Margaret V. Root Kustritz, DVM, PhD, DACT

AnSc/VM 435F – 612-624-7290 –  rootk001@umn.edu


Embryology is the study of development of the fertilized egg to a complete individual. An understanding of embryology allows clearer understanding of production and effects of hormones during fetal life and after birth.

The fertilized egg undergoes (1) cell proliferation to increase the number of identical cells present, (2) cell growth, with synthesis of new cellular substance and increase in water uptake by the cells, (3) differentiation of cells into different types, and (4) integration, formation of different types of cells into systems that are cooperative (for example, the circulatory system).

The first stage, consisting of creation of a large number of identical cells, is called cleavage. The original fertilized egg is one cell, which cleaves to form two cells. The two cells cleave to form four and so on. Once 32 cells are present, it is called a morula. The 32 cells are within the zona pellucida, the tough outer covering of the egg.

During the second stage the cells move to the periphery of the sphere, against the inside of the zona pellucida, to form a hollow ball called a blastocyst. Those cells on the outside go on to form the placenta and those attached inside, the inner cell mass, go on to form the embryo. The zona pellucida ruptures to allow a great increase in cell size and number; this is called blastocyst hatching.

A sheet of cells from the inner cell mass grows out in all directions to line the interior of the blastocyst. This is endoderm tissue, which goes on to form the primitive gastrointestinal tract of the embryo. At this point, the developing embryo is called a gastrula. This endoderm tissue eventually develops into the tissue lining the gastrointestinal and respiratory tracts, the liver, and the pancreas.

Most of the cells of the inner cell mass rise to the surface to form the embryonic disc. This tissue forms ectoderm tissue, which eventually develops into this skin, nervous system, sensory parts of the eye and ear, mammary glands, and parts of the placental tissue.

Finally, a layer of cells from the inner cell mass grows out in all directions between the ectoderm and endoderm. This tissue, the mesoderm, eventually develops into the blood vessels, urogenital system, and all connective and supporting tissues, including all types of muscle.

The ectoderm thickens where the head will form, creating the neural plate. Folds develop longitudinally to form the neural tube. Mesoderm on either side of the neural tube segregates into masses called somites, which will form muscle and bone. Mesoderm lateral to the somites forms into the major organ systems. At this point, about 20 days from fertilization of the egg, the embryo is called a tailbud embryo and it contains both ancestral and adult forms of the individual.

The tailbud embryo assumes a “C” shape. The neural tube is aligned along the midline of the dorsal, convex surface. The internal body cavity is formed and a primitive heart is beating. The neural tube separates into the parts of the brain and spinal cord. The eyes, major blood vessels, abdominal organs and limbs begin to form and function. By 30 days, embryonic development is virtually complete and by day 35 the offspring is more correctly termed a fetus; at this point, development is finished and growth is the primary occurrence for the rest of gestation.

The early embryo is indifferent, meaning it is of no specific gender. Cells from the primitive gut migrate to a ridge near the primitive kidneys termed the gonadal ridge. These cells can go on to become either testes or ovaries and so are called primordial germ cells. Whether the individual will become male or female is dependent on the chromosomes that individual has.

Male individuals have both an X and a Y chromosome. In the presence of a Y chromosome, cords of tissue grow from the surface of the indifferent gonad into the center, entrapping primitive germ cells. These cords of tissue differentiate into seminiferous tubules and the primitive germ cells differentiate into spermatogonia (immature spermatozoal cells). Cells that develop between the seminiferous tubules, called interstitial cells or Leydig cells, secrete testosterone. Sertoli cells, which develop along the basement membrane of the seminiferous tubules, secrete the hormone Müllerian inhibiting factor.

Two ductal systems develops in the embryo, near the developing kidney. In males, the paramesonephric ducts, also called the Müllerian ducts, are inhibited from further development by Müllerian inhibiting factor, and the mesonephric ducts, or Wolffian ducts, are stimulated to develop by presence of testosterone and its metabolite, dihydrotestosterone.

Female individuals have two X chromosomes. Lack of testosterone stimulates ingrowth of cords into the interior of the ovary in two waves. Primitive germ cells are entrapped in these sex cords, which break up to form individual germ cells surrounded by a single layer of cells, termed primordial follicles. In the absence of Müllerian inhibiting factor, the paramesonephric ducts develop and the Wolffian ducts regress.


Endocrinology is the study of hormones. Endocrinology is a huge subject and a comprehensive review is well beyond the scope of this lecture. Some generalities of endocrinology and the glands and hormones of interest to small animal reproduction will be described.

Hormones are chemical substances secreted from glands into surrounding tissue or carried to other tissues through the bloodstream. These chemicals affect only certain tissues and therefore evoke a specific response. Tissues cannot respond to a hormone unless that tissue contains receptors for that hormone. Receptors act like gates on the surface of the cell with hormone acting like a key; if the key fits the gate, a response occurs within that cell.


Hormone secretion is not constant. A physiologic or neurologic stimulus promotes or prevents release of a given hormone from its site of production or storage. Positive stimuli cause release of a hormone while negative stimuli prevent its release. Physiologic stimuli that may cause hormone release include changing hormone concentrations in the body and changes in other physiologic parameters in the body, such as blood pressure or concentration of sugar in the blood. Neurologic stimuli include physical stimulation, such as suckling of puppies, and environmental changes, such as increase in daylength or stress; one study described how the stress of transportation of a large cat colony across the country induced heat in 76% of the queens within 18 to 31 days.

Pheromones, chemical substances that alter behavior or physiology of another animal through olfaction, are an important part of reproductive behavior in animals. Some pheromones induce a signaling effect, in which the pheromone induces a behavior change in another animal not associated with a physiologic change. Others induce a primer effect, in which the pheromone induces a physiologic change in another animal. It is assumed , but not proven, that pheromones induce hormonal changes in dogs and cats associated with reproduction. Pheromone effects are well described in the mouse (Table 1). In the dog, the pheromone produced by females identifying her as being in heat has been chemically elucidated (methyl-p-hydroxybenzoate) and a commercial product containing this compound marketed for enhancing male libido. An associated compound, methylparaben, often is used as a preservative in shampoos and may be a cause of male dogs exhibiting sexual interest in female dogs that are not in heat. Female dogs housed together eventually will go through heat together (“dormitory effect”). Male dogs identify female dogs as being in heat by investigating the female’s urine, anal gland secretions and vulvar secretions, which presumably contain these products. The dog flicks his tongue rapidly against the inside of the upper incisors to present these secretions to the vomeronasal organ. In cats, the vomeronasal organ is located within the nasal cavity; male cats will investigate a female and then stand with head outstretched, breathing in slowly, to present these volatile compounds to the vomeronasal organ. This behavior is called flehmen.

Table 1. Phermone-based effects of murine reproduction



Bruce effect

Introduction of a male to females pregnant by another male induces pregnancy loss and subsequent return to estrus

Whitten effect

Introduction of a strange male to a group of females induces synchronous estrus

Lee-Boot effect

Females housed in groups exhibit longer estrous periods than do females housed alone

Vandenbergh effect

Juvenile females exposed to males exhibit earlier onset of puberty

Koyama S, 2004

Males exposed to soiled bedding from estrous females have increased density of spermatozoa in their ejaculate

Another important concept in control of hormone secretion is feedback. Rising concentrations of a given hormone in the bloodstream usually “feed back” to the stimulus that caused release of the hormone, stopping further production. An example is testosterone secretion. The release of interstitial cell stimulating hormone from the pituitary gland stimulates release of testosterone from the testes. As concentrations of testosterone rise in the bloodstream, this signals the pituitary to stop release of interstitial cell stimulating hormone, which in turn stops further production of testosterone This is called a negative feedback loop. When concentration of testosterone in serum falls, the pituitary is again stimulated to release interstitial cell stimulating hormone and the cycle starts again. This is an important concept because it alters how we use hormones for therapy. Any hormone introduced into the body may exert negative or positive feedback on glands and tissues, altering the animal’s ability to produce or secrete that hormone now and in the future. Caution must be employed when introducing hormones into the body.



The pineal gland lies on top of the brain. The primary hormone produced by the pineal gland is melatonin. Melatonin secretion is coupled to daylength; decreasing daylength stimulates melatonin release. Melatonin decreases reproductive activity in most species and has been associated with decreased production of estrogen and other reproductive hormones from the ovary and with an increased refractory time after mating before a subsequent mating occurs.

Melatonin secretion and its effect on reproduction are important in understanding the effect of environmental clues for seasonal breeders. Domestic dogs are not seasonal breeders; litters are born during all months of year. Wild dogs, coyotes, wolves and one breed of domestic dog, the Basenji, are seasonal, usually cycling once yearly. Time of year that seasonal breeders cycles varies with geographic location. At this latitude, wild dogs cycle in the spring. Cats are seasonal breeders; cats are considered to be seasonally polyestrous, meaning they go through heat cycles continuously over the breeding season unless that is interrupted by pregnancy or disease. The seasonal effect of reproductive activity in cats lessens as one nears the equator. Cats at this latitude show reproductive activity from about January to October. This is very dependent on daylength. Cats can be induced to be reproductively active all year long if housed so that they never see natural light and get artificial light for at least 12 hours daily. Male cats may or may not show a seasonal effect. In both domestic cats and big cats, it has been shown that semen quality and concentration of testosterone in blood may be lower during the non-breeding season but not so decreased as to make the cat infertile.


The hypothalamus lies at the base of the brain. It has a unique blood system connecting it to the pituitary gland, which lies beneath it. This blood system, the hypophysial portal system, carries hormones from the hypothalamus directly to the pituitary without circulating it elsewhere in the body. This means that only very tiny concentrations of hypothalamic hormones need be produced to stimulate an effect in the pituitary gland.

The hormones produced and released from the hypothalamus all cause a direct positive or negative effect on secretion of hormones from the pituitary. The hypothalamic hormone of greatest interest in small animal reproduction is gonadotropin releasing hormone (GnRH). GnRH is a very small protein hormone. The stimuli causing release of GnRH are not well defined but probably include environmental stimuli and feedback from secretion of pituitary hormones. GnRH causes release of follicle stimulating hormone (FSH) and luteinizing hormone (LH) form the pituitary. Release of GnRH is not continuous but occurs as regular pulses over the day. These pulses of GnRH cause pulsatile release of LH and FSH, which in turn cause pulsatile release of testosterone from the tests. This pulsation is important; continuous administration of GnRH is associated with decreased production of spermatozoa from the testes. Control of secretion of GnRH is not completely understood. Compounds that may stimulate or inhibit GnRH secretion include dopamine, serotonin and other brain chemicals. In female cats, GnRH is released after breeding. Queens will not ovulate unless induced to do so. It has been shown that after having been bred an average of four times, virtually all queens produce significant enough amounts of GnRH to stimulate release of LH from the pituitary and subsequent ovulation.


The pituitary is a bilobed gland at the base of the brain. The two lobes are the posterior lobe, sometimes called the neurohypophysis, and the anterior lobe, or adenohypophysis. The two lobes arise from different tissue during embryologic development. There are no hormone-producing cells in the neurohypophysis. Rather, nerve cells that lie in the hypothalamus produce hormones that are stored in the posterior lobe. The anterior lobe is a true gland; hormones are produced and stored in the anterior lobe.

The hormones of the posterior lobe are oxytocin and vasopressin. Vasopressin is involved in kidney function and will not be discussed. Oxytocin is a peptide hormone, released from the posterior pituitary secondary to neurologic stimulation. The two stimuli best documented to cause oxytocin release are pressure of the head of a puppy into the cervix during whelping (Ferguson’s reflex) and suckling of the mammary glands by the pups.

During breeding, oxytocin may cause smooth muscle contractions in the female’s reproductive tract, improving movement of spermatozoa to the site of fertilization. Oxytocin causes synchronized contractions of the uterus during parturition and contributes to further dilation of the cervix. During lactation, oxytocin stimulates letdown of milk into the mammary glands. Oxytocin concentrations are high in dams while they are nursing. Oxytocin also has been associated with increased sexual receptivity in some species and may be involved in formation of the normal pair-bond between dams and offspring. In humans, inhalation of oxytocin stimulates trust in social relationships.

The anterior lobe of the pituitary produces and stores the majority of the hormones in the body. For this reason, the pituitary sometimes is called “the master gland of the body.” Hormones secreted from the anterior pituitary include FSH; LH; prolactin; thyroid stimulating hormone (TSH); and adrenocorticotropic hormone (ACTH).

FSH is a large protein hormone secreted both in males and females. It is named for its role in the female, in which is stimulates maturation of ovarian follicles in preparation for ovulation. In males, FSH stimulates production of a protein in the testes that binds testosterone tightly within the testis to promote spermatogenesis.

LH, another large protein, also is named for what it does in the female. Luteinization in dogs occurs in the ovarian follicles around the time of ovulation. In cats, LH release occurs after breeding. LH also is called interstitial cell stimulating hormone, named for its function in males. In male dogs, LH stimulates release of testosterone from the testes, which is necessary for continuing production of spermatozoa. LH production is controlled by a feedback loop with testosterone, as previously described.

Prolactin is a protein hormone that induces milk production and stimulates normal maternal behavior. Its production is stimulated during pregnancy and concentrations of prolactin in blood rise sharply at the time of whelping, as other hormone concentrations abruptly decrease. Hyperprolactinemia has been associated with abnormal reproductive behavior and function in humans. Prolactin also may be involved in the refractory period exhibited by most animals for a time after mating; in humans, it has been shown that prolactin concentrations in blood increase after orgasm.


Which hormone the ovary is producing is dependent on the structures present on it. Mature follicles, also called Graafian follicles, contain a single ovum suspended on a hillock of cells within a pool of fluid. The wall of the follicle is lined with granulosa cells, which secrete estrogen. The predominant estrogen secreted in dogs is estradiol-17β. Estrogen causes the physical and behavioral changes we associate with estrus, or heat, in dogs, including vulvar swelling, exudation of serosanguinous vulvar discharge, and attraction of male dogs.

There are two types of estrogen receptors, named estrogen receptor α (ER α) and ER β. Distribution of estrogen receptors within the brain varies, presumably because activation of these receptors elicits different behaviors. Concentration of estrogen receptors in the brain also varies with physiologic status of the animal; fasted animals show a decline in receptor number and a decline in mating behavior. Estrogen may stimulate genes for production of opioids and oxytocin; these are thought to provide mild pain relief and to decrease anxiety. This may be what allows a female to stand to be bred when such contact usually would be considered an attack. Finally, naturally occurring products in plants that activate estrogen receptors, phytoestrogens, may be associated with irregular heat cycles in some species.

After ovulation, the ruptured follicles fill with blood and the granulose cells are transformed into luteal cells to form corpora lutea, which secrete progesterone. Progesterone is responsible for pregnancy maintenance and to that end, it promotes secretion of intrauterine glands to support the fertilized eggs, stimulates development of the mammary glands and induces maternal behavior. Progesterone can, in some circumstances, activate oxytocin receptors. Progesterone receptors are activated more quickly if the animal is primed with estrogen. Behavioral effects of excessive progesterone are inhibitory; feral cats receiving an oral progesterone-type compound in food showed fewer heat cycles, lack of sexual interest and loss of “social status.” These changes probably are due to negative feedback of progesterone to the hypothalamus and pituitary, decreasing pulsatile release of GnRH and FSH. Abrupt decline in progesterone concentrations and a subsequent rise in prolactin cause many of the normal mothering behaviors seen in dogs, including nesting and maternal aggression. Loss of progesterone at birth is thought to be part of the complex causing “the baby blues” in women.


During embryonic development, the testes secrete Müllerian inhibiting factor, which prevents development of the female tubular reproductive tract, and testosterone, which stimulates development of the male tubular reproductive tract. Continuing secretion of testosterone is necessary for continuing spermatogenesis in male dogs. Testosterone and its metabolite, dihydrotestosterone (DHT) are responsible for normal breeding behavior.


As described above, normal male dogs and cats are exposed to significant amounts of testosterone and DHT during development. Normal female dogs and cats are exposed to significant amounts of estrogen. Timing and concentration of testosterone and estrogen exposure must be correct for normal fetal development to occur. Occasionally, a bitch or queen will be exposed to chemicals, including medications and products naturally occurring in plants, which exert a hormonal effect and cause defects in the puppies or kittens.

If the bitch or queen is exposed to testosterone or other androgens during pregnancy, the female offspring will be masculinized. Physical abnormalities of the internal reproductive tract may be present as may abnormal external genitalia. Severely masculinized females have no vulva or vagina and a clitoris so enlarged as to resemble a penis. These females never show signs of heat or normal mating behavior. A similar circumstance has been reported to occur in female pups or kittens that lie between two male littermates in the uterus, presumably because the female is exposed to the male siblings’ testosterone. However, this effect is unlikely to be significant in the majority of cases because the placentas of puppies and kittens do not connect and there is no transfer of blood directly between them.

Feminization of offspring due to exposure of the dam to estrogen occurs very rarely. Feminization more commonly is associated with abnormalities of chromosome number. Calico (orange, black and white) and tortoiseshell (black and orange) cats provide the best example. The gene controlling the colors black and orange is on the X chromosome. Since male cats should have only one X chromosome (38, XY), they can be black or orange but not both. Male cats demonstrating both colors must have more than one X chromosome (39, XXY or 38, XY / 38, XY chimera). These cats usually are infertile and may or may not show normal mating behavior.

Male dogs and cats require exposure to testosterone before and immediately at the time of birth if they are to show normal development and normal reproductive behavior as adults. The exposure to testosterone at birth is a priming effect; animals not exposed to testosterone at birth could not respond to the presence of testosterone at puberty with normal breeding behavior.

Exposure to estrogen at the time of birth may be important for later reproductive success in both males and females. In male laboratory animals, lack of estrogen exposure at birth has been associated with delayed onset of puberty and decreased frequency of mounts when breeding.

Oxytocin is first secreted to a significant extent in rodents in the early postnatal period. However, experimental administration of oxytocin has been demonstrated to delay puberty onset in female rats. The exact role of oxytocin secretion at birth in dogs and cats, if any, is unclear.


Sexually dimorphic behaviors are those that differ due to gender (Table 2). Be aware that sexually dimorphic behaviors may occasionally be seen in animals of the “wrong” gender in normal circumstances. For example, although male dogs exhibit urine marking more frequently than female dogs in general, intact female dogs are more likely to urinate frequently and to direct urine toward objects when they are away from their home area.

Table 2. Sexually dimorphic behaviors of dogs and cats




Urine marking (frequent urination, directing urine at objects)



Lifting leg to urinate



Squatting to urinate



Urine spraying (vertical surface)






Mounting other animals, people, inanimate objects



Aggression towards individuals of the same gender



Investigation of urine, hindquarters of individuals of opposite gender



Flagging, standing to be bred



Nesting and other maternal behaviors




Sexually dimorphic behaviors also may vary depending on the location and physiologic state of the animals involved. For example, dispersal of juvenile wild dogs from the territory into which they were born varies with number of animals in the home territory and presence or absence of the parent of the opposite gender. Presumably, this variation helps prevent inbreeding and competition for mating opportunities. Aggression between animals is much more common in males than in females and occurs in both dogs and cats. However, when space is not at a premium, dogs show minimal aggression to strange male dogs. Aggressive encounters were much more common when dogs encountered unfamiliar dogs more frequently due to space constraints or when females in heat were present, stimulating competition for mating. The presence of unfamiliar males also is more common when a bitch is in estrus; males have been documented to roam up to 5 miles to find a bitch in heat.

Castration and ovariohysterectomy (OHE) often are suggested as means to control problem behaviors in dogs and cats. Dogs that have undergone OHE no longer exhibit the behaviors of heat which, while normal, are annoying to many owners. OHE is not associated with beneficial changes in behavior, nor is it associated with detrimental changes, except perhaps an increase in indiscriminate appetite. Conversely, allowing bitches to go through one heat or to be bred before OHE also has not been demonstrated to cause beneficial changes in behavior.

Castration can be an effective technique for controlling some negative behaviors of male dogs and cats. The effect is greatest for sexually dimorphic behaviors, such as urine spraying in male cats, and urine marking, roaming and mounting behaviors in male dogs. Neither the age of the animal nor the duration of time he had exhibited the undesirable behavior prior to castration were correlated with likelihood of resolution. Castration has not been demonstrated to have a significant impact on aggressive behaviors in male dogs but will decrease intermale aggression in male cats. Finally, castration may be associated with faster progression of age-related cognitive impairment in dogs, suggesting a protective effect of testosterone on brain function at some level.


A) PUBERTY             

Puberty is defined in females as the first heat period and in males as the first appearance of semen quality adequate to effect pregnancy and normal breeding behavior. In most animals, puberty occurs when animals reach about 80% of their adult body weight. As you might expect, puberty onset is earlier in small breed than in large breed dogs. Average age at puberty in male and female dogs is 7 months, with a range of 6 to 24 months.

Puberty onset in cats is dependent on body weight, age, breed, and season of the year. Most cats will not enter puberty any earlier than 4 months of age. If they reach the appropriate age and weight during the non-breeding season of the year, they will not become reproductively active until the breeding season begins. In general, short-haired breeds enter puberty earlier and cycle for more of the year, while long-haired breeds and Manx cats enter puberty later and are more likely to have a prolonged anestrus (non-cycling) portion of the year.

The first, or pubertal, heat of bitches is not characteristic of those that will come later. Oftentimes, bitches do not show the entire range of physical changes expected of heat and will not stand to be bred. Since most bitches are too young to be bred at the time of their pubertal heat, this is not a concern in clinical practice. The pubertal estrus of most queens is a fertile estrus and queens can be successfully bred. This is important to stress to cat owners, to ensure siblings are separated early in life to prevent accidental inbreeding.


The estrous, or heat cycle, consists of four stages (Tables 3 and 4). The hormonal and behavioral aspects of each stage will be described in detail for dogs and cats.

Table 3. The estrous cycle of the bitch






Estrogen rising

Vulva swollen and turgid, bloody vulvar discharge

Male attracted, sniffs at bitch’s urine and vulvar secretions, licks her perineal area, may stop eating and exhibit increased aggression toward other males and general increased activity. Female will not allow mounting and breeding, may snap at male.


Estrogen falling, progesterone rising

Vulva swollen and soft, bloody to straw-colored vulvar discharge

Male attracted as above. Bitch shows flagging (lateral elevation of the tail) and allows male to mount and breed.


Progesterone high

Slight mucoid discharge?






Table 4. Estrous cycle of the queen






Estrogen rising


Male attracted, licks queen’s perineal area, may stop eating and exhibit increased aggression toward other males and general increased activity. Female will not allow mounting and breeding, usually passive if male attempts breeding. May show increased rubbing, perceived as affection by owner.


Estrogen falling, progesterone rising if cat induced to ovulate


Male attracted as above. Female shows lordosis (elevation of hindquarters, lateral deviation of tail), increased vocalization and increased rubbing. Female allows male to mount and breed.






Progesterone high








Proestrus, the first stage of the estrous cycle, is a time of preparation for fertile breeding. The follicles on the ovaries are maturing to the point where ovulation can occur. The vaginal lining thickens to form a layer of cornified cells, presumably to decrease discomfort for the bitch or queen during breeding. Inexperienced males are interested during this stage but experienced males often are not. Male dogs determine if a bitch is in heat by licking and sniffing at her urine, anal gland secretions and vulvar secretions. The pheromones in these secretions are pulled into the vomeronasal organ in the mouth and nasal cavity. Some species of animal exhibit “flehmen”, a pulling back of the lips and nostrils that enhances movement of inhaled substances to the olfactory bulb. Male dogs and cats do not exhibit flehmen because their lips are too firmly attached to their upper jaw to allow much movement. Instead, many animals will stand with their head erect and neck extended for a moment. Frequency of urination of the bitch increases as she progresses through proestrus and oftentimes the male will urinate over the bitch’s urine, presumably to “hide” her scent from competing males. Some male dogs are able to determine not just if the bitch is in estrus but if she’s at the optimal time for breeding; presumably this, too, is pheromone-mediated.

Male cats determine if a queen is in heat by sniffing and licking at her perineal area. Investigation of urine is difficult due to the normal, secretory urinary habits of cats and queens do not produce vulvar secretions during proestrus and estrus. Most queens do not go through a discernible proestrus and stand to be bred as soon as the male is interested. The sniffing behavior shown by male dogs and cats is associated with an increase in the concentration of testosterone in their blood.

Estrogen stimulates these physical changes and these behaviors. It has been demonstrated that information from the olfactory area of the brain and from the genitalia stimulates pathways to the spinal cord and the brain, readying the animal for copulation. Estrogen induces development of new neural connections along this pathway.


Estrus is standing heat. By its simplest definition, an animal is not in estrus unless they will allow breeding to occur.

Female dogs may passively accept the attentions of the male or may show soliciting behavior, consisting of licking at the male, approaching with her perineum exposed or running away (“teasing). The male dog investigates the female and, if she stands and is not hostile, will attempt to mount. Normal breeding behavior takes practice; 97% of experienced males mount the hindquarters of estrous bitches, while only 39% of inexperienced males mount the bitch at her hindquarters. The amount of time before the male mounts and the number of mounts he attempts before intromission (introduction of the penis into the vagina) occurs is variable and is not associated with fertility. The male dog clasps his forelimbs just in front of the bitch’s hindlimbs and thrusts vigorously for several minutes. Spermatozoa are deposited into the vagina at that time. As the male’s penis continues to engorge, it becomes too large to be withdrawn from the vagina and the copulatory lock, or tie, is formed. The male, with his penis still within the vagina of the bitch, steps off and lifts one hindleg over her back so the two dogs are standing “butt to butt”. The tie usually lasts about 15 minutes. During this time, the female’s vagina is contracting and the male is sending pulses of prostatic fluid down the vagina; these actions promote forward movement of spermatozoa. Most animals stand quietly during the tie but it is recommended that the dogs not be left unattended so neither is damaged by the other walking around. Some bitches show short-lived aggression as the tie is broken, presumably due to pain. Some male dogs that do not achieve a tie will show a “phantom tie”, where they stand quietly for a time after breeding and ejaculate prostatic fluid although the penis is not caught within the vulvar lips.

It is best to minimize the number of males present when attempting to breed a bitch. If one or two males are present, copulation occurs within 30 minutes 75 to 80% of the time. If three or more males are present, copulation occurs only 5% of the time and average time until copulation averages 150 minutes. This is due to increased intermale aggression and possibly to mate preference, which is fairly common in older bitches. Male dogs may attempt mating several times in one day but more commonly breed once daily. It has been demonstrated that libido of male dogs suffers if they are used for breeding or have semen collected more than once daily. Bitches will tolerate breeding more than once daily. The average bitch is in estrus for 9 days but normal bitches may stand for as few as 3 days or as many as 21 days. Bitches may be bred by more than one male and will carry pups from different sires (superfecundation).

Queens should be brought to the male’s territory for breeding. Territoriality is important to male cats and many will not breed away from their home. Queens in heat exhibit soliciting behavior by assuming the posture of lordosis, treading their hindlimbs and presenting their perineal area to the tom cat. The tom cat may show “caterwauling”, a harsh, drawn-out cry that tells other males this is his territory and lets the queen know he’s present. Usually the tom cries out softly just prior to mating. The tom cat grasps the scruff of the neck of the queen with his teeth. This grip does not break the skin and is not an aggressive act but rather serves to stabilize the male and allow him to position the queen for copulation. Intromission and ejaculation last only seconds. Recorded time intervals for breeding in cats are 5 to 50 seconds for mounting and grasping of the neck by the male, 1 to 30 seconds for introduction of the penis and ejaculation and 0 to 1 second for the male to dismount. The queen cries out as ejaculation occurs, then immediately rolls over and bats at the male, who jumps away. The queen then goes through a period of frantic rolling and licking at her vulva termed the after-reaction. The reason for the after-reaction is not known; some hypothesize it is a mechanism that promotes forward movement of spermatozoa. In a survey of 120 copulations, 54% of queens screamed when bred, 77% struck out at the tom immediately after breeding, 92% licked at their vulva and 100% rolled frantically. Reported duration of the after-reaction is 1 to 7 minutes. When the queen appears to have completed the after-reaction, the male will approach again. Domestic cats mated an average of about 2 times per hour in one study. Most domestic cats breed 15 or fewer times in a day. A given tom cat can service as many as 20 queens in a harem. Big cats may mate hundreds of times in one day, especially those species that are solitary by nature.

Length of estrus averages about 7 days in cats. Studies disagree as to whether or not being bred, and subsequently being induced to ovulate, decrease estrus length in domestic cats. Estrus length is shorter in the fall, just prior to the end of the breeding season.

A phenomenon has been described in which cats that had been successfully bred went back into estrus during pregnancy and were bred again, creating kittens of very different age within the queen. This phenomenon, called superfetation, has never been proven to occur in domestic cats.


Post-estrus occurs only in cats and other induced ovulators. If the queen is not induced to ovulate, she will come back into estrus within about 14 days.


Diestrus is the stage of the cycle during which the animal is pregnant if bred successfully. All bitches go through a 2 month diestrus period, regardless of whether they were bred. This long exposure to the hormone progesterone and subsequent exposure to prolactin is associated with physical (mammary development, milk production) and behavioral changes of pregnancy (nesting, protection of inanimate objects as puppies). Queens that are induced to ovulate but are not pregnant also may go through a false pregnancy but rarely show physical or behavioral changes.


This is the period of reproductive quiescence. Bitches go through anestrus at the end of every diestrus period, such that the average bitch goes through estrus every 7 months. Queens cycle continuously until they enter anestrus associated with short daylength.



Some male dogs are hesitant to breed bitches and rarely show investigative behavior (licking or sniffing at urine spots, flehmen, etc). Proven causes for poor libido in male dogs and cats include: (1) negative breeding experience (aggressive female, injury to the penis during mating, constant discipline for mounting behavior outside of breeding), (2) pain when attempting to mount or ejaculate (prostate disease, spinal or rearlimb orthopedic injury, hair rings in cats), (3) chromosomal abnormality (calico or tortoiseshell cat), (4) improper breeding environment (tom cat outside of his territory, area too busy), (5) mate preference (some dominant male dogs only will breed with the dominant bitch or only will breed females of their breed) and (6) improper age (too young or too old). A hypothesized cause for poor libido in animals is hyperprolactinemia, which is associated with poor libido in human males and females. Low serum testosterone concentration has never been reported as a cause of poor libido in animals, although some males will show more interest in females when treated with testosterone. Routine treatment with testosterone is not recommended; remember that testosterone must be secreted in pulses for normal production of spermatozoa to occur and if you periodically provide the dog with a large amount of testosterone, you disrupt that pulsatile release and exert negative feedback to the pituitary, stopping secretion of LH and subsequent secretion of testosterone for an unpredictable length of time.


Dogs and cats may occasionally exhibit inappropriate mounting and masturbation behaviors. Male dogs intended for breeding never should be excessively disciplined for mounting behavior because that may make them less willing to mount and breed bitches when you want them to. Castrated male cats that exhibit breeding behaviors may have a tumor of the adrenal gland that secretes testosterone. Male cats that have significant amounts of testosterone in their system develop spines on their penis; examination of the penis for spines is a quick test for testosterone in male cats. Male dogs may exhibit mounting, extrusion of the penis and penile licking and inability to retract the penis into the prepuce (paraphimosis). If the dog is intact and is not intended for breeding, he should be castrated. If the dog is neutered, do not discipline him excessively as he may take that as positive reinforcement. Try to avoid conditions that cause the behavior, exercise him more and teach him obedience commands and make him “sit” or “drop” when he starts to show the behavior. Treatment with progesterone-type drugs may lessen the behavior. Very occasionally, female dogs will show mounting and thrusting behavior. This probably is masturbation, as they stimulate the clitoris. Treatment is as described for male dogs.


3) Refusal of female to stand for breeding

Some females never show true standing behavior. Causes for this include: (1) dominant bitch, (2) physical abnormalities of the bitch or queen (vaginal stricture, spinal or rearlimb pain) and (3) improper breeding timing (bitch or queen not in estrus). Physical problems and improper breeding timing can be ruled out with a complete physical examination, including a digital vaginal examination in bitches, and by measures to determine optimal breeding timing, including measurement of progesterone in blood.




Bitches and queens do not exhibit any specific behaviors during pregnancy. They are not reported to get “morning sickness” but many bitches will experience 7 to 10 days of inappetance about three weeks after breeding. Dogs and cats may get more particular about their diet when pregnant but as far as we know they do not get cravings. With advanced pregnancy, as the enlarged uterus pushes on the stomach, bitches and queens eat smaller meals less frequently and become less active. Many have an increased respiratory rate or pant, again due to pressure from the enlarged uterus. Cats rarely pant and any open-mouth breathing in queens is cause for concern.


Bitches and queens begin to exhibit nesting behavior about one week before giving birth. Most want to be in a secluded area but the occasional bitch or queen wants to be near the owner. Pick a spot for them or they will pick their own nest, invariably your closet or someplace else inconvenient. You want an area that is private but that can be observed easily and that can be cleaned. Commercial whelping and queening boxes are available. Children’s wading pools (dogs) and plastic dishpans (cats) work well, too. Some sort of nesting material that cannot be ingested by the dam and is easily laundered or replaced is recommended. Newspaper can be used but may stain the neonates and dam. Towels work well. Feral cats often change nest sites, presumably to prevent predation of kittens. Domestic cats given the run of the house may do the same.


Parturition is giving birth, termed whelping in dogs and queening in cats. Parturition is split into three stages.

Stage I labor is the long stage during which the cervix is dilating. The bitch or queen is uncomfortable and restless. She usually will not eat and may vomit. Bitches often pant. This stage may last up to 12 hours in normal bitches and queens.

Stage II labor is passage of the offspring. Overt abdominal contractions are evident and the bitch or queen obviously is bearing down. Bitches and queens rarely cry out when giving birth. The puppy or kitten usually is passed within the amniotic sac. The bitch or queen bites through the amniotic membrane and vigorously licks the pup or kitten to stimulate breathing. Then the bitch or queen chews through the umbilical cord and pushes the pup or kitten beneath them. Puppies and kittens usually are passed every 15 to 60 minutes. In dogs, if a pup has not been passed in two hours, veterinary intervention is recommended. However, normal queens may take a much longer time to pass all their kittens; in one survey, median queening time was 16 hours with a range of 4 to 42 hours. Bitches often tolerate the presence of humans they know well and some bitches seek out the owners at this stage. Queens like to be left alone and may stop labor if bothered too much.

Stage III labor is passage of the placentas. Stages II and III usually happen concurrently, with a bitch or queen passing one or two offspring, then some placentas, then some more offspring, etc. Bitches and queens often eat the placentas. This is an adaptation to keep the den clean and prevent predation. However, the placentas offer no benefit to the bitch or queen and may cause gastroenteritis with vomiting and inappetance. Placentas should be removed unless the bitch or queen is excessively agitated by presence of a human.



Most bitches and queens will not allow offspring to nurse until all have been born. Pups and kittens at birth are not capable of much movement, cannot control their own body temperature and must nurse within the first 8 to 12 hours of life to ensure they get adequate nutrition, do not dehydrate and receive antibodies to protect them from disease. The suckling reflex is very strong in normal puppies and kittens and can be elicited easily. Puppies and kittens find the dam’s nipples by trial and error and by olfaction. The bitch or queen may use her limbs to push the offspring into the area of the mammary glands. Queens often nearly form a circle while lying on their side, keeping the kittens warm and promoting their ability to suckle. While nursing, puppies and kittens knead the mammary gland; this may help promote milk letdown.  Puppies generally do not show a preference for which mammary gland they use for nursing and are easily rotated so the owner can be sure that all pups get to nurse off the larger rear mammary glands that produce more milk. Kittens do show a preference for a specific nipple but queens generally make the same amount of milk from all mammary glands so this is not a concern. The bitch or queen should allow the offspring to nurse and should lick the genitals after feeding to stimulate urination and defecation; this behavior should continue until the pups or kittens are mobile, at about three weeks of age. Bitches and queens rarely leave the offspring for the first week postpartum, leaving only to eat and to urinate and defecate. By about 4 weeks, as the puppies and kittens become more mobile and their teeth start to erupt, the queen and bitch become more evasive, standing when the offspring attempt to suckle and leaving the offspring alone for longer periods of time.

Cross-fostering is placement of puppies or kittens with a dam that is not their own. This may be necessary because of disease or death of the dam or because the litter size is very large. Cross-fostering easily is accomplished in dogs and cats, both of which are species with natural tendencies to cross-foster. It appears that dams identify their offspring by olfaction, perhaps linked to release of oxytocin. Cross-fostering is easiest if attempted in the first week of life and if the dam and surrogate dam are related.

Male dogs of domestic species virtually always ignore puppies. The very occasional male may attack pups but most do not seem to be aware of their existence. Similarly, domesticated male cats usually ignore kittens. Siamese males are reported to groom kittens of their breed on occasion. In some breeds of big cat, most notably the lion, an incoming male will kill the cubs in the pride and many of the females in the pride will then be induced to come into heat, ensuring the new male will get a chance to procreate. In domestic cats, loss of kittens is not consistently associated with the queen coming into heat so there would not evolutionary advantage for tom cats to kill the kittens.


False pregnancy, or pseudocyesis, is a normal phenomenon in bitches. At the end of diestrus, when progesterone concentrations fall and prolactin concentrations rise, the bitch exhibits mammary development, produces milk and may show behavioral changes as if she’s whelped including nesting and mothering of inanimate objects. Some bitches will be aggressive. This is most likely an evolutionary adaptation. In a pack of wild dogs, all females eventually cycle at the same time. The females low in the hierarchy in the pack may not be bred. If they go through false pregnancy and are lactating at the same time as related females that have given birth, they can cross-foster the pups and in that way ensure their genetics are carried on.



Postpartum aggression is uncommon in queens. In bitches, postpartum aggression can occur and may be unassociated with premonitory signs. Aggression may be exacerbated by vocalization of pups. Mild sedation occasionally is necessary to ensure safety of humans and other animals in the household. All bitches showing aggressive behavior should be seen by a veterinarian to ensure their behavior does not reflect pain, for example from mastitis (inflammation of the mammary gland).


The occasional bitch or queen will cannibalize their offspring. This behavior also is called kronism. In bitches, causes include pain (usually associated with mastitis) and low blood calcium (also called hypocalcemia or eclampsia). Cannibalism is more common in cats and has been reported to account for 12.5% of kitten deaths prior to weaning. Causes include pain, too large a litter, stress and overcrowding. Occasionally queens will kill kittens from other females in a facility; it is thought this is a predator-prey response since they do not recognize those kittens as their own and kittens are the size of prey.


Some bitches and queens reject their offspring. If only one or two offspring are repeatedly pushed away from the nest or carried away and hidden, this suggests there is something wrong with that puppy or kitten and that it should be examined by a veterinarian. Bitches and queens routinely reject offspring that are cold or are not active. If the entire litter is being rejected, that suggests something is wrong with the queen. Things to look for are signs of disease in the dam (mastitis, metritis [uterine infection], eclampsia, high fever from any cause) and environmental factors that may be stressing the dam (overcrowding, large litter). Some dams are poor mothers for their first litter, especially if they are a “high-strung” animal. Many of these dams go on to be good mothers with subsequent litters.

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