question: Why do most animals have many babies at once but humans usually only have one at a time?
comment: R vs k selection. Humans put tons of energy into one offspring at a time so it has a higher chance of survival. Some other animals put their energy into having tons of offspring but don't spend much energy on them each individually after birth, so even if some or most (in some cases) die, it's a net win
passages: Multiple birth
  
  A multiple birth is the culmination of one multiple pregnancy, wherein the mother delivers two or more offspring. A term most applicable to placental species, multiple births occur in most kinds of mammals, with varying frequencies. Such births are often named according to the number of offspring, as in "twins" and "triplets". In non-humans, the whole group may also be referred to as a "litter", and multiple births may be more common than single births. Multiple births in humans are the exception and can be exceptionally rare in the largest mammals.
   Recent history has also seen increasing numbers of multiple births. In the United States, it has been estimated that by 2011, 36% of twin births and 78% of triplet and higher-order births resulted from conception by assisted reproductive technology.
  
  Section::::Human multiple births.:Twins.
  
  Twins are by far the most common form of multiple births in humans. The U.S. Centers for Disease Control and Prevention report more than 132,000 sets of twins out of 3.9 million births of all kinds each year, about 3.4%, or 1 in 30.
   In humans, the average length of pregnancy (two weeks fewer than gestation) is 38 weeks with a single fetus. This average decreases for each additional fetus: to thirty-six weeks for twin births, thirty-two weeks for triplets, and thirty weeks for quadruplets. With the decreasing gestation time, the risks from immaturity at birth and subsequent viability increase with the size of the sibling group. Only as of the twentieth century have more than four all survived infancy.
   There are a wide range of reproductive strategies employed by different species. Some animals, such as the human and northern gannet, do not reach sexual maturity for many years after birth and even then produce few offspring. Others reproduce quickly; but, under normal circumstances, most offspring do not survive to adulthood. For example, a rabbit (mature after 8 months) can produce 10–30 offspring per year, and a fruit fly (mature after 10–14 days) can produce up to 900 offspring per year. These two main strategies are known as K-selection (few offspring) and r-selection (many offspring). Which strategy is favoured by evolution depends on a variety of circumstances. Animals with few offspring can devote more resources to the nurturing and protection of each individual offspring, thus reducing the need for many offspring. On the other hand, animals with many offspring may devote fewer resources to each individual offspring; for these types of animals it is common for many offspring to die soon after birth, but enough individuals typically survive to maintain the population. Some organisms such as honey bees and fruit flies retain sperm in a process called sperm storage thereby increasing the duration of their fertility.
   Multiple gestation. Pregnancy with twins, triplets, or more, referred to as a multiple gestation, increases the risk of infants being born prematurely (before 37 weeks of pregnancy). Having infants after age 30 and taking fertility drugs both have been associated with multiple births. Having three or more infants increases the chance that a woman will need to have the infants delivered by cesarean section. Twins and triplets are more likely to be smaller for their size than infants of singleton births. If infants of multiple gestation are born prematurely, they are more likely to have difficulty breathing.
   Large mammals, such as primates, cattle, horses, some antelopes, giraffes, hippopotamuses, rhinoceroses, elephants, seals, whales, dolphins, and porpoises, generally are pregnant with one offspring at a time, although they may have twin or multiple births on occasion. In these large animals, the birth process is similar to that of a human, though in most the offspring is precocial. This means that it is born in a more advanced state than a human baby and is able to stand, walk and run (or swim in the case of an aquatic mammal) shortly after birth. In the case of whales, dolphins and porpoises, the single calf is normally born tail first which minimises the risk of drowning. The mother encourages the newborn calf to rise to the surface of the water to breathe.
   Human infants are also almost always born with assistance from other humans because of the way that the pelvis is shaped. Since the pelvis and opening of birth canal face backwards, humans have difficulty giving birth themselves because they cannot guide the baby out of the canal. Non-human primates seek seclusion when giving birth because they do not need any help due to the pelvis and opening being more forward. Human infants depend on their parents much more and for much longer than other primates. Humans spend a lot of their time caring for their children as they develop whereas other species stand on their own from when they are born. The faster an infant develops, the higher the reproductive output of a female can be. So in humans, the cost of slow development of their infants is that humans reproduce relatively slowly. This phenomenon is also known as cooperative breeding.
   Large mammals which give birth to twins is much more rare, but it does occur occasionally even for mammals as large as elephants. In April 2018, approximately 8-month old elephant twins were sighted joining their mother's herd in the Tarangire National Park of Tanzania, estimated to have been born in August 2017.
  
  Section::::Mammals.:Human childbirth.
   Infanticide, the killing of the offspring by adult individuals, has been reported in many mammalian species and it is considered as an adaptive strategy to enhance fitness. Infanticide as the result of male–male competition for reproduction will occur under the following conditions: 
  
  BULLET::::- the male only kills unrelated infants
  
  BULLET::::- premature loss of her infant will result in the female going into oestrus sooner
  
  BULLET::::- the male's chance of siring the next offspring is high. Infanticide by competing males is relatively common in mammals due to the length of gestation and lactation.
   Fertility rates across the globe have steadily declined. This trend, known as the demographic transition, began in the 1700s and continues today. It is strongly correlated with increased industrialization in a society. This trend is now seen in almost all cultures, resulting in some societies with below replacement fertility. Below replacement fertility is when the rate of childbirth in a society is less than the amount needed for each woman to have at least one daughter. Since the chance of having a daughter is 50/50, there must be at least two children for every adult woman in the population.
   The number of multiple births has increased over the last decade. For example, in Canada between 1979 and 1999, the number of multiple birth babies increased 35%. Before the advent of ovulation-stimulating drugs, triplets were quite rare (approximately 1 in 8000 births) and higher-order births much rarer still. Much of the increase can probably be attributed to the impact of fertility treatments, such as in-vitro fertilization. Younger patients who undergo treatment with fertility medication containing artificial FSH, followed by intrauterine insemination, are particularly at risk for multiple births of higher order.
   For most species, the amount a fetus grows before birth determines the length of the gestation period. Smaller species normally have a shorter gestation period than larger animals. For example, a cat's gestation normally takes 58–65 days while an elephant's takes nearly 2 years (21 months). However, growth does not necessarily determine the length of gestation for all species, especially for those with a breeding season. Species that use a breeding season usually give birth during a specific time of year when food is available.
   There have been a few sets of nonuplets (9) in which a few babies were born alive, though none lived longer than a few days. There have been cases of human pregnancies that started out with ten, eleven, twelve or fifteen fetuses, but no instances of live births. The pregnancies of the 10, 11 and 15 fetuses have all resulted from fertility medications and assisted reproductive technology (ART). However, there has been one documented case when 12 fetuses were conceived naturally.
  
  Section::::Causes and frequency.
   Most male carnivorans have a baculum, although it is relatively short in felids, and absent in hyenas. Carnivorans tend to produce a single litter annually, but some produce multiple litters a year, and larger carnivorans, like bears, have gaps of 2–3 yr between litters. The average gestation period lies between 50 and 115 days, although the ursids and mustelids have delayed implantation, thus extending the gestation period six to nine months beyond the normal period. Litter sizes are usually small, ranging from one to 13 young, which are born with underdeveloped eyes and ears. In most species, the mother has exclusive or at least primary care of the offspring. Many species of carnivorans are solitary, but a few are gregarious.
   Other studies have hypothesized that LRC is likely to influence sex ratios in roe deer, as well as primates. Consistent with these hypotheses, the sex-ratios in roe deer and several primates have been found to be skewed towards the sex that does not compete with mothers.
  
  Section::::Interactions between relatives.:Local mate competition.
   Section::::Causes.:Tempo effect.
  
  Total fertility rate (TFR) is affected by a phenomenon called the tempo effect, which describes "distortions due to changes in the timing of births." John Bongaarts and Griffith Feeney have suggested that this tempo effect is driving the decline of measured fertility rate in the developed world.
  
  Section::::Causes.:Type of partnership.
   Human multiple births can occur either naturally (the woman ovulates multiple eggs or the fertilized egg splits into two) or as the result of infertility treatments such as IVF (several embryos are often transferred to compensate for lower quality) or fertility drugs (which can cause multiple eggs to mature in one ovulatory cycle).
   Development at birth varies considerably among animals, and even among mammals. Altricial species are relatively helpless at birth and require considerable parental care and protection. In contrast, precocial animals are born with open eyes, have hair or down, have large brains, and are immediately mobile and somewhat able to flee from, or defend themselves against, predators. Primates are precocial at birth, with the exception of humans.
  
  The duration of gestation in placental mammals varies from 18 days in jumping mice to 23 months in elephants. Generally speaking, fetuses of larger land mammals require longer gestation periods.
   Shorter gestation lengths are found in the greater short-nosed fruit bat ("Cynopterus sphinx") with a period of three months. The litter size of all megabats is usually one offspring.
   Selective reduction was developed in the mid-1980s, as people in the field of assisted reproductive technology became aware of the risks that multiple pregnancies carried for the mother and for the fetuses.
  
  Section::::Management.:Care in pregnancy.
  
  Women with a multiple pregnancy are usually seen more regularly by midwives or doctors than those with singleton pregnancies because of the higher risks of complications. However, there is currently no evidence to suggest that specialised antenatal services produce better outcomes for mother or babies than ‘normal’ antenatal care.
  
  Section::::Management.:Nutrition.
   Animals frequently display grouping behavior in herds, swarms, flocks, or colonies, and these multiple births derive similar advantages. A litter offers some protection from predation, not particularly to the individual young but to the parents' investment in breeding. With multiple young, predators could eat several and others could still survive to reach maturity, but with only one offspring, its loss could mean a wasted breeding season. The other significant advantage is the chance for the healthiest young animals to be favored from a group. Rather than it being a conscious decision on the part of the parents, the fittest and strongest baby competes most successfully for food and space, leaving the weakest young, or runts, to die through lack of care.
   Gestation, called "pregnancy" in humans, is the period of time during which the fetus develops, dividing via mitosis inside the female. During this time, the fetus receives all of its nutrition and oxygenated blood from the female, filtered through the placenta, which is attached to the fetus' abdomen via an umbilical cord. This drain of nutrients can be quite taxing on the female, who is required to ingest slightly higher levels of calories. In addition, certain vitamins and other nutrients are required in greater quantities than normal, often creating abnormal eating habits. The length of gestation, called the gestation period, varies greatly from species to species; it is 40 weeks in humans, 56–60 in giraffes and 16 days in hamsters.
   The preterm births also result in multiples tending to have a lower birth weight compared to singletons.
  
  Among the exceptions are the Kupresak triplets of Mississauga, Ontario, Canada; their combined weight at birth in 2008, of 17 lbs, 2.7 ounces, set a world record.
  
  Section::::Risks.:Cerebral palsy.
  
  Cerebral palsy is more common among multiple births than single births, being 2.3 per 1,000 survivors in singletons, 13 in twins, and 45 in triplets in North West England. This is likely a side effect of premature birth and low birth weight.
  
  Section::::Risks.:Incomplete separation.
   Section::::Trivers-Willard hypothesis.:Food availability.
   All anthropoids have a single-chambered uterus; tarsiers have a bicornate uterus like the strepsirrhines. Most species typically have single births, although twins and triplets are common for marmosets and tamarins. Despite similar gestation periods, haplorhine newborns are relatively much larger than strepsirrhine newborns, but have a longer dependence period on their mother. This difference in size and dependence is credited to the increased complexity of their behavior and natural history.
  
  Section::::Etymology.
  