Here is an article I wrote on female dogs, and the hormones which govern sexual behavior:
Like most mammals, dogs are ruled hormonally by their pituitary gland. One of the most important hormones in a bitch's life is the hormone FSH, which stimulates the production of estrogen. In all mammals EXCEPT humans, an increase in estrogen production is needed to allow a full display of sexual behavior. Under the influcence of estrogen, bitches might be more active, will urinate more, at least certainly in the presence of dogs, and will behave in a receptive manner with male dogs or even with their owners. A female in heat will sometimes cock her leg to urinate and seem to increase her territory markings with urine. Under the influence of estrogen, some bitches will whine and moan more, others will be more nervous and irritable, some will even mount males or non-estrous females. There are, however, certain false myths about breeding in bitches as there are with dogs. It is not necessary, for example, for a bitch to have a litter. It will not make her more emotionally mature or stable.
Under FSH influence, estrogen is seasonally produced in the ovaries of the dog but then, after ovulation, the other female hormone, progesterone, becomes dominant. Progesterone is an amazing chemical, thought to have a calming effect on the dog's mind. Certainly in large doses, it has a sedative effect. In fact, modified only very slightly, it is actually used as an anaesthetic in animals. While estrogen increases in the dog's body for a short length of time, progesterone remains in the circulation, influencing the brain for two months after each estrus and can have a dramatic effect on canine behavior. The most common behaviors are those associated with pregnancy: nest building, guarding possessions, and milk production. In most instances the behavior is not associated with a true pregnancy, but rather with a false hormonal pregnancy, a progesterone induced false pregnancy.
The behavior of a bitch experiencing a false or phantom pregnancy can change dramatically. The sedative effect of progesterone on some bitches can be stultifying and it is not uncommon that pet owners bring in their post-estrous bitches to the vet thinking that they are seriously ill when in fact they are only subdued. This is a very subjective diagnosis. One bitch I owned, Leonora, became overwhelmingly subdued for two months after each season, so much so that I was not afraid to call it a depression. She sat or lay under tables or behind furniture and flatly refused to play with anyone or anything. Then, as if by magic, over a period of a week or so, she snapped out of this state of mind and reverted back to her natural playful state.
Some bitches will experience changes in their taste buds. After ovulation, some will only eat specific things, such as meat, and won't touch anything else. A favorite treat will not tempt them in the slightest.
Guarding toys, dolls, rags, slippers or anything else that can be carried is another common behavioral consequence of the surge in progesterone. Progesterone, like estrogen, influences behavior by acting on the brain. This is why it is now frequently used as a treatment in behavior modification in male dogs. Dog pups, as mentioned in the earlier article titled "Castration of the Dog: Pros and Cons", have a surge of testosterone right before or right after birth that 'masculinizes' their brains. Females don't have the equivalent. The lack of the secretion of ANY hormone at birth allows for the development of a 'female' nervous system.
Giving estrogen to a male dog can actually stimulate male sexual activity, but progesterone on the other hand seems to have a sedating effect and it is likely that this sedative effect makes the dog more amenable to behavior modification therapy. The calming effect of the high level of progesterone is also the reason why it is best to avoid spaying bitches for two months after esterus. Spaying during this time can result in a precipitous drop in progesterone levels with possible accompanying emotional disturbances, irritability, aggression and depression.
Whether to spay, and if so when to spay, is a perennial question in veterinary medicine. If pet owners do not plan to breed from their bitches, most vets strongly recommend spaying in most instances before the dog's first season. Unlike male dogs, females come under no sex hormone influence until they reach puberty. Suddenly a short sharp surge of estrogen, then a prolonged two month surge of progesterone, dramatically alter the dog's behavior until both hormones subside and the dog gets back to "normal" again, back to 'anestrus'. A male dog lives with a constant supply of male hormone circulating in his body and influencing his mind. Females, on the other hand, only come under sex hormone influence typically twice yearly for a total of four months, although there are some bitches that will come into estrous three times yearly.
During that time, and specifically, under the influence of progesterone, ther can be dramatic departures from previous behavior patterns. The most unpleasant is the possesiveness that can develop over objects. Once they develop this hormone induced behavior, it becomes a learned behavior that can continue for life, independent of whenther the bitch is spayed in the future or not. As long as a female is anatomically mature and doesn't have an infantile vulva, then the safest time to spay her is before her first season. That way she will probably never suffer from mammary tumors, the most common tumors in bitches, and she will never suffer the emotional upheavals of estrus and the consequent and normal phantom pregnancy. Spaying affects a dog's ability to work only in that it saves the dog from losing the ability to work because of hormonal surges. Spaying doesn't change the personality of a bitch. If anything, it preserves the natural personality of the animal. The only potential behavioral consequence of spaying is that inherently dominant bitches might become more dominant. Spaying prevents estrous cycles but it does not make a dog less feminine. It can't because in the absence of the testosterone influence at birth, a dog's brain simply IS feminine, as is the human and other mammilian brains. Spaying can, however, stimulate a dog's appetite and alter her metabolic rate. This is why some spayed dogs become overweight. This potential problem can easily be avoided by reducing the calorie intake by around 10-20% after spaying, and by ensuring that the bitch continues to get sufficient exercies.