I think
virtually everyone is used to regard his pet dog as a domesticated wolf. And
the majority of scientists does so as well, as the “wolf hypothesis” seems to
be well-supported by the evidence - seems.
But there are in fact several arguments that suggest the wolf hypothesis is not
as water proof as widely assumed. Some authors propose the domestic dog is in
fact a descendant of a now extinct wild canid species, a wild-type Canis familiaris. Perhaps we might have to change our view on the origin
of our “best friends”, or at least get more open-minded for alternative hypotheses.
I came across this interesting re-consideration of dog domestication thanks to
the user Joao Ferro.
English
Wikipedia on the origin of the domestic dog states: “Some
authors wrote as if domestic dogs were descended from a species of now-extinct
wild dog distinct from wolves [...] but that has been disproved.“ This
is the only sentence treating this alternate hypothesis, and it is anything but
disproved. Koler-Matznick [1] suggests that the actual ancestor of the domestic
dog might have been a now-extinct medium-sized generalist canid, comparable to
the Golden jackal or coyote. In this article I am going to give an overview over the arguments and evidence pro
and contra both the wolf hypothesis and this alternative hypothesis, some of
these come very recent studies. Let’s go over it step by step.
Morphological evidence
Characterising
differences between the dog and the wolf are cranial features such as the
shortened rostrum, elongated frontals, broader skulls and smaller teeth,
smaller body size and floppy ears [1]. Assuming that the dog descended from the
wolf, these features can be explained with a pleiotropic effect because of selection
for tame behaviour and relaxed selection, the way we see it in other
domesticated species [1]. Apart from that, there is considerable overlap
between dogs and wolves in respect to these features. Interestingly, a 1986
study comparing the skull morphology of dog skulls which do not show a
considerable relative shortening with wild canid species showed that the
cranium of the dog is in fact more basal compared to the specialized wolf and
similar to those of jackals, coyotes or the dole. This is not only not in
accordance with the explanation with neoteny but also not with the wolf
hypothesis. A study from 1942 concludes that the shape of the dog’s brain case
resembles the coyote closer than the wolf, and a work from 1983 found that
cranial and dental data from dogs resembles smaller canids closer than any wolf
subspecies [1]. Some dog characters that are diagnostic are not related to
paedomorphism, two of them are mandibular [1]. These characters are shared with
C. l. chanco and C. l. pallipes [1], what can either be explained by descent or
introgression. Gonzalez 2012 find morphologic similarities between C. l.
pallipes and the dog which is not shared by C. l. lupus based on bivariate
analyses, and which is probably not the result of hybridization and further recognized cranial similarities between the dingo and Canis lupaster [2].
Interestingly,
the superficial brain morphology of dogs is closest to the coyote, which itself
is close to the jackal, and that of the wolf is distinct from that of the those.
So it seems
that wolves are specialized canids related to their ecological niche and that
dogs share morphologic characters not necessarily connected to paedomorphism
that they share with more basal canids [1]. If you wonder why the postcranial
skeleton plays little role in these examinations, I think the reason for it is
that the postcranial skeleton shows way less variation within Canis and that most of the differences
are a product to allometry, but that’s my personal guess.
You might
wonder why I didn’t cover canid fossils yet. The problem with fossils is that we
are talking about only very scanty and dubious data here with taxonomic
uncertainties [1]. Early domestic dogs might have resembled their ancestors to
a large extent so that they are not distinguishable, or putative domestic dog
skulls might actually be single exceptional wolf skulls [1]. And if canid
material definitely shows signs of domestication, it is still uncertain if they
belong to the modern lineage of dogs or represent a now extinct, independently
domesticated lineage [1]. Distinguishing closely related species from limited osteologic
samples is highly problematic and questionable, see the controversy around C. variabilis (or C. l. variabilis) which also was purported as possible dog ancestor
[1,3].
But what
can be said is that 4000 years old pariah dog skulls are virtually identical to
modern counterparts, what indicates phenotypic consistency over millennia [1,2,3]
and moreover, the oldest European domestic dogs known by the 1940s and 1961
were claimed to resemble the Dingo [1], although I don’t know what modern
literature says in this case. The oldest genetically confirmed domestic dog is
from the Altai 33,000 years ago [10], so we should use this date as a working
hypothesis for the first domestic dogs.
Ecologic and behavioural evidence
One of the
most interesting aspects of the controversy around the dog’s ancestry is their
ecologic niche. A recent paper from 2013 showed that dogs have genes that
enable them to digest higher amounts of starch in their diet than wolves do [4].
This can be either an effect of domestication, as the authors assume, or an
ancestral trait inherited from an ancestor with an ecologic niche different
from wolves. We can’t say more yet.
Darren
Naish, author of the marvellous blog Tetrapod Zoology, writes in a post on dog
origins from 2006 that domestic mammals usually “revert back to wildtype after
being feral for a few generations” [3]. While this statement is surely very simplified
and in my opinion not true in all respects, but it is certainly correct that most
feral domestic animals occupy the same ecologic niche and show a behaviour very
similar to that of their wild type. These behavioural aspects include social
structure, predation or defence against predators or interaction with man.
Feral dogs are opportunistic, voluntary human commensals, just like other
generalist canids such as foxes, coyotes and jackals, which readily reproduce
in human vicinity [1]. The wolf, however, never is a voluntary human commensal and
avoids contact with humans [1]. Furthermore, wolves have a very broad prey spectrum,
varying from animals the size of a hare to that of a bison. The fact that they
are the largest living canids is certainly helpful for this purpose. Feral dogs
on the other hand are rather small compared to most wolves (10-20 kg) and
comparable, once again, with other generalist canids [1]. Despite domestic dogs
show a considerable variation regarding body size, there is no feral dog
population I know which reaches the average size of wolves. As far as I know, feral
dogs are hypo- to mesocarnivores that usually prey on animals smaller or as
large as themselves and do not take down such large animals as wolves do. (The dingo might be
an exception, perhaps as an adaption to being the only apex predator on the
Australian mainland since their arrival, but this is my personal speculation.)
The social structure
of feral dogs is flexible, less complex and less hierarchic than that of
wolves, and they are not as gregarious [1]. One could argue that this is the
result of altered brain structures as a result of domestication, but once
again, their social behaviour is congruent with that of smaller,
less-specialized canids than the wolf [1].
Vocalization
The most
remarkable dogs regarding their vocalization are the dingo-type New guinea
singing dogs. They howl at high frequency in bird-like manner. This “singing”
is not known for any other living canid (different from the dingo and very
different from wolves), although similar sounds have been described for a dhole
at the Moscow zoo [8].
Genetic evidence
The
contribution of female coyotes or jackals to dog domestication can be ruled out
based on all genetic studies carried out so far. As far as I know, the dhole
and C. simensis were not tested yet,
and the contribution of male members of other species cannot be ruled out by
mtDNA [1]. The wolf proved to be the genetically closest living canid to the
dog, but this alone does not confirm it as its ancestor. A direct proof would
be if the domestic dog clustered within modern wolves. Molecular clock analyses
based on mtDNA implicate that wolf and dog separated about 76,000 to 136,000
years before present [5]. A study from 1997 showed that some of the domestic
dog’s mitochondrial haplotypes intergraded with wolves, indicating an ancestral
relationship [1]. Another study from the same year found wolves and dogs to be
sister clades, sharing only one of 52 haplotypes[5], but this does not rule out
that both descended from ancient members of C.
lupus now extinct.
A paper
from 2007 [11] states that genetic studies found C. l. chanco and C. l.
pallipes to be actually outside the dog + Holarctic wolf clade and that
they diverged much earlier, 800.000 and 400.000 years BP. This indicates that
the domestic dog is indeed nested within the wolf. A study published this year
[10] confirmed the sister clade relationship between wolves and dogs again, but
this study did not include pallipes
and chanco in its phylogenetic
analysis. The assumption that dogs were domesticated from wolves thus seems
confirmed, but read on.
Another interesting fact: if dogs
are domestic wolves, it should be assumed that feral dogs and wolves readily
interbreed in the wild and create a panmictic population [1], like it is the
case in other domestic species and, noticeable, in the Australian dingo along
with abandoned domestic dogs. Although wolf-dog hybridizations are known to
occur, the shared number of mtDNA haplotypes is very small according o Vila et
al. 1997 [5], and Indian wolves and pariah apparently do not often hybridize in
the wild [1]. Vila et al further write: “The behavioral and
physiological differences between domestic dogs and gray wolves may be
sufficiently great such that mating is unlikely and hybrid offspring rarely
survive to reproduce in the wild.“ Furthermore,
Gonzales 2012 states that there are examples of hybrids in the wild but these
are not common and populations are not affected by it [2]. However, he also
writes: “Some
dog and wolf populations from South Asia, the Middle East and North America
appear to be inextricably linked, by a long chain of hybridization events,
which may have contributed greatly to the current biological identity of both
groups“. Introgression into wild wolves by domestic dogs is likely also for
North American populations [1], and abandoned dogs and wolves seemingly also
intermix in Eastern Europe [6 (yeah, not the best reference, but I also heard
on other sources that hybrids are common in these areas)]. Also Freedman et al.
2014 suggest post-divergence gene flow between wolves and dogs, and that both
underwent severe bottlenecks after their divergence [9]. The ability to produce fertile hybrid offspring is the main character of
the “biologic”* species concept [1], but I think we should remember that wolves
and coyotes also hybridize in the wild and yet they are still considered
separate species based on other biologic criteria.
Koler-Matznik 2002 writes: “The
question of how humans created a domesticated wolf between 15,000 YBP, the
oldest estimate from the fossil record for DD (Clutton-Brock 1995), and 135,000
YBP, the highest estimate for separation of DD/wolf from mtDNA (Vilà et al.
1997), is rarely addressed“ [1]. I think this is not an inconsistency. The
moment of separation of the two clades (domestic dog + modern wolves) does not
implicate that domesticated dogs themselves are of that age, just that the
clade they belong to separated from the modern wolf clade about 100.000 years
ago.
* I have always been thinking the term “biologic”
species concept is rather awkward. All species concepts are biologic. “Genetic
species concept” might be a better and more definite term.
Behavioural suitability for domestication and
purpose of domestication
Gregariousness
often is considered a prerequisite for why the domestication of the dog was so
successful. However, Charles Hamilton Smith (this man strangely keeps showing
up on my blog) reported the case of an Andean fox, which belongs to a solitary
species, tamed by South American people and used as hunting aid. Therefore the
ancestor of the domestic dog was not necessarily gregarious.
There are
several hypotheses that try to explain why and how humans domesticated the
wolf, assuming it is the ancestor of the dog. One that is quite popular is that
they were used as a hunting aid. There are several problems with this hypothesis.
Wolves are chase hunters of medium-sized to large prey while early human
hunters were “ambush predators”, so the wolves would have chased their prey
away [1]. Furthermore, wolves are strongly food-possessive, so trying to secure
the kill would be a dangerous game [1].
Pariah dogs
and smaller canids may be a more conclusive model for canids as hunting aid. Aboriginal
people prevent their tamed dingoes from following when hunting kangoroos
because they would chase them off, but dingoes are very useful to locate smaller
prey either on the ground or in trees [1]. Also the Basenji hunts alongside
humans. Furthermore, early humans could have been kleptoparasitic by following inoffensive
small to medium-sized canids and stealing off their prey, as still some people
do with pariah dogs and also the dhole today [1]. This could be a reasonable
reason for early humans to raise and keep these canids and use them for hunting
purposes.
Another
hypothesis claims that wolves might have served as guards; this is unlikely
considering that all canids make poor guards and even abandon their offspring
if their personal life is threatened [1]. Furthermore, it is not unlikely that
wolves considered Pleistocene humans as prey. Indian wolves are known to prey
upon children, and before long-range weapons and persecution they would have
been bolder [1]. It is known that wolves do attack people, even adult ones and
especially in the past, and a predatory motivation is one among many [7].
Many
authors propose that wolves domesticated themselves by voluntary becoming a
human commensal, feeding of camp refuse and leftovers [1]. According to them,
this different ecological niche separated these commensal wolves from their
conspecific, so that they went different evolutional ways, one leading to the
domestic dog [1]. Those wolves would have become increasingly tame and humans
started to breed them for whatever purpose. This hypothesis is problematic
because early nomadic hunter-gatherers probably used as much nutritious
portions of a carcass as possible, like modern hunter-gatherers do, so that it
is unlikely that there was enough nutrition to support a reproducing population
of wolves [1], and it is very dubious why a large, specialized big game hunting
predator would niche shift to scavenger of scarce refuse [1]. A generalized,
smaller canid in the pariah niche is more likely to become a human commensal
[1].
Summary and conclusion
Let’s put
it all together: the dog is not phenotypically closer to the wolf than to other
canids, actually it has features that cannot be put aside as results of neoteny
but likely are plesiomorphic and similar to other, less derived canid species,
suggesting that the wolf is a more specialized canid than the other species,
including the dog. This is also the case in the brain structure of the dog and
the social structure of feral dogs, which is more flexible, less gregarious and
less hierarchical than that of wolves, like in other smaller and
less-specialized canids. Dogs are adapted to a more starch-rich diet than
wolves, this is either an adaption to their domestic niche or a trait inherited
from their wild ancestors. Generalist canids and feral dogs are opportunistic,
voluntary human commensals that reproduce in humans neighbourhood, while wolves
never do so voluntarily. The former usually are meso- to hypocarnivores, while
wolves, being the largest extant canid, regularly take down big game. The
vocalization of the New guinea singing dog is exceptional among living canids,
it might or might not be an ancestral trait (far-fetched because no other
pariah dog displays it).
Genetically,
modern wolves and dogs are close relatives, but they share only few haplotypes
and seem to be sister clades and diverged about 100.000 years ago. C. l. chanco
and C. l. pallipes seemingly are considerably more basal, being outside the
northern wolves + dogs clade and diverged about 400.000 years before present. Therefore,
dogs cluster within C. lupus
genetically. Hybridizations occur in the wild, sometimes also leaving tracks in
each other’s populations, but this occurs also between other canid species.
It is
unlikely that wolves are suitable as a hunting aid. Paria dogs and other less-specialized
canids are more suitable for this purpose. It is also unlikely that large
predators like wolves voluntarily became human commensals feeding on scarce
refuse (if there was any) of hunter-gatherer camps, while many less-specialized
canids already are. Any canids make poor guards and it is possible that wolves
considered early humans prey.
So, now my
personal interpretation of all this. Behavioural, morphological and ecological
data seems to contradict the wolf hypothesis, or at least makes it unlikely.
Genetic data, on the other hand, seems to strongly confirm it (unless you
consider chanco and pallipes separate species). How does that fit together? I
discovered something that seems to be overlooked in this discussion. Chanco and
pallipes are not only more basal regarding their phylogenetics, but also
regarding their ecology and behaviour. They are less gregarious and their
social structure is less hierarchical and very similar to dingoes, they are
hypo- to mesopredators and take down larger prey on occasion, just like pariah
dogs. They are, simply spoken, less specialized and more plesiomorphic, like
pariah dogs and unlike the northern wolf clade. Parsimony thus dictates that
the common ancestor of the northern wolf + dog clade was so as well. It is also
interesting to note that the coat of both chanco and pallipes is lighter in
colour on average than that of northern wolves, having a sand-coloured to
reddish tint, slightly resembling dingo-type pariah dogs. Therefore I consider
it likely that domestic dogs descend from basal, plesiomorphic wolves less
specialized than modern northern wolves, what also was also concluded by the
authors on recent papers on wolf/dog genetics. However, I take one speculative
step and state it might be possible that within the approximately 100.000 years
between the split from primitive wolves, the familiaris-lineage might have undergone
evolution on species level (therefore, wild-type Canis familiaris) to a niche and phenotype similar to dingo-type
pariah dogs, since the colour of their coat is an effective camouflage in their
environment and it would explain the almost uniform appearance of dingo-type
pariah dogs, ranging from Israel (Canaan Dog) over South East Asia, Australia
to North America (Carolina dog). However, that’s just a guess of mine. But
regarding the species status of domestic dogs, I think it is premature to label
them under Canis lupus, especially since there is no general opinion regarding
the species status of chanco and pallipes either.
By the way,
that being said does not mean that northern wolves played absolutely no role in
the domestication of the dog. The use of wolf-dog hybrids in the creation of
certain breeds, such as the German Shepherd, is documented. Genetically, such
dog breeds show a stronger relationship to wolves than the main dog group
consisting of a large part of the domestic dog, including dingo-type pariah
dogs as well as popular, highly derived
breeds like the collie and retriever [5].
Literature
[1] Koler-Matznick,
2002: The Origin of the dog revisited.
[2]
Primitive dogs
[3] Erik
Axelsson et al.: The genomic signature of
dog domestication reveals adaption to a starch-rich diet. 2013.
[4] http://darrennaish.blogspot.co.at/2006/10/controversial-origins-of-domestic-dog.html
[6] Shaun Ellis: The
man who lives with wolves. 2010.
[7] https://en.wikipedia.org/wiki/Wolf_attacks_on_humans
[8] Kohler-Matznick
et al.: An updated description of the New
Guinea singing dog (Canis hallstromi, Troughton 1957). 2003
[9]
Freedman et al. 2014: Genome Sequencing highlights
the dynamic early history of domestic dogs.
[10]
Druzhkova et al. 2013: Ancient DNA
analysis affirms the Canid from Altai as a primitive dog
[11]
Aggarwal et al. Mitochondrial DNA coding
region sequences support the phylogenetic distinction of two indian wolf
species. 2007.
