Sometimes
commenters on my blog ask me on what I think about the idea of using extant,
wild cattle species for breeding an accurate aurochs substitute. One could even
go further and try to replace the aurochs with other, related bovine species
instead of “breeding-back” results and/or dedomesticated cattle. For other
exterminated species that did not leave domestic descendants this option is
considered (such as using extant water buffalo for Bubalus murrensis). However,
domestic cattle are probably way closer to the aurochs in behaviour and ecology
than extant wild bovines (see here). Nevertheless these could be used in order
to obtain wild traits that domestic cattle lack altogether or to increase the
fitness for survival in nature instead of waiting for those traits to
re-develop via dedomestication. There is a good chance that the aurochs shared
some wild traits or even the same alleles with its closely related species that
domestic cattle have lost, and they could re-enter the gene pool via
hybridization. This would make it easier for new “breeding-back” populations to
establish in harsh regions and perhaps also a higher morphological resemblance
could be achieved.
Looking at
extant wild bovines, I see three species which could qualify as useful for
hybridization: wisent, wild yak and the Java banteng. I am going to explain
which traits make these species useful; of course, being completely different
species, they also have a lot of undesired traits that would occasionally
reappear in the hybrids such as the beard of a bison or the white “socks” of
banteng. But those could be bred out just as any undesired domestic trait such
as white spots on the belly or deviant horn shapes as well, one could argue. With
this post, I want to go over the pro’s and con’s of using hybridization with
wild bovines for “breeding-back” in detail (I already did one in 2015, but this
post will be more comprehensive).
Wisent
Bison bonasus
 |
| The wisent has, of all extant wild bovines, the most aurochs-like proportions and body shape |
As other
wild bovines, wisents have a strongly marked sexual dimorphism in size. The wisent
is not only as large as large aurochs (bulls reaching 180cm at the shoulders)
but also has bodily proportions rather similar to those of the aurochs, with a
squarely built trunk, the hump is high but not as high as in American bison and
they have a neck bulge similar to that of taurine cattle (that the aurochs
probably also had). These similarities in body shape are not coincidental. In
fact, mitochondrial and nuclear DNA have confirmed the wisent as an aurochs
hybrid that evolved at the end of the Pleistocene and it is estimated that
about 10% of the nuclear genome of the wisent is of aurochs legacy (see here).
Wisent and domestic cattle have been hybridized repeatedly, the results are
called Zubron in Poland. Although first generation calves have to be born by
caesarean section, female F1 hybrids are fully fertile and males from the
second generation onwards. Some zubrons in the internet show interesting
morphs, such as this bull with his high-legged athletic build or this cow with
its large hump and aurochs-like colour. Both these zubrons were probably bred
using ordinary domestic cattle, therefore hybridization between aurochs-like
cattle and wisent could produce even better results. One quarter wisent could
boost the morphology of “breeding-back” results quite dramatically, and
concerning the aurochs legacy of Bison
bonasus, why not?
The problem
is that aurochs/cattle and wisent are sympatric species, what means that they
theoretically share the same ecosystems. Therefore, their niches differ (an
ecological principle that is called competitive exclusion). Wisents are more
adapted to dryer habitats on higher altitudes and aurochs/cattle to more humid
habitats on lower altitude. Furthermore, wisents are so-called
semi-intermediary feeders, which means that their food choice includes more
browsing than in cattle, which are more grazers. Also, wisents also slightly
differ in behaviour, including vocalization. Therefore, when using wisents for
“breeding-back”, you would not only have to select out animals based on their
external appearance but you also have to watch food choice, habitat choice,
behaviour and even vocalization, otherwise many biological aspects of the
aurochs substitute would be messed up. Furthermore, wisent and cattle differ in their number of spine elements, which cannot be observed in living individuals without an X-ray.
The next
problem that is a result of both species being sympatric comes to play when
both these “breeding-back” results with wisent introgression and wisents would
be released in the same reserve. Wisent and aurochs shared the ecosystem and
naturally probably did not or extremely rarely interbreed. If the
“breeding-back” results are wisent hybrids it could increase the frequency of
interbreeding and reduce fertility barriers. This would be a danger for the
biologic integrity of the endangered wisent. And in fact, the danger of
hybridization between cattle and wisent is sometimes used as an argument not to
release both species in one reserve although the danger is virtually
nonexistent1.
Wild yak
Poephagus* mutus
 |
| The horns of wild yaks are as good as identical to those of aurochs |
* Yes, I
have the yak in its own “genus” here. It is phylogenetically actually closer to
bison than cattle (I explain why I write “genus” under quotation marks in an
upcoming post).
The wild
yak is a large bovine of the right size (bulls having a shoulder height of
1,6-2m) and sexual dimorphism (cows being 30% smaller in linear dimensions
according to Wikipedia) that also has a large shoulder hump. But what is most
striking is that wild yaks tend to have horns that match those of the aurochs
exactly in size, diameter and curvature. Almost identical, one could dare to
say. Considering the fact that finding breeds that have strongly
inwards-curving horns in both sexes is rather difficult for “breeding-back”,
and that most “breeding-back” cattle are deficient in this respect, using wild
yaks would be a great aid for achieving this trait. I did this sketch of what a
wild yak without hair would look like, and it resembles the morphology of an
aurochs pretty well, so the benefit of crossbreeding with aurochs-like breeds
would be obvious.
What is
more striking is the yak’s adaptions to harsh climate. Yaks need less food
intake than domestic cattle due to their larger rumen and survive longer
without food and water. These traits could be beneficial for establishing
“breeding-back” populations in harsh areas. For example, the Puszta of
Hortobagy is not ideal cattle habitat but the best area available for
large-scale natural grazing, which is why Taurus cattle had problems during
winter there during the first couple of years. Yak introgression would help the
cattle to deal with the conditions of the Puszta.
Fertility is
a problem in yak hybrids as well. As it is often the case in hybrids, the
heterogametic sex (males in the case of mammals) is infertile (Haldan’s rule),
but in the case of the yak not only in the first generation but also in the
second generation.
Wild yaks
are adapted to way colder temperatures than cattle. They get overheat problems
above 15°C due to a higher amount of body fat and fewer sweat glands. Yaks are
furthermore more suited to higher altitudes than cattle.
They also
differ in behaviour. Yaks, when they feel threatened, erect their tufted tail
in order to appear larger – a display behaviour that cattle do not do. So when
selecting hybrids, one would have to pay attention to that too.
Java banteng
Bos javanicus javanicus
 |
| Java banteng have the same colour dimorphism as the aurochs |
The Java
banteng is way more closely related to the aurochs and domestic cattle than
both yak and bison are. As far as I know, they can interbreed with cattle
without fertility problems and there is a hybrid breed that was bred by
intermixing with zebus, Madura cattle. Java banteng have the right size and
body with a large shoulder hump and long legs, and the sexual dimorphism in
size is considerable as in all wild bovines. But the biggest advantage would be
their sexual dimorphism in colour: bulls are very dark brown to black while
cows have an orange-reddish brown colour, which is very reminiscent of the
aurochs. This sharp contrast is present universally in the subspecies, while
even in those domestic cattle breed with a good sexual dichromatism (such as
Maronesa) it is reduced and tends to vary from individual to individual. The
colour of Java banteng differs in having white “socks” and buttocks, but they
can be bred out. The historical range of aurochs and banteng did not overlap,
and there is a chance that they might have been ecologically equivalent.
Banteng and cattle/aurochs differ slightly in combat behaviour, though. Banteng
rely more on display than actual fights, which is why they have a higher profile
with higher shoulder spines, a longer dewlap (thermoregulation might play a
role here as well) and upright horns.
So there
are pro’s and con’s for all those three species. Theoretically you could even
build a look-alike by hybridizing these three species alone taking the horns of
a yak, aurochs-like colour of banteng and aurochs-like proportions of wisent,
but that resemblance would be only vague and superficial, and it also would not
be an ideal ecological substitute for the aurochs. In order to reach the goal
that is replacing the aurochs as authentically as possible in morphological,
behavioural and ecological respect, hybridization should only be used in small
doses otherwise it would be a step away from the goal. But apart from the fact
that introducing other species into the gene pool makes the selection criteria
far more comprehensive and complicated, the biggest problem would be that
hybrids are stigmatized. Although hybrids were a part of speciation in many
cases (for example, sheep, bovines including the wisent, canids, geese and even
humans), happen in nature in many tetrapod groups all the time (within genera
such as Bombina, Pelophylax and Canis just
to name a few; more on hybridization in evolution in a future post), hybrids
are still regarded as Frankenstein creations done by humans for no particular
good. Especially in conservation hybridization is often considered “genetic
pollution”, often with a good reason but in some cases this purity fanaticism
can actually be harmful and do more harm than good, such as in the case of the
wisent population in the Caucasus. “Breeding-back” results with introgression
from other cattle breeds would loose their legitimation as native species and
there will surely be tabloids with screaming headlines like “hybrid monster
cattle to be released into European nature”. Both will not help the academic
and public acceptance of these “breeding-back” results.
For these
reasons, I would not recommend using existing wild bovine species for “breeding-back”.
I think nothing speaks against an experimental project using hybrids of
aurochs-like cattle and one of these bovine species (although I do not
recommend using wisent because both species are sympatric) and in order to see
how it works out, although I think the danger of the hybrids becoming a
behavioural and ecological mosaic is given.
1 Frans Vera: Do European bison and cattle cross spontaneously? 2002. Vakblad
Natuurbeheer.
