Canine Endocrinology and the Hormones of Reproduction: A Lay Person's Primer

There are numerous websites describing the hormones of reproduction in the dog. Many are very good, but many of these same sites deal with hormones of reproduction from a very clinical, biological, and text book perspective. This page is meant to be different in that I deal with the topic of canine endocrinology for the lay person. To begin then, let's do away with the biological term canine endocrinology and use an alternative, hormones of reproduction in the dog. The two phrases means pretty much the same thing...but the latter term, hormones of reproduction, is more focused as we will not concern ourselves with any hormones that have no bearing on reproduction.

Estrus

stage when a bitch is receptive to breeding

Estrogen

hormone responsible for many of the outward/physical indications of the heat cycle such as flagging and interest from stud dogs¹

Luteinizing hormone (LH)

hormone that causes follicles to be released from the ovaries

Ovulation

the release of a mature oocyte from a follicle

Proestrus

stage when the body prepares for estrus. Indications are bleeding/discharge, elevated estrogen levels, and gradual cornifying of vaginal cells

Progesterone

hormone responsible for maintaining reproductive organs and pregnancy

Dealing only with proestrus and estrus let's look at the graph. When the bitch begins her heat cycle she is in proestrus. As breeders, we note the onset of proestrus with the first spotting of blood. This we say is the first day of proestrus or day one of her heat cycle. During proestrus, estrogen is elevated but progesterone remains at baseline levels (certrainly less than 1.0 and frequently just above 0.0 [example: 0.12]). For this reason progesterone levels themselves are not a good indication that the bitch is in season, but breeders can rely upon blood discharge and vaginal cytology (determining the proportion of cornified vaginal cells) to tell us the bitch is in heat.

At the end of proestrus, LH spikes. This is due to a feedback system triggered by the prolonged elevation of estrogen. The LH spike is very brief however and while medical science can test for LH levels very accurately, the test would need to be ran once or twice daily to catch the LH spike. The LH spike is important for it indicates that follicles have been released from the ovaries and estrus has begun.

Now, a follicle is not an egg and cannot be fertilized. It has a ways to go and some changes to make first. While making these changes, progesterone levels increase along a fairly predictable pattern and once progesterone levels hit 5 ng/ml we know that ovulation has occured. At this point, we have an immature oocyte and not a follicle...but still the oocyte cannot be fertilized. No...we have a few more days to wait.

At this point we are on day one of ovulation but may be on day 28 of the bitch's heat cycle. It is important to stop thinking in terms of days now and start thinking in terms of hours. You see, it takes 48 to 72 hours for the immature eggs to mature (and with the brief LH surge we know the follicles were all released over a very short time span). Once matured at 72 hours (that's day three for those still thinking in days) they are ready to be fertilized. The eggs will remain viable for about 48 hours and by the end of this period the eggs are dead (well, they were never really alive, so we should say they are no longer viable). This marks the end of estrus and then end of the heat cycle.

What can we learn from all this? Well, there is one key point, if you are using anything but natural breeding you will want to inseminate on day three or four...anything later can result in missing a breeding. For this reason, it is important to know when the bitch ovulated and home based kits just do not have the accuracy to give a reliable answer.

If the wheels have been turning in your head while reading this, you probably have a questions that goes something like this:

• Why wait to breed on day 3 after ovulation...why not inseminate as soon as possible after ovulation and just let the sperm hang around while the eggs mature?
• This sounds very similar to the standard dating game. You know ,the guy shows up at the girl's front door but she is not quite ready to leave...just a few more minutes. For a young man this is Ok, but waiting is not good for sperm...they are on a schedule you see, and under natural breedings they can stick around for maybe four days. However, with fresh chilled semen they cannot wait that long as these sperm are only viable for about two days. Frozen semen has an even shorter lifespan and it is safe to assume only 24 hours of viability when using frozen semen.