In
domestic dogs, sexual maturity begins to happen around age six to twelve months
for both males and females, although this can be delayed until up to two years
old for some large breeds. This is the time at which female dogs will have
their first estrous cycle. They will experience subsequent estrous cycles
semiannually, during which the body prepares for pregnancy. At the peak of the
cycle, females will come into estrus, being mentally and physically receptive
to copulation. Because the ova survive and are capable of being fertilized for
a week after ovulation, it is possible for a female to mate with more than one
male.
Fertilization
typically occurs 2–5 days after ovulation; 14–16 days after ovulation, the
embryo attaches to the uterus, and after 7-8 more days the heart beat is
detectable.
Dogs
bear their litters roughly 58 to 68 days after fertilization, with an average
of 63 days, although the length of gestation can vary. An average litter consists
of about six puppies, though this number may vary widely based on the breed of
dog. In general, toy dogs produce from one to four puppies in each litter,
while much larger breeds may average as many as twelve.
Some
dog breeds have acquired traits through selective breeding that interfere with
reproduction. Male French Bulldogs, for instance, are incapable of mounting the
female. For many dogs of this breed, the female must be artificially inseminated
in order to reproduce.
Neutering
Neutering
refers to the sterilization of animals, usually by removal of the male's
testicles or the female's ovaries and uterus, in order to eliminate the ability
to procreate and reduce sex drive. Because of the overpopulation of dogs in
some countries, many animal control agencies, such as the American Society for
the Prevention of Cruelty to Animals (ASPCA), advise that dogs not intended for
further breeding should be neutered, so that they do not have undesired puppies
that may have to later be euthanized.
According
to the Humane Society of the United States, 3–4 million dogs and cats are put
down each year in the United States and many more are confined to cages in
shelters because there are many more animals than there are homes. Spaying or
castrating dogs helps keep overpopulation down. Local humane societies, SPCAs,
and other animal protection organizations urge people to neuter their pets and
to adopt animals from shelters instead of purchasing them.
Neutering
reduces problems caused by hypersexuality, especially in male dogs. Spayed
female dogs are less likely to develop some forms of cancer, affecting mammary
glands, ovaries, and other reproductive organs. However, neutering increases
the risk of urinary incontinence in female dogs, and prostate cancer in males,
as well as osteosarcoma, hemangiosarcoma, cruciate ligament rupture, obesity,
and diabetes mellitus in either sex.
Inbreeding depression
A
common breeding practice for pet dogs is mating between close relatives (e.g.
between half- and full siblings). In a study of seven different French breeds
of dogs (Bernese mountain dog, basset hound, Cairn terrier, Epagneul Breton,
German Shepherd dog, Leonberger, and West Highland white terrier) it was found
that inbreeding decreases litter size and survival. Another analysis of data on
42,855 dachshund litters, found that as the inbreeding coefficient increased,
litter size decreased and the percentage of stillborn puppies increased, thus
indicating inbreeding depression.
About
22% of boxer puppies die before reaching 7 weeks of age. Stillbirth is the most
frequent cause of death, followed by infection. Mortality due to infection was
found to increase significantly with increases in inbreeding. Inbreeding
depression is considered to be due largely to the expression of homozygous deleterious
recessive mutations. Outcrossing between unrelated individuals, including dogs
of different breeds, results in the beneficial masking of deleterious recessive
mutations in progeny.
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