What is the period of fertility in mammals that only occurs once or twice a year?
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Estrus: is in some-most mammals, but not primates who have menstrual cycles.  It is spelt a couple ways if you want to do more searches on it: estrus, oestrus. Refers to period of fertility (usually with a bloody show) when female is "in heat," is usually willing to mate and has viable-mature eggs ready and waiting. Some animals are spontaneous ovulators AND have an oestrus cycle.  Presence of penis in vagina (mechanical stimulation) causes the eggs to be released at mating.  Cats for example....notice the barbs on the male cat's penis before?
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Female Reproductive System.\n\nDescription:\nThis is the human's reproductive system, that is capable of producing a new being to the world. This is situated in the female gender of any species. (But we do find certain exceptions like, Sea Horses, where the female sea-horse simply produces the eggs, yet hands them over to the male sea-horse to take care of! mostly mistaken to believe that male sea-horses do have a reproductive system.) Its the only natural way of reproducing a specific being to the world.(Their are other ways, to reproduce species, in a lab i.e. cloning, tube.) \n\nBenifit:\nReproduction of a specific specie would keep them away from the Red-Data-Book, where all the endangered species are listed.\n\nThis is a very basic explanation.\n\nHope it is helpful!\nGood Luck!
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Yes.  A whale is a mammal, and one characteristic of all mammals is that they reproduce sexually.
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ovulation lasts from 24 to 36 hours during the middle of your cycle. if you are regular, ovulation will on the 14 day (day 1 starting the first day of your period).
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When 2 full moons occur in any calendar month which happens on average once every 2.5 years. See:\n\nhttp://scienceworld.wolfram.com/astronomy/BlueMoon.html
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There is potential for your dog to become pregnant during the entire heat cycle, which is about 28 days. The highest chance of her to get pregnant is right about in the middle of this cycle. If you have a male dog around, you will be able to distinguish this "peak" time by his behavior. He will be extremely restless and will be panting and pacing all over the place. This time usually lasts about 1 week.\n\nI have a male and female dog and I have had the pleasure of experiencing 2 of her cycles with them. The first heat she had, we ended up getting the male fixed. It was horrible trying to keep them away from each other. He was a mess, and so were we because he would NOT sleep and therefore, would not let us sleep. The 2nd time she went into heat was much much better for everyone. The male was not effected too much, except during her peak time, and it still was not nearly as bad as when he was still in tact.\n\nWe just had her spayed and all is well now. I suggest getting your dog spayed to avoid unwanted litters as well as other disease and problems that she may have if you don't get her fixed.
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The most frequent animal/human sex is in excess of eight times a week. The age group that is the most frequent is less than 15 years old. This is a fact that I found for you on a site. The link below will let you know more about the history of animal/human sex. It also tells you all kinds of percentages! Very interesting. It is illegal in most places.
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2010.\n\nIt's every 10 years.
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Do you mean "Why does it occur" or do you mena "How does it occur".\n\nHow it occurs is essentially ovarians failure. The ovaries stop producing hormones and there is a cascade of biological effects as a result, eveythhing  from cessationn of menstruation through to a loss of bone density and mood changes.\n\n(BTW menopause has nothing whatoever to with running out of eggs as Dr_n_ur_friend implied. A woman at menopause stillhas hundreds of thousands of viable eggs available.)\n\nWhy it occurs is because pregnancy is quite risky, women are esential in the raising of their offspring in the early years and human babie mature very slowly. As a result humans evolved a system whereby women couldn't risk there lives through pregnancy if they weren't going to live long enough to ensure the children survived to adulthood.\n\nMenopause is simply a way of stopping a woman having a child  if she is likely to die before the child turns 12. Instead she is able to devote all here resoucres to rasing any previous children or assisting in the care of grandchildren.\n\nToday of women commonly live far more than 12 years after menopause, but in a 'natural' state that owuld have been rare, and our bodies are still evolutionarily stick in the neolithic no matter how advanced our technology may be.
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The estrous cycle (also oestrous cycle; originally derived from Latin œstrous) refers to the recurring physiologic changes that are induced by reproductive hormones in most mammalian placental females; humans and great apes are the only mammals who undergo a menstrual cycle instead. Estrus cycles start after puberty in sexually mature females and are interrupted by anestrous phases. Typically estrus cycles continue until death. Some animals may display bloody vaginal discharge, often mistaken for menstruation.
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go to http://www.mymonthlycycles.com and put in your period info and it will calculate when you will ovulate and be fertile.\n\n\nOne of the hallmarks of the current generation is that women are choosing to have children later in life and it is likely that this trend will continue. On average, women today are having two children with approximately four years time between the first and second births. Between 1985 and 1994 the proportion of births to women in their 20s decreased from 62 percent to 53 percent, while the proportion of births to women 30 - 44 years of age increased from 25 percent to 34 percent. Since the mid 1970s there has been a four-fold increase in the percent of first births to women 30 years and older. \n\nOvulation is the term used to define the release of an egg (usually one, though sometimes more) from a woman’s ovary. Conception occurs when a sperm fertilizes the egg. While sperm can survive for several days inside a woman’s reproductive tract, an egg can only survive for up to 24 hours after ovulation. This puts a limitation on the time when conception is most likely to occur. According to a recent survey conducted at the World Congress on Fertility and Sterility, 20 percent of the problems in conceiving are due to a couple’s failure to time intercourse correctly during the woman’s fertile time period.\n\nThe Opinion Research Corporation International (ORC) found that most women don’t know when they can become pregnant. Nearly all women (98 percent) of reproductive age are unclear about how many days each month it is possible to get pregnant, and more than half (52 percent) of women aren’t sure when those fertile days occur during their cycle. Most parents do not know the day(s) on which their children were conceived.\n\nTo increase chances of becoming pregnant, women should monitor their body’s fertile time period - the time that sperm can survive within a woman’s reproductive tract until ovulation and fertilization can occur. A woman’s most fertile period is typically several days (usually no more than 5-6) ending on the day after ovulation. Because menstrual cycles vary from one woman to another and even from one cycle to the next in the same woman, identifying a woman’s fertile time period can be challenging. However there are several methods a woman can use to determine when she is most likely to conceive: \n\nCalendar - A woman can chart her menstrual cycle on a calendar to determine the days she is most likely to ovulate. \nBasal Body Temperature (BBT) - The BBT is a woman’s temperature first thing in the morning, before getting up or doing any activity. In a normal menstrual cycle, a woman’s BBT rises almost one degree from just after ovulation through the beginning of her next cycle. \nCervical Mucus - Most of the time the cervix produces a mucus that protects the body from sperm, bacteria and other organisms. For several days each month, the consistency of the mucus changes to allow sperm to enter the fallopian tubes. By monitoring these changes, a woman can determine her fertile time period. \nSymptothermal - Charting a combination of fertility indicators like cervical secretions and BBT can often help to indicate a woman’s fertile time period. \nThe above charting methods require women to record the secondary signs of fertility caused by changes in hormones. Because these conditions can be affected by factors such as infections or medications, they require careful monitoring and analysis, and they are never 100% reliable. An alternative to these methods is measuring changes in hormone levels. An increase in the level of estrogen marks the beginning of most women’s fertile phase, while an increase in the luteinizing hormone (LH) level signals that ovulation will occur soon - usually within approximately 24 to 36 hours.\n\nThe fertility monitor is a relatively new approach designed to provide daily information about a woman’s fertile time period. The monitor recor
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Multimammate rats (also called multimammate mice) in the genus Mastomys have 24 nipples.  Litters sizes are often greater than 12 per female (though each female could presumably manage 24 offspring).  Sexual maturity is reached in 50-60 days.  They can get pregnant again right after birth.  Gestation is just over 30 days.\n\nSo, assuming a 50-50 sex ratio, no mortality, and unlimited resources, a single reproductive pair could generate a population size of 268,586 in one year.
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Human sperms take about 65 days to completely develop.
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Nearly all animals undergo some form of sexual reproduction. Adults are diploid or occasionally polyploid. They have a few specialized reproductive cells, which undergo meiosis to produce smaller motile spermatozoa or larger non-motile ova. These fuse to form zygotes, which develop into new individuals.\n\nMany animals are also capable of asexual reproduction. This may take place through parthenogenesis, where fertile eggs are produced without mating, or in some cases through fragmentation.\n\nA zygote initially develops into a hollow sphere, called a blastula, which undergoes rearrangement and differentiation. In sponges, blastula larvae swim to a new location and develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement. It first invaginates to form a gastrula with a digestive chamber, and two separate germ layers - an external ectoderm and an internal endoderm. In most cases, a mesoderm also develops between them. These germ layers then differentiate to form tissues and organs.\n\nAnimals grow by indirectly using the energy of sunlight. Plants use this energy to turn air into simple sugars using a process known as photosynthesis. These sugars are then used as the building blocks which allow the plant to grow. When animals eat these plants (or eat other animals which have eaten plants), the sugars produced by the plant are used by the animal. They are either used directly to help the animal grow, or broken down, releasing stored solar energy, and giving the animal the energy required for motion. This process is known as glycolysis.
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Females reach sexual maturity around age 3 and mate around age 4 or 5. Males begin to rut by age 3, but do not reach full sexual maturity until age 6. Typically, males and females are seasonal breeders. Breeding occurs in winter and overlaps with the rainy season; both vary in respect to the camel's geographic range. The onset of the breeding season is believed to be cued by nutritional status of the camel and the daylength. The gestation period typically lasts for a period of 15 months, followed by the birth of a single calf.
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Irregular Cycles: Metrorrhagia \nOne of the most common gynecological problems is an irregular menstrual cycle. But before you jump to the conclusion that you're irregular, it's important to remember that being regular doesn't mean your cycle is the same number of days each time. One month your cycle may be twenty-nine days, and the next month it may be thirty-one days. This is still considered the norm. It's also normal to be lighter one month and heavier the next. So long as you're menstruating every twenty to forty days, it's a sign that you're ovulating. Another common misperception about irregular cycles is the belief that unless you have a period every four weeks (again, the statistical average) you're irregular. This is not true. Some women menstruate every three weeks, which is normal for them; some menstruate every five weeks, which is normal for them. The only time you should be concerned is if your period consistently yo-yos: three weeks, then four weeks, then five weeks, then three weeks, and so on. When this happens, it's usually a sign that you're not ovulating regularly. This is common in young girls after they first begin menstruating. If your period only jumps around once or twice a year, there isn't anything to worry about. Occasional stress is usually the culprit when this happens.
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It depends on the organism.  Humans are just built that way.  Other species like frogs continually generate oocytes (the egg precursor).  What happens in humans is usually not an absolute truth for other animals.  Some animals are asexual (no sperm needed).  All sorts of variations on the theme exists.