[QUOTE=Damrock Farm;6152935]
why does this same thing not happen with small animals? Not really understanding how removing the ovaries, which also cause a mare to SHOW heat as well as NOT show heat, could cause her to go permanently into showing estrus.[/QUOTE]
The simple answer to this, and without getting too deep into the physiological differences between species, is that they are different species and the hormones behave differently in the different species! 
The more complicated version is as follows :
With a cycling mare, immediately prior to onset of estrus, PGF2a is released by the endometrium which destroys the CL and causes a drop in the circulating levels of progesterone - which hitherto has been causing a negative response to a stallion’s advances - and the mare enters a couple of days of proestrus condition. during this time frame, the response to a stallion will go from NO!! through “Ho Hum…”. As the developing dominant follicle grows and starts to secrete increasing levels of estrogen, the mare will become receptive to the stallion, as in the mare progesterone causes resistance to a stallion’s advances, while estrogen causes receptivity. Towards the end of the estrus phase, there is an increase in LH levels, which peak just after ovulation (which is stimulate in part by the LH) and those in turn stimulate the production of luteal tissue in the form of the CL, which secretes progesterone causing the mare to return to a resistant condition to stallion’s advances. This - as long as the mare remains non-pregnant and cyclical - continues for about another 12-14 days at which time the PGF2a is again release, the CL is again destroyed and the whole process is repeated.
In the bitch (for example), during the proestrus phase - which lasts around 9 days in the bitch - estrogen levels are elevated, but peak just prior to the onset of estrus and then decline. Note the difference here between bitch and mare. Estrogen doesn’t peak in the mare until about 24-48 hours prior to ovulation.
Once estrus commences in the bitch there is very quickly an LH surge (within about 2 days of the onset which brings about a preovulatory rise in progesterone caused by estrual luteinization of follicles and a ‘backup’ in progesterone from the estrogen synthesis. Progesterone levels continue to rise throughout estrus and level out only after the end of estrus and the onset of diestrus. Again, this is significantly different from the mare, in whom the LH peak does not occur until after ovulation and progesterone levels do not rise until after ovulation (typically of a single follicle) and subsequent formation of the CL.
Ovulation in the bitch occurs about 24-48 hours after the LH surge and takes about 24 hours for the multiple oocytes to be released. These oocytes however are not yet ready for fertilization as they are diploid (2N) and must undergo reduction division to become 1N, which takes 2-3 days. The mare however ovulates an oocyte already at Metaphase II and capable of being fertilized immediately. There is a difference therefore in receptiveness between the species, wherein a bitch is receptive with elevating levels of progesterone (they still gradually elevating during ovulation and oocyte maturation), whereas the mare ovulates prior to elevations in progesterone levels, those elevations being created by the CL the forms following ovulation.
Following the fertile period of estrus, both species enter diestrus and both have elevated progesterone levels. The difference here however is that in the mare the elevated progesterone level will remain that way for only the 12-14 days of diestrus (until the PGF2a release and destruction of the CL), whereas in the bitch, progesterone levels will decline to less than 1 ng/ml 60-100 days after the first day of diestrus, at which point she enters anestrus which (combined with the diestrus duration) will last through to something in the region of 180 days.
It is clear from the above that progesterone in the different species produces different effects - the mare is resistant in the face of elevated levels of progestins, while the bitch has the ability to be receptive and still breedable. In the bitch, resistance to a dog’s advances is not linked to elevated progesterone level, just as receptivity is not linked to elevated estrogen levels - estrogen level peaks in the bitch during the proestrus period, before the onset of estrus; in the mare, receptivity is stimulated by elevated estrogen levels which peak shortly before ovulation. The stimuli in the bitch are therefore not specifically ovarian-related (although there may be some connection), but are probably related to other sources, with adrenal sources being a major consideration. Removal of the ovaries therefore in the bitch will not significantly affect the receptivity/resistance levels, with the exception of the fact that there is no estrus phase, and therefore no periodic stage of significant receptivity. In the mare, the interplay of progesterone and estrogen produces a greater effect in resistance and receptivity (respectively) and therefore ovarian removal is going to have a greater impact.
In the mare the issue is exacerbated by the issue of adrenal hormone production. This effect is seen in mares in winter anestrus that still appear to “cycle” even though they are not ovulating. In fact, this is a very useful analogy related to your question. If we were to think of winter anestrus mares as ovariectomized mares (they not having functional ovaries), we could come up with a not unreasonable comparison. In the winter anestrus mare - that is the mare that has truly “shut down” cyclically and is not ovulating - one will see differing responses if teased by a stallion. Some of those mares will have a “ho hum” attitude, not showing receptivity or resistance; some may show a degree of resistance (although typically not as pronounced as when they are cycling); while others will actually tease and be openly receptive to the stallion. This is because of adrenal secretion of estrogens causing a degree of receptiveness, or progesterone causing resistance. If the ovaries are removed rather than being inactive, the same effect can be seen - one will see some mares that have periods or resistance coupled with periods of receptivity; others will be “ho hum” pretty much all of the time; while others - as I noted in my previous post of the ovariectomized mare - may demonstrate continued receptivity.
Hopefully the above answers your question adequately!
On the subject of Depo-provera (depot medroxyprogesterone acetate): McKinnon et. al. showed in multiple research projects that the drug was not capable of maintaining pregnancy in the absence of an endogenous progestin source. This then has to raise the question of it’s ability to raise progesterone levels >1.8 ng/ml. In other research (I believe it was McCue et. al.) it was demonstrated that it did not have an effect in estrus suppression. Empirical evidence by some offers a suggestion to the contrary, although typically this is not provided under controlled circumstances (such as those offered by McCue if that was who it was that did the research) and one therefore cannot rule out a placebo effect for the person evaluating the situation. Bottom line on does it work? Science says no, some people say yes.
Hope this helps on all counts!
