zebras developed their unique coat, and if it evolved several times independently within Africa, a connection to a special
environmental condition is very likely. The hypothesis that it evolved in order
to remain undetected for tsetse flies and horseflies is the most popular one,
and indeed it has been shown that polarized striped patterns is less attractive
to tabanid flies [3]. According to the hypothesis, zebras needed this
protection because their relatively recent arrival on the African continent did
not give them the time to evolve efficient immune defence against diseases transmitted
by these parasites. And that is how we finally come to the Quagga: far in the
south, where the tsetse fly is not present, the stripe pattern allegedly got
useless and therefore these zebras reduced it again. But I have several
problems with this scenario. First of all, there is no real correlation between
the range of the tsetse flies and that of the zebras. The modern range of the Mountain zebra’s range
is totally outside (and it actually occurs as southwards as the Quagga did), as
is a large part of that of the Plains zebra, and I wouldn’t be surprised if the
former range of the African wild ass and that of the tsetse fly also
overlapped. Furthermore, I don’t know of studies suggesting that zebras are more
vulnerable to trypanosomiasis than other African herbivores, making the passive
protection through their striped coat necessary. I think the “fly hypothesis”
is plausible, but not yet really convincing for me (please correct me if my
criticism is erroneous).
Regardless
of why the Quagga had reduced stripes, they were its most remarkable feature
(there is the possibility that this condition actually is a basal one, but I’ll
address that later on). Sometimes the quagga is described as looking like half
zebra and half horse, but I don’t think that is an ideal portrayal of its
appearance. I would rather describe it as a brown zebra with white legs and
tail and stripes fading away at the posterior half of its body. On its
brownish rear, the Quagga displayed an eel stripe just like phenotypically bay
dun wild equines. Most stuffed specimen show brownish stripes, but that’s
likely due to discolouration because all of them are more than hundred years
old. Based on contemporaneous paintings and photographs of one individual, I
think that the lighter space between the stripes on the neck and head area were
white or whitish, in some individuals maybe brownish. With this (baaadly rendered) drawing, which
is also featured on the Quagga Project’s website, I tried to portray the individual variation within the Quagga:
The preserved specimen of the Quagga, which are all listed here, might give you a good idea of the variation within the population as well. Considering
the unique pelage characteristics and also alleged craniometric differences between the
Quagga and the Plains zebra [4]*, it is understandable that it has been
regarded as a separate species for a long time. However, genetic studies found
the Quagga to cluster within the Plains zebra (thus indicating that its
ancestors were fully striped) and having little genetic diversity. It is likely
that the Quagga split from other lineages between 290.000 and 120.000 years
ago, possibly due reproductive isolation during the last glacial, what also
provides a possible scenario for the radically different pelage colour (genetic
drift) [3]. Most importantly, the Quagga does not share any haplotypes with
other Plains zebras according to this study, indicating that there was no
genetic exchange after their isolation [4]. This means that the Quagga is a
genetically distinct subspecies rather than a mere colour variant, although
more material might be needed.
* Some have
disputed the craniometric differences because available horse or donkey skulls
were used for some mounted zebra skins instead of genuine skulls. However, I
know that there is at least one preserved Quagga skull (on the only mounted
Quagga skeleton); I wonder which material those cranimetric comparisons are
based on. If you want to judge it for yourself, here is the confirmed Quagga
skeleton with the skull in profile view, and a Plains zebra skull.
However,
the Quagga’s outer appearance was not homogeneous, there actually was
considerable variation concerning the degree of striping among the mounted Quagga
skins. Some museum specimen show a very brownish coat with stripes only
recognizable at the shoulder, neck and head area, others display stripes that
go right to the hips and a comparably light background colouration (as far as
we can tell from those skins). Interestingly, the living plains zebras also
show a degree of striping varying from north to south, with those in the north
having the most polarized stripe pattern (E.
q. boehmi) and those in the south showing a tendency to reduced striping on
the rear and legs (E. q. burchelli). Concerning the pelage characteristics, the
quagga might represent the end of a cline, although it seems to be genetically distinct.
Because of
the obvious variation in modern Burchell’s Zebras striping, Lutz Heck proposed that selective breeding could rebreed the Quagga’s
outer appearance (phenotype) in a book published in 1955. Eventually a project with this aim was formed by
zoologists, veterinarians and museum personnel under the initiative of Reinhold
Rau in 1987 [4]. They selected wild zebras, mostly from the Etosha national
park and exclusively from the burchelli
subspecies that already showed a promising-looking amount of stripe-reduction. The
current herd has 83 of zebras resulting from this selective breeding programme
living in several locations near Cape Town. The youngest individuals are from
the fourth generation. Keep in mind that zebras have a longer generation span
than domestic horses so breeding with them takes longer. The project selected
for a reduced striping and a more brownish background colour by crossing the
chosen wild zebras and choosing the offspring with the least striping for
further breeding. Surprisingly, some animals also showed stronger striping, but
those were selected out. The breeding progressed surprisingly fast, already the third
generation showd considerable reduction of stripes on the legs and rear, a
white tail and a slightly more brownish background colour. The fourth
generation (some of them were born in 2012) show an even greater approximation
towards the Quagga’s pelage characteristics. The project developed a counting
system in order to quantify the progress, and the results show that the
reduction of striping is happening fast, but the darkening of the background
colour progresses only slowly (this aspect is especially hard to quantify
because of dust, sun et cetera). Here are some photos and a video of some of
these “Rau zebras”**:
Individual "Freddy", F4 |
Individual "Henry", F3 |
Individual FM12, F4 |
** Funnily,
I independently came up with the idea of calling these animals “Rau zebra”, the
Quagga Project itself wants to name their results “Rau Quagga”, but I think
calling them zebras is more objective.
If you are
interesting in seeing more photos of these Rau Zebras, you can have a look at
the homepage www.quaggaproject.com
or the project’s facebook page. In contrast to aurochs or wild horse projects,
where all desired phenotypic features are present in living animals (which descend from the desired archetype) and “just”
have the be united, the Quagga Project wants to achieve features that are not
present in any living zebras (which do not descend from the Quagga). If you’d imagine a bell curve, the Boehm zebra is
on the one end and the Quagga is on the other end, and the Burchell zebras are
somewhere in between. Nevertheless, the optical resemblance that already has been
achieved is impressive to me. Some of these individuals have show a
significant resemblance to the Quagga already, like “Freddy”, “Henry”, DJ10 and
FM12. The stripes on the legs are greatly reduced or absent, the stripes on the rear are reduced, and the background colour is more brownish than on average Bruchells zebras. Imagine how Quagga-like the F5 and F6 generation might become. But can the project truly rebreed all the Quagga’s phenotypic features,
or even the Quagga as a whole?
The
Quagga’s appearance often is equalized with its coat, but there are other
features that yet have to be evaluated. For example, there is no osteometric study working with that one complete skeleton and that of living Plains zebras,
and since the use of skulls from stuffed skins is questionable, one should
compare the genuine Quagga skull with that of living zebras in order to verify
the statement that there are no osteometric/craniometric differences between
the Quagga and other members of that species. I think it is well plausible that
there are, because of the apparent founder effect during its evolution, but
this simply requires testing.
Furthermore,
there is more that defines a subspecies than just a differing phenotype. The
argument “the Quagga was merely a subspecies, so it can be rebred” that is
sometimes brought up in connection with the project is erroneous. Some of the Youtube commenters seemingly have a very simplified idea of species and little clue
about population genetics. Species that are divided into several subspecies are
not genetically homogenous, connected populations but more like a branched
tree, with each subspecies being an evolutionary distinct clade, defined by
whatever differences that are laid down in the genome. And apparently there was
not much genetic exchange between the Quagga and other Plains zebra populations
for at least 120.000 years (although more evidence might be needed). This means
that E. q. quagga was genetically
different from E. q. burchelli just
like burchelli is from E. boehmi and so on. To make a
comparison on a similar level, you cannot rebreed the Polar wolf by selecting
white Timber wolves. Also, you cannot rebreed the Boehm Zebra by selecting
those Burchell’s Zebra with the heaviest striping. Different subspecies mean a
different genetic make-up (although not as marked as on species level, of
course) and therefore a different animal. Consequently, the Rau zebra will
always differ from the Quagga because the populations have a different
ancestral history. These differences are at least genetically, and perhaps also
in ecologic, behavioural or phenotypic respects that are either unknown or not
evaluated, although I personally think these aspects are overall roughly the same the within the
Plains zebra.
We probably
cannot say anything about the exact ecologic role of the Quagga because there
are no living representatives left to study. But probably, since they belong to
the same species, they were largely similar if not ecologically identical to
other Plains zebras. I read in some sources that South Africa’s flora has a
higher degree of endemism than farther in the north, but I don’t know if that
is true (does anybody have literature on that?). The Quagga project claims that
the flora is not significantly different from that of the habitat of living
members of the species. I think that many conservationist would be in favour of
reintroducing the Plains zebra into its former range in South Africa, as
subspecies are obviously no barrier for reintroduction projects, especially
when the native type is lost (as the reintroduction of Przewalski horses in
some parts of Europe or recent projects with big cats have shown). The ecologic functionality and physical characteristics of the Rau zebras are the same as in other
plains zebras, but they are the only lineage that resembles the type that once
was native in that region. If the Plains zebra is to return into this area –
what would surely make sense from a conservational view – the Rau zebras from
the Quagga Project certainly are the most desirable option (just like the
project says on their web page).
I think it
is very helpful that the Quagga Project methodologically quantifies their
selective breeding progress; also, it’s the only breeding-back project that has
published papers explaining their “materials and methods”, making the whole
process much more scientific and transparent (see here and here). Apart from that, I am also curious to see other effects on these zebras because of the selective breeding, and perhaps also new mutations will show up.
Although
the Quagga itself is lost, I am very happy that the Quagga Project is trying to
establish zebras resembling this remarkable subspecies. It will be interesting
to see how far they can get and indeed the similarities that already have been
achieved are astonishing. Some of these Rau zebras already show an amount of
stripe reduction that is shared by some Quagga mounts, and if they had the same brownish background colour they would be virtually indistinguishable. Because the progress in stripe reduction has
been fast but not the change of the background colour, the project decided to put
more focus on that feature [5].
Also, the
Rau zebra would be a very useful tool if it will be possible to clone
one or more Quagga individuals one day, however unlikely that scenario is. But these
few cloned Quaggas could constantly be merged into the Rau zebra’s population
that provides a genetic base, coming as close to the Quagga as technically
possible. However, this is a fantasy scenario of mine (yet?).
Literature
- [1] Orlando et al.: Revising the recent evolutionary history of equids using ancient DNA. 2009
- [2] Samantha A. Price und Olaf R. P. Bininda-Emonds: A comprehensive phylogeny of extant horses, rhinos and tapirs (Perissodactyla) through data combination. 2009.
- [2] Knight, Kathryn: How the Zebra Got its stripes. 2012.
- [3] Leonard et al.: A rapid loss of stripes: the evolutionary history of the extinct quagga. 2005
- [4] Eric Harley et al.: The restoration of the Quagga: 24 years of selective breeding. 2013.
Are the aurochs back breeding programs using objective measurements to score individuals and to guide future breeding? If so, I would love to learn about that.
ReplyDeleteUntil now, no aurochs effigy-breeding projects do that; but it is possible that Tauros Project is going to do something like that. I already asked Henri Kerkdijk-Otten about that. IMO, Tauros Project yet needs to make their "material and methods" publicly, just like the Quagga Project did.
Deletehttp://www.arkive.org/explore/species/all/extinct
ReplyDeletehttp://www.arkive.org/quagga/equus-quagga-quagga/
The Quagga:
ReplyDeletehttps://quaggaproject.org/
https://www.quaggaproject.org/
https://www.atlasvirtual.com.br/quagga.htm
http://www.avph.com.br/quagga.htm
https://prehistoric-fauna.com/Equus-quagga-quagga
https://prehistoric-fauna.com/Quagga