Why Incest Isn’t Best
Originally published in USASA Journal,
Nov/Dec 2002
by
|
…the breeding of purebred dogs is akin to [breeding laboratory
mice]…[most breeds] are becoming progressively more
inbred. My observation is that most are on the road to extinction, but most
breeders do not even realize they are part of an experiment. John B. Armstorng, PhD |
Incest,
the marriage of close relatives, is forbidden on moral and legal grounds in
most human societies. But the prohibitions also make biological sense. Inbred
individuals tend to suffer from inbreeding depression. Among humans, the
ancient Egyptian pharaohs and the more recent Hawaiian and European royal
families provide the better known human examples. Inbreeding depression has not
been studied much in dogs but it is well researched in other species.
Many dog breeders know very little about inbreeding depression. Purebred dogs,
including the Australian Shepherd, are commonly bred to related individuals and
occasionally bred to relatives close enough that among humans such a match
would be considered incest. These are dogs, not people, so the moral issues are
not the same. However, we ignore the biological impacts of the practice at our
peril—or more accurately the peril for our dogs.
Inbreeding depression is the loss of viability or function resulting from
excess inbreeding. The signs of inbreeding depression most frequently cited are
reproductive failures. Anything that impacts reproduction is an up-front
concern for dog breeders. But inbreeding depression may also manifest itself as
poor health. Its effects are often so subtle that breeders or even
veterinarians may not recognize the root cause and not every inbred animal will
show signs of it.
Inbreeding-related reproductive failures include lack of libido and low sperm
count in males while females may fail to get pregnant, have unusually small
litters, or exhibit poor mothering ability. Non-reproductive indications of
inbreeding depression can be manifested as a high frequency of immune-mediated
diseases, significantly higher incidence within a line or breed of one or more
diseases than is seen in the species as a whole, or even things as subtle as
dogs that is are “poor-keepers” or that seem to catch every little bug that
comes along. In really severe cases, a very inbreed strain may go extinct.
The level of inbreeding is usually measured using a formula called Wright’s
Coefficient of Inbreeding. It calculates the probability that genes may have
been inherited from both sides of an individual’s pedigree. It is far too
complex to do by hand over more than two or three generations, but some of the
better pedigree software will calculate coefficient of inbreeding (COI) for
you. The usual 3-5 generation pedigree won't give sufficient information for a
useful calculation. For Australian Shepherds the author has found 10
generations to be the best indicator. Few Aussies have a complete 10-generation
pedigree, so this will give you pretty much all there is to know. While some
individual lines of descent may go back 20-30 generations, running the
calculation for more than ten does not result in any significant change in the
result. Running it for fewer than 10 frequently causes the number to drop,
giving an incorrectly optimistic result.
Modern breeders should know the COI of each of their dogs and determine what
the COI will be on planned litters. The average COI is for the breed, or at
least the portion of it with which you work, is also important. Show line
Aussies average around 12-14%. This is roughly equivalent to all of them being
half-brothers and sisters. Working line Aussies are variable. Many have very
low COIs, under 6%, but some are as high or higher than the show line average. Inbred dogs,
especially those with a COI of 25% or higher, that are experiencing problems of
the kind described above may be exhibiting inbreeding depression.
Not surprisingly, most the research on the effects of inbreeding in mammals has
been done on farm animals, laboratory animals like mice, and endangered wild
species. One study on cattle done by Dr. John Pollock at
Laboratory mice are often pointed to as proof that extreme inbreeding works.
Lab mice are arguably the most inbred of domestic mammals; so much so that
members of a strain are near clones of one another. This extreme inbreeding is
necessary so researchers will know exactly what to expect from that particular
strain.
Strains are developed by breeding mice brother to sister for many generations,
producing levels of inbreeding unheard of in dog circles. But there is a price
to be paid for this. In 20 generations, 80% of the lines descendant from the
original pair will have gone extinct due to lethal health problems or an
inability to reproduce. Even those that make it through the bottleneck are
hardly the mice their ancestors were. Take any pair of field mice put them in a
laboratory cage and they survive very well, living longer than they would in
the wild. Take any pair of lab mice and put them in a field and their “wild”
life expectancy is zip. They are suitable only to the very controlled
environment of a laboratory, where the “weather” never changes, food, water and
housing are provided and there are no predators.
Wild species of all kinds employ a number of behavioral
strategies to avoid inbreeding. If Nature does something so consistently, you
can bet she has good reason. Among social animals, the young of one or both
genders may disperse to form or join other groups. Dominant breeding males may
hold their position only a short time. Solitary animals tend to be territorial,
at least in breeding season, with a male’s territory overlapping that of
several females. Their offspring must disperse and seek territory elsewhere,
sometimes travelling long distances to do so. But even in nature, conditions
are occasionally such that an animal has no choice but to mate with a relative.
The cheetah is a very inbred species. In the
Pleistocene, cheetahs roamed
The cheetah’s inbreeding problem is “background.” Due to that long-ago
bottleneck, they are all closely related even though there are now several
thousand of them. A number of breeds of dog have experienced similar but more
recent bottlenecks due to things like war. Other breeds have very few founders,
which is akin to a wild species living on an island.
Islands that are well away from the nearest mainland gain species only rarely,
when a very few individuals arrive through some accidental circumstance. If
they survive the initial inbreeding depression they adapt to their new
environment, sometimes to the point of forming entirely new species as can be
seen with the finches and tortoises of the
Wolves are more relevant to dog breeders than tortoises or finches and they
provide an excellent example of this process. For the most part, wolf
populations are scattered across great landmasses of the northern hemisphere.
Normally, only the alpha pair of a pack will breed and most offspring disperse
during their first or second year to seek new territory. Those that remain are
strongly discouraged from breeding by higher-ranking pack members. But there is
a population of wolves on
There are moose on the island, so for many years the fortunes of the
increasingly inbred wolves followed the boom and bust of the moose population
cycle. The moose and wolves of
The ultimate decision was to let things be and the wolves managed to pull
through, with 19 counted in 2001. The population appears healthy, but the next
canine plague may again leave their fate hanging in the balance.
Our dogs don’t have to hunt for their dinners or survive in the elements. They
are provided with comfortable environments, sometimes to the extent of
air-conditioning and a space on the couch. Obtaining a meal requires no more
effort than a trot to the food bowl. It is neither moral nor ethical for a dog
breeder to subject his animals to the kind of harsh culling process exacted by
Nature. However, we should give health, fitness and reproductive issues much
greater importance in our selection process.
Australian Shepherds are blessed with around 300 founders, so our background
inbreeding is very low. However, our breed like most others has experienced
historic inbreeding stemming from the choices made by recent generations of
breeders who have used one sire more than others or frequently sought the
output of a particular kennel. Early preference for the Flintridge-type dog in
the show ring is a prime example in Aussies. Even in working lines there are
early sires and kennels whose names occur with considerable frequency.
Any time a single dog or kennel is responsible for producing a significant
portion of the breeding animals in a breed, the breed gene pool is skewed
toward whatever genes those dogs had. Breeders do this because they want to
skew things toward genes that produce the desirable traits they found in those
animals. But along with those genes come others that are less desirable.
Inbreeding depression isn’t a matter of specific genes that cause the sort of
health problems we are always on the lookout for, like bad hips or eye disease.
It lies in genes that have a subtler effect. Such genes work only a little less
well than their “good” versions. A breeder isn’t likely to notice a 5-10%
reduction in bodily function. But as levels of inbreeding rise these genes may
accumulate. Over time reproductive and health problems increase. The changes
are so gradual they are often blamed on diet, pollution and other environmental
causes. All of these may contribute but it is genes that make dogs susceptible.
Every individual has 3-5 “lethal equivalents.” These are an accumulation of
mutated genes that, if matched with like genes, would either kill the organism
early in life or prevent it from reproducing. The portion of lethality carried
by any one gene can range from 1-100%. A single lethal equivalent may be one
gene, 10 that reduce fitness by 10%, or a hundred that reduce it 1%. The mutt
that lives down at the corner probably has in excess of 100 such genes. Your
purebred Aussie likely has more because they have been inadvertently concentrated
by inbreeding.
Each puppy will inherit half of its lethal load from each parent. If the
parents are unrelated, there may be little or no impact on that puppy. But if
they are, the risk of pairing up genes that are part of the load increases. The
more ancestors there are common to both sides of the pedigree, the greater the
risk. This is why monitoring COI is important.
Historic inbreeding and recent or “close” inbreeding (matings
that would be considered incestuous among humans) are the primary factors
contributing to inbreeding depression in purebred dogs, with the historic
inbreeding being the greater factor since many don’t
recognize it as inbreeding at all.
There is a strong possibility that inbreeding depression may be becoming a
factor in Australian Shepherds. Reproductive problems do occur. Almost 25% of
the breeders who responded to the 1999 Australian Shepherd Club of America
breed health survey reported having dogs with reproductive failures (low sperm
count, lack of libido, failure to conceive, small litters or poor mothering
ability). This has sobering implications for the breed. There is no way from
this data to know the COIs
of the affected dogs, but such a large response indicates that breeders need to
pay more attention to reproductive issues.
Autoimmune disease data was gathered in the same survey. (Allergy data is being
excluded here because the survey did not distinguish between mild or isolated
allergy attacks and severe chronic conditions.) 17% of the respondents reported
having had at least one dog with autoimmune disease, another sobering
statistic.
Understanding the problem is one thing, but what can a breeder do about it?
Very few practice close inbreeding generation after generation. And breeders
aren’t in the business of breeding COIs, they want to
produce quality dogs. This can be accomplished by giving preference to assortative mating—the breeding of individuals of the
desired phenotype with preference given to those least related.
Say your bitch as a COI of 12%. You have looked at possible studs and narrowed
your list to three that you think are equally good. Dog A is from the same line
as your bitch; the puppies would have a COI of 21%. Dog B, an older stud who
hasn’t been bred often but has produced consistently good offspring, would
produce puppies that are 10%. Dog C, an up-and-coming star already booked for a
number of bitches, would produce puppies that are 8%.
Dog C might seem like the way to go at first glance, but he has high potential
for becoming a popular sire. Your puppies might wind up half-siblings to
everybody else’s. Dog A causes a significant increase in inbreeding. Dog B may
not give the lowest COI, but he does produce puppies that will be lower than
their dam and you are more likely to find less-related mates for them because
he has not been over-used.
We also need to do what we can to reduce the lethal load carried by our dogs.
We should not use animals that exhibit signs of inbreeding depression.
Consistent reproductive failures are an excellent argument against further
breeding. There is nothing more natural than reproduction. The dog that is
unable to produce, deliver or rear puppies without veterinary intervention or
considerable help from the breeder should not be bred. Chronically unhealthy or
unthrifty dogs are not good breeding candidates, even if they have no
identifiable hereditary disease. Nor are dogs with severe allergies or any
chronic autoimmune disease. Some of these diseases can affect reproduction,
thyroid disease being a prime example. In the author’s opinion, the
administration of thyroid hormone to bitches that will not settle so they can
produce puppies is foolishly short-sighted at best and highly unethical if done
by someone knowledgeable.
Even with the rapid rise of inbreeding levels in the Australian Shepherd to the
point that so many of our dogs are virtual half-siblings, our short history as
a registered pure breed and our broad founder base has given us the potential
to keep our breed healthy and viable. The Aussie is in much better shape than
so many other breeds and need not risk the extinction warnings of the opening
quotation. It is up to breeders to see that it stays that way.
Copyright 2002
C. A. Sharp. All rights reserved.
C.A. Sharp is editor of the "Double Helix
Network News", the quarterly newsletter for those interested in genetics
and hereditary disease in the Australian Shepherd.
Return
Home