Anatomy and Physiology of Animals/Reproductive System
After completing this section, you should know:
- the role of mitosis and meiosis in the production of gametes (sperm and ova)
- that gametes are haploid cells
- that fertilization forms a diploid zygote
- the major parts of the male reproductive system and their functions
- the route sperm travel along the male reproductive tract to reach the penis
- the structure of a sperm and the difference between sperm and semen
- the difference between infertility and impotence
- the main parts of the female reproductive system and their functions
- the ovarian cycle and the roles of FSH, LH, oestrogen and progesterone
- the oestrous cycle and the signs of heat in rodents, dogs, cats and cattle
- the process of fertilization and where it occurs in the female tract
- what a morula and a blastocyst are
- what the placenta is and its functions
Reproductive System 
In biological terms sexual reproduction involves the union of gametes - the sperm and the ovum - produced by two parents. Each gamete is formed by meiosis (see Chapter 3). This means each contains only half the chromosomes of the body cells (haploid). Fertilization results in the joining of the male and female gametes to form a zygote which contains the full number of chromosomes (diploid). The zygote then starts to divide by mitosis (see Chapter 3) to form a new animal with all its body cells containing chromosomes that are identical to those of the original zygote (see diagram 13.1).
Diagram 13.1 - Sexual reproduction
The offspring formed by sexual reproduction contain genes from both parents and show considerable variation. For example, kittens in a litter are all different although they (usually) have the same mother and father. In the wild this variation is important because it means that when the environment changes some individuals may be better adapted to survive than others. These survivors pass their “superior” genes on to their offspring. In this way the characteristics of a group of animals can gradually change over time to keep pace with the changing environment. This “survival of the fittest” or “natural selection” is the mechanism behind the theory of evolution.
In most fish and amphibia (frogs and toads) fertilisation of the egg cells takes place outside the body. The female lays the eggs and then the male deposits his sperm on or at least near them.
In reptiles and birds, eggs are fertilized inside the body when the male deposits the sperm inside the egg duct of the female. The egg is then surrounded by a resistant shell, “laid” by the female and the embryo completes its development inside the egg.
In mammals the sperm are placed in the body of the female and the eggs are fertilized internally. They then develop to quite an advanced stage inside the body of the female. When they are born they are fed on milk excreted from the mammary glands and protected by their parents until they become independent.
Sexual Reproduction In Mammals 
The reproductive organs of mammals produce the gametes (sperm and egg cells), help them fertilise and then support the developing embryo.
The Male Reproductive System 
The male reproductive system consists of a pair of testes that produce sperm (or spermatozoa), ducts that transport the sperm to the penis and glands that add secretions to the sperm to make semen (see diagram 13.2).
The various parts of the male reproductive system with a summary of their functions are shown in diagram 13.3.
Diagram 13.2. The reproductive organs of a male dog
Diagram 13.3 - Diagram summarising the functions of the male reproductive organs
The Testes 
Sperm need temperatures between 2 to 10 degrees Centigrade lower and then the body temperature to develop. This is the reason why the testes are located in a bag of skin called the scrotal sacs (or scrotum) that hangs below the body and where the evaporation of secretions from special glands can further reduce the temperature. In many animals (including humans) the testes descend into the scrotal sacs at birth but in some animals they do not descend until sexual maturity and in others they only descend temporarily during the breeding season. A mature animal in which one or both testes have not descended is called a cryptorchid and is usually infertile.
The problem of keeping sperm at a low enough temperature is even greater in birds that have a higher body temperature than mammals. For this reason bird’s sperm are usually produced at night when the body temperature is lower and the sperm themselves are more resistant to heat.
The testes consist of a mass of coiled tubes (the seminiferous or sperm producing tubules) in which the sperm are formed by meiosis (see diagram 13.4). Cells lying between the seminiferous tubules produce the male sex hormone testosterone.
When the sperm are mature they accumulate in the collecting ducts and then pass to the epididymisbefore moving to the sperm duct or vas deferens. The two sperm ducts join the urethra just below the bladder, which passes through the penis and transports both sperm and urine.
Ejaculation discharges the semen from the erect penis. It is brought about by the contraction of the epididymis, vas deferens, prostate gland and urethra.
Diagram 13.4 - The testis and a magnified seminiferous tubule
Semen consists of 10% sperm and 90% fluid and as sperm pass down the ducts from testis to penis, (accessory) glands add various secretion...
Accessory Glands 
Three different glands may be involved in producing the secretions in which sperm are suspended, although the number and type of glands varies from species to species.
Seminal vesicles are important in rats, bulls, boars and stallions but are absent in cats and dogs. When present they produce secretions that make up much of the volume of the semen, and transport and provide nutrients for the sperm.
The prostate gland is important in dogs and humans. It produces an alkaline secretion that neutralizes the acidity of the male urethra and female vagina.
Cowper’s glands (bulbourethral glands) have various functions in different species. The secretions may lubricate, flush out urine or form a gelatinous plug that traps the semen in the female reproductive system after copulation and prevents other males of the same species fertilizing an already mated female. Cowper’s glands are absent in bears and aquatic mammals.
The Penis 
The penis consists of connective tissue with numerous small blood spaces in it. These fill with blood during sexual excitement causing erection.
Penis Form And Shape 
Dogs, bears, seals, bats and rodents have a special bone in the penis which helps maintain the erection (see diagram 13.2). In some animals (e.g. the bull, ram and boar) the penis has an “S” shaped bend that allows it to fold up when not in use. In many animals the shape of the penis is adapted to match that of the vagina. For example, the boar has a corkscrew shaped penis, there is a pronounced twist in bulls’ and it is forked in marsupials like the opossum. Some have spines, warts or hooks on them to help keep them in the vagina and copulation may be extended to help retain the semen in the female system. Mating can last up to three hours in minks, and dogs may “knot” or “tie” during mating and can not separate until the erection has subsided.
Sperm are made up of three parts: a head consisting mainly of the nucleus, a midpiece containing many mitochondria to provide the energy and a tail that provides propulsion (see diagram 13.5).
Diagram 13.5 - A sperm
A single ejaculation may contain 2-3 hundred million sperm but even in normal semen as many as 10% of these sperm may be abnormal and infertile. Some may be dead while others are inactive or deformed with double, giant or small heads or tails that are coiled or absent altogether.
When there are too many abnormal sperm or when the sperm concentration is low, the semen may not be able to fertilize an egg and the animal is infertile. Make sure you don’t confuse infertility with impotence, which is the inability to copulate successfully.
Sperm do not live forever. They have a definite life span that varies from species to species. They survive for between 20 days (guinea pig) to 60 days (bull) in the epididymis but once ejaculated into the female tract they only live from 12 to 48 hours. When semen is used for artificial insemination, storage under the right conditions can extend the life span of some species.
Artificial Insemination 
In many species the male can be artificially stimulated to ejaculate and the semen collected. It can then be diluted, stored and used to inseminate females. For example bull semen can be diluted and stored for up to 3 weeks at room temperature. If mixed with an antifreeze solution and stored in “straws” in liquid nitrogen at minus 79oC it will keep for much longer. Unfortunately the semen of chickens, stallions and boars can only be stored for up to 2 days.
Dilution of the semen means that one male can be used to fertilise many more females than would occur under natural conditions. There are also advantages in the male and female not having to make physical contact. It means that owners of females do not have to buy expensive males and the possibility of transmitting sexually transmitted diseases is reduced. Routine examination of the semen for sperm concentration, quality and activity allows only the highest quality semen to be used so a high success rate is ensured.
Since the lifespan of sperm in the female tract is so short and ova only survive from 8 to 10 hours the timing of the artificial insemination is critical. Successful conception depends upon detecting the time that the animal is “on heat” and when ovulation occurs.
The Female Reproductive Organs 
The female reproductive system consists of a pair of ovaries that produce egg cells or ova and fallopian tubes where fertilisation occurs and which carry the fertilised ovum to the uterus. Growth of the foetus takes place here. The cervix separates the uterus from the vagina or birth canal, where the sperm are deposited (see diagram 13.6).
Diagram 13.6. - The reproductive system of a female rabbit
Note that primates like humans have a uterus with a single compartment but in most mammals the uterus is divided into two separate parts or horns as shown in diagram 13.6.
The Ovaries 
Ovaries are small oval organs situated in the abdominal cavity just ventral to the kidneys. Most animals have a pair of ovaries but in birds only the left one is functional to reduce weight (see below).
The ovary consists of an inner region (medulla) and an outer region (cortex) containing egg cells or ova. These are formed in large numbers around the time of birth and start to develop after the animal becomes sexually mature. A cluster of cells called the follicle surrounds and nourishes each ovum.
The Ovarian Cycle 
The ovarian cycle refers to the series of changes in the ovary during which the follicle matures, the ovum is shed and the corpus luteum develops (see diagram 13.7).
Numerous undeveloped ovarian follicles are present at birth but they start to mature after sexual maturity. In animals that normally have only one baby at a time only one ovum will mature at once but in litter animals several will. The mature follicle consists of outer cells that provide nourishment. Inside this is a fluid-filled space that contains the ovum.
A mature follicle can be quite large, ranging from a few millimetres in small mammals to the size of a golf ball in large animals. It bulges out from the surface of the ovary before eventually rupturing to release the ovum into the abdominal cavity. Once the ovum has been shed, a blood clot forms in the empty follicle. This develops into a tissue called the corpus luteum that produces the hormone progesterone (see diagram 13.9). If the animal becomes pregnant the corpus luteum persists, but if there is no pregnancy it degenerates and a new ovarian cycle usually.
Diagram 13.7 - The ovarian cycle showing from the top left clockwise: the maturation of the ovum over time, followed by ovulation and the development of the corpus luteum in the empty follicle
The Ovum 
When the ovum is shed the nucleus is in the final stages of meiosis (cell division). It is surrounded by few layers of follicle cells and a tough membrane called the zona pellucida (see diagram 13.8).
Diagram 13.8 - An ovum
The Oestrous Cycle 
The oestrous cycle is the sequence of hormonal changes that occurs through the ovarian cycle. These changes influence the behaviour and body changes of the female (see diagram 13.9).
Diagram 13.9 - The oestrous cycle
The first hormone involved in the oestrous cycle is follicle stimulating hormone (F.S.H.), secreted by the anterior pituitary gland (see chapter 16). It stimulates the follicle to develop. As the follicle matures the outer cells begin to secrete the hormone oestrogen and this stimulates the mammary glands to develop. It also prepares the lining of the uterus to receive a fertilised egg. Ovulation is initiated by a surge of another hormone from the anterior pituitary, luteinising hormone (L.H.). This hormone also influences the development of the corpus luteum, which produces progesterone, a hormone that prepares the lining of the uterus for the fertilised ovum and readies the mammary glands for milk production. If no pregnancy takes place the corpus luteum shrinks and the production of progesterone decreases. This causes FSH to be produced again and a new oestrous cycle begins.
For fertilisation of the ovum by the sperm to occur, the female must be receptive to the male at around the time of ovulation. This is when the hormones turn on the signs of “heat”, and she is “in season” or “in oestrous”. These signs are turned off again at the end of the oestrous cycle.
During the oestrous cycle the lining of the uterus (endometrium) thickens ready for the fertilised ovum to be implanted. If no pregnancy occurs this thickened tissue is absorbed and the next cycle starts. In humans and other higher primates, however, the endometrium is shed as a flow of blood and instead of an oestrous cycle there is a menstrual cycle.
The length of the oestrous cycle varies from species to species. In rats the cycle only lasts 4-5 days and they are sexually receptive for about 14 hours. Dogs have a cycle that lasts 60-70 days and heat lasts 7-9 days and horses have a 21-day cycle and heat lasts an average of 6 days.
Ovulation is spontaneous in most animals but in some, e.g. the cat, and the rabbit, ovulation is stimulated by mating. This is called induced ovulation.
Signs Of Oestrous Or Heat 
- When on heat a bitch has a blood stained discharge from the vulva that changes a little later to a straw coloured one that attracts all the dogs in the neighbourhood.
- Female cats “call” at night, roll and tread the carpet and are generally restless but will “stand” firm when pressure is placed on the pelvic region (this is the lordosis response).
- A female rat shows the lordosis response when on heat. It will “mount” other females and be more active than normal.
- A cow mounts other cows (bulling), bellows, is restless and has a discharge from the vulva.
Breeding Seasons And Breeding Cycles 
Only a few animals breed throughout the year. This includes the higher primates (humans, gorillas and chimpanzees etc.), pigs, mice and rabbits. These are known as continuous breeders.
Most other animals restrict reproduction to one or two seasons in the year-seasonal breeders (see diagram 13.10). There are several reasons for this. It means the young can be born at the time (usually spring) when feed is most abundant and temperatures are favourable. It is also sensible to restrict the breeding season because courtship, mating, gestation and the rearing of young can exhaust the energy resources of an animal as well as make them more vulnerable to predators.
Diagram 13.10 - Breeding cycles
The timing of the breeding cycle is often determined by day length. For example the shortening day length in autumn will bring sheep and cows into season so the foetus can gestate through the winter and be born in spring. In cats the increasing day length after the winter solstice (shortest day) stimulates breeding. The number of times an animal comes into season during the year varies, as does the number of oestrous cycles during each season. For example a dog usually has 2-3 seasons per year, each usually consisting of just one oestrous cycle. In contrast ewes usually restrict breeding to one season and can continue to cycle as many as 20 times if they fail to become pregnant.
Fertilisation and Implantation 
The opening of the fallopian tube lies close to the ovary and after ovulation the ovum is swept into its funnel-like opening and is moved along it by the action of cilia and wave-like contractions of the wall.
Copulation deposits several hundred million sperm in the vagina. They swim through the cervix and uterus to the fallopian tubes moved along by whip-like movements of their tails and contractions of the uterus. During this journey the sperm undergo their final phase of maturation so they are ready to fertilise the ovum by the time they reach it in the upper fallopian tube.
High mortality means only a small proportion of those deposited actually reach the ovum. The sperm attach to the outer zona pellucida and enzymes secreted from a gland in the head of the sperm dissolve this membrane so it can enter. Once one sperm has entered, changes in the zona pellucida prevent further sperm from penetrating. The sperm loses its tail and the two nuclei fuse to form a zygote with the full set of paired chromosomes restored.
Development Of The Morula And Blastocyst 
As the fertilised egg travels down the fallopian tube it starts to divide by mitosis. First two cells are formed and then four, eight, sixteen, etc. until there is a solid ball of cells. This is called a morula. As division continues a hollow ball of cells develops. This is a blastocyst (see diagram 13.11).
Implantation involves the blastocyst attaching to, and in some species, completely sinking into the wall of the uterus.
The Placenta And Foetal Membranes 
As the embryo increases in size, the placenta, umbilical cord and foetal membranes (often known collectively as the placenta) develop to provide it with nutrients and remove waste products (see diagram 13.12). In later stages of development the embryo becomes known as a foetus.
The placenta is the organ that attaches the foetus to the wall of the uterus. In it the blood of the foetus and mother flow close to each other but never mix (see diagram 13.13). The closeness of the maternal and foetal blood systems allows diffusion between them. Oxygen and nutrients diffuse from the mother’s blood into that of the foetus and carbon dioxide and excretory products diffuse in the other direction. Most maternal hormones (except adrenaline), antibodies, almost all drugs (including alcohol), lead and DDT also pass across the placenta. However, it protects the foetus from infection with bacteria and most viruses.
Diagram 13.11 - Development and implantation of the embryo
Diagram 13.12. The foetus and placenta
The foetus is attached to the placenta by the umbilical cord. It contains arteries that carry blood to the placenta and a vein that returns blood to the foetus. The developing foetus becomes surrounded by membranes. These enclose the amniotic fluid that protects the foetus from knocks and other trauma (see diagram 13.12).
Diagram 13.13 - Maternal and foetal blood flow in the placenta
Hormones During Pregnancy 
The corpus luteum continues to secrete progesterone and oestrogen during pregnancy. These maintain the lining of the uterus and prepare the mammary glands for milk secretion. Later in the pregnancy the placenta itself takes over the secretion of these hormones.
Chorionic gonadotrophin is another hormone secreted by the placenta and placental membranes. It prevents uterine contractions before labour and prepares the mammary glands for lactation. Towards the end of pregnancy the placenta and ovaries secrete relaxin, a hormone that eases the joint between the two parts of the pelvis and helps dilate the cervix ready for birth.
Pregnancy Testing 
The easiest method of pregnancy detection is ultrasound which is noninvasive and very reliable Later in gestation pregnancy can be detected by taking x-rays.
In dogs and cats a blood test can be used to detect the hormone relaxin.
In mares and cows palpation of the uterus via the rectum is the classic way to determine pregnancy. It can also be done by detecting the hormones progesterone or equine chorionic gonagotrophin (eCG) in the urine. A new sensitive test measures the amount of the hormone, oestrone sulphate, present in a sample of faeces. The hormone is produced by the foal and placenta, and is only present when there is a living foal.
In most animals, once pregnancy is advanced, there is a window of time during which an experienced veterinarian can determine pregnancy by feeling the abdomen.
Gestation Period 
The young of many animals (e.g. pigs, horses and elephants) are born at an advanced state of development, able to stand and even run to escape predators soon after they are born. These animals have a relatively long gestation period that varies with their size e.g. from 114 days in the pig to 640 days in the elephant.
In contrast, cats, dogs, mice, rabbits and higher primates are relatively immature when born and totally dependent on their parents for survival. Their gestation period is shorter and varies from 25 days in the mouse to 31 days in rabbits and 258 days in the gorilla.
The babies of marsupials are born at an extremely immature stage and migrate to the pouch where they attach to a teat to complete their development. Kangaroo joeys, for example, are born 33 days after conception and opossums after only 8 days.
Signs Of Imminent Birth 
As the pregnancy continues, the mammary glands enlarge and may secrete a milky substance a few days before birth occurs. The vulva may swell and produce thick mucus and there is sometimes a visible change in the position of the foetus. Just before birth the mother often becomes restless, lying down and getting up frequently. Many animals seek a secluded place where they may build a nest in which to give birth.
Labour involves waves of uterine contractions that press the foetus against the cervix causing it to dilate. The foetus is then pushed through the cervix and along the vagina before being delivered. In the final stage of labour the placenta or “afterbirth” is expelled.
Adaptations Of The Foetus To Life Outside The Uterus 
The foetus grows in the watery, protected environment of the uterus where the mother supplies oxygen and nutrients, and waste products pass to her blood circulation for excretion. Once the baby animal is born it must start to breathe for itself, digest food and excrete its own waste. To allow these functions to occur blood is re-routed to the lungs and the glands associated with the gut start to secrete. Note that newborn animals can not control their own body temperature. They need to be kept warm by the mother, littermates and insulating nest materials.
Milk Production 
Cows, manatees and primates have two mammary glands but animals like pigs that give birth to large litters may have as many as 12 pairs. Ducts from the gland lead to a nipple or teat and there may be a sinus where the milk collects before being suckled (see diagram 13.14).
Diagram 13.14 - A mammary gland
The hormones oestrogen and progesterone stimulate the mammary glands to develop and prolactin promotes the secretion of the milk. Oxytocin from the pituitary gland releases the milk when the baby suckles. The first milk is called colostrum. It is a rich in nutrients and contains protective antibodies from the mother. Milk contains fat, protein and milk sugar as well as vitamins and most minerals although it contains little iron. Its actual composition varies widely from species to species. For example whale’s and seal’s milk has twelve times more fat and four times more protein than cow’s milk. Cow’s milk has far less protein in it than cat’s or dog’s milk. This is why orphan kittens and puppies cannot be fed cow’s milk.
Reproduction In Birds 
Male birds have testes and sperm ducts and male swans, ducks, geese and ostriches have a penis. However, most birds make do with a small amount of erectile tissue known as a papilla. To reduce weight for flight most female birds only have one ovary - usually the left, which produces extremely yolky eggs. The eggs are fertilised in the upper part of the oviduct (equivalent to the fallopian tube and uterus of mammals) and as they pass down it albumin (the white of the egg), the membrane beneath the shell and the shell are laid down over the yolk. Finally the egg is covered in a layer of mucus to help the bird lay it (see diagram 13.15).
Most birds lay their eggs in a nest and the hen sits on them until they hatch. Ducklings and chicks are relatively well developed when they hatch and able to forage for their own food. Most other nestlings need their parents to keep them warm, clean and fed. Young birds grow rapidly and have voracious appetites that may involve the parents making up to 1000 trips a day to supply their need for food.
Diagram 13.15 - Female reproductive organs of a bird
- Haploid gametes (sperm and ova) are produced by meiosis in the gonads (testes and ovaries).
- Fertilisation involves the fusing of the gametes to form a diploid zygote.
- The male reproductive system consists of a pair of testes that produce sperm (or spermatozoa), ducts that transport the sperm to the penis and glands that add secretions to the sperm to make semen.
- Sperm are produced in the seminiferous tubules, are stored in the epididymis and travel via the vas deferens or sperm duct to the junction of the bladder and the urethra where various accessory glands add secretions. The fluid is now called semen and is ejaculated into the female system down the urethra that runs down the centre of the penis.
- Sperm consist of a head, a midpiece and a tail.
- Infertility is the inability of sperm to fertilize an egg while impotence is the inability to copulate successfully.
- The female reproductive system consists of a pair of ovaries that produce ova and fallopian tubes where fertilisation occurs and which carry the fertilised ovum to the uterus. Growth of the foetus takes place here. The cervix separates the uterus from the vagina, the birth canal and where the sperm are deposited.
- The ovarian cycle refers to the series of changes in the ovary during which the follicle matures, the ovum is shed and the corpus luteum develops.
- The oestrous cycle is the sequence of hormonal changes that occurs through the ovarian cycle. It is initiated by the secretion of follicle stimulating hormone (F.S.H.), by the anterior pituitary gland which stimulates the follicle to develop. The follicle secretes oestrogen which stimulates mammary gland development. luteinising hormone (L.H.) from the anterior pituitary initiates ovulation and stimulates the corpus luteum to develop. The corpus luteum produces progesterone that prepares the lining of the uterus for the fertilised ovum.
- Signs of oestrous or heat differ. A bitch has a blood stained discharge, female cats and rats are restless and show the lordosis response, while cows mount other cows, bellow and have a discharge from the vulva.
- After fertilisation in the fallopian tube the zygote divides over and over by mitosis to become a ball of cells called a morula. Division continues to form a hollow ball of cells called the blastocyst. This is the stage that implants in the uterus.
- The placenta, umbilical cord and foetal membranes (known as the placenta) protect and provide the developing foetus with nutrients and remove waste products.
Test Yourself 
1. Add the following labels to the diagram of the male reproductive organs below.
- testis | epididymis | vas deferens | urethra | penis | scrotal sac | prostate gland
Diagram of the Male Reproductive System
2. Match the following descriptions with the choices given in the list below.
- accessory glands | vas deferens or sperm duct | penis | scrotum | fallopian tube | testes | urethra | vagina | uterus | ovary | vulva
- a) Organ that delivers semen to the female vagina
- b) Where the sperm are produced
- c) Passage for sperm from the epididymis to the urethra
- d) Carries both sperm and urine down the penis
- e) Glands that produce secretions that make up most of the semen
- f) Bag of skin surrounding the testes
- g) Where the foetus develops
- h) This receives the penis during copulation
- i) Where fertilisation usually occurs
- j) Ova travel along this tube to reach the uterus
- k) Where the ova are produced
- l) The external opening of the vagina
3. Which hormone is described in each statement below?
- a) This hormone stimulates the growth of the follicles in the ovary
- b) This hormone converts the empty follicle into the corpus luteum and stimulates it to produce progesterone
- c) This hormone is produced by the cells of the follicle
- d) This hormone is produced by the corpus luteum
- e) This hormone causes the mammary glands to develop
- f) This hormone prepares the lining of the uterus to receive a fertilised ovum
4. State whether the following statements are true or false. If false write in the correct answer.
- a) Fertilisation of the egg occurs in the uterus
- b) The fertilised egg cell contains half the normal number of chromosomes
- c) The morula is a hollow ball of cells
- d) The mixing of the blood of the mother and foetus allows nutrients and oxygen to transfer easily to the foetus
- e) The morula implants in the wall of the uterus
- f) The placenta is the organ that supplies the foetus with oxygen and nutrients
- g) Colostrum is the first milk
- h) Young animals often have to be given calcium supplements because milk contains very little calcium
- http://www.anatomicaltravel.com/CB_site/Conception_to_birth3.htm Anatomical travel. Images of fertilisation and the development of the (human) embryo through to birth.
- http://www.uchsc.edu/ltc/fert.swf Fertilisation. A great animation of fertilisation, formation of the zygote and first mitotic division. A bit advanced but still worth watching.
- http://www.uclan.ac.uk/facs/health/nursing/sonic/scenarios/salfordanim/heart.swf Sonic. An animation showing the foetal blood circulation through the placenta to the changes allowing circulation through the lungs after birth.
- http://en.wikipedia.org/wiki/Estrus Wikipedia. As always, good interesting information although some terms and concepts are beyond the requirements of this level.