Reproductive cycle in pets – part i

Reproductive Cycle in Pets: Canine
Just after the shortest day, we would like to give you a bit of an advance on spring and one of
the more normal phenomena that happen when the weather warms and the days grow longer.
We, predictably, experience a rash of phone calls from panicked owners concerning the
sudden onset of odd behaviours in their pets. Oftentimes, they report a decreased appetite,
whining, crying, frequent urination or odd postural reactions. A spring virus? An injury?
Infection? The first question asked by your veterinarian will likely be "Is your pet spayed?" If
the answer is "No," then read on for a lesson in your pet’s reproductive cycle.
Your pet’s reproductive cycle is regulated by hormones produced both in the brain and the
ovary. These hormones not only produce the changes in the reproductive organs needed for
pregnancy but cause some dramatic departures in your pet’s normal behaviour as well.
Hormones influence fertility and reproductive behaviour in both the dog and the cat, although
their heat and reproductive cycles vary by environmental and breeding behaviour.
Canine Reproductive Cycle
There is basically three phases to the female dog's reproductive cycle: the follicular, luteal
and quiescent phases. Each phase is under the influence of the dominant hormone produced.
It is really an amazing system of timing and feedback from the brain to the ovary and back.
The dog's ovary is a collection of eggs in different stages of maturation. Each individual egg
is actually enclosed in a tiny fluid filled sac called a follicle. A hormone produced by the
pituitary gland at the base of the brain sends a signal for the eggs to develop within their
follicles. This hormone is called follicle stimulating hormone (FSH) and causes the eggs to
mature. Once the eggs have fully developed, they too have the ability to produce their own
hormones. Follicles produce a hormone called estradiol 17B, and it is the dominant influence
on the "follicular phase" of the reproductive cycle.
In the luteal phase, the pituitary gets busy again and secretes a hormone called luteinizing
hormone (LH). The production of this hormone is triggered by the maturing follicles. LH acts
on the follicle to stimulate ovulation, the time when the egg breaks through the follicle so that
it can be fertilized. The egg then begins to travel down the Fallopian tube and the empty
follicle undergoes its own physical change. It becomes a corpus luteum, the Latin name for
"yellow body." It is named so because once the egg has ruptured, the follicle enlarges and
turns an easily recognizable yellow colour in contrast with the less mature follicles. The
corpus luteum now has its own job to do, and that is to produce progesterone, the hormone of
the luteal phase. It is in this phase that your dog exhibits mating and receptive behaviour or
signs of heat. You may see swelling of the vulva and bloody vaginal discharge that signal her
fertility to potential mates.
Since dogs have multiple births, several eggs mature and are released at the same time. If
fertilized, the eggs will implant on the uterine wall. This implantation results in a continued
release of progesterone, the hormone responsible for maintaining pregnancy. If none of the
eggs is fertilized, the body becomes aware that no eggs have implanted and progesterone is
no longer released. This results in the quiescent or quiet phase. This stage may last several
months, in which the dog will show no signs of sexual behaviour.
Most dogs cycle twice a year in this manner, but it is safe to say that the reproductive cycle is unique to the individual. There will be variations in the number of heat cycles per year, the level of behavioural change, and interest in mating.

Source: http://www.landmarkvets.co.nz/site/landmarkvets/files/Reproductive%20Cycle%20in%20Pets_Canine.pdf