question: why dont babies walk almost immediately likemostmammals..Not an expert by any means (obviously), but from watching documentaries like planet Earth.. it seems like most mammals become almost independent within weeks or even days whereas humans are pretty vulnerable for at least the first 5-6 ye
comment: Our brains are among our most important physical organs--specifically, the size and complexity with which they develop. But because of the way we've developed to walk, giving birth to a baby that's more developed (and therefore has a larger head) would shatter a woman's pelvis and kill the baby in the process. The solution nature came up with was to give birth earlier and have mothers care for pretty much defenseless babies until they're more developed.
passages: 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.
   Physiological prematurity
  
  The term physiological prematurity refers to the fact that compared to most animals, humans are born in a premature biological state. Although sensory organs and skeletal and muscular systems are largely developed prenatally, human babies at the time of their birth are completely helpless and dependent on intensive care. This is in contrast to the maturity at birth found in other higher mammals (e.g. elephants, horses). 
   At the opposite end of the spectrum are precocial animals in which the young have open eyes, have hair or down, have large brains, and are immediately mobile and somewhat able to flee from, or defend themselves against, predators. For example, with ground-nesting birds such as ducks or turkeys, the young are ready to leave the nest in one or two days. Among mammals, most ungulates are precocial, being able to walk almost immediately after birth. Beyond the precocial are the superprecocial animals, such as the megapode birds, which hatch with full flight feathers.
  
  Section::::Basis.
   Another example is the blue wildebeest, whose calves can stand within an average of six minutes from birth and walk within thirty minutes; they can outrun a hyena within a day. Such behavior gives them an advantage over other herbivore species; they are 100 times more abundant in the Serengeti ecosystem than hartebeests, their closest taxonomic relative. Hartebeest calves are not as precocial as wildebeest calves and take up to thirty minutes or more before they stand, and as long as forty-five minutes before they can follow their mothers for short distances. They are unable to keep up with their mothers until they are more than a week old.
   There is no sharp limit of development, gestational age, or weight at which a human fetus automatically becomes viable. According to studies between 2003 and 2005, 20 to 35 percent of babies born at 24 weeks of gestation survive, while 50 to 70 percent of babies born at 25 weeks, and more than 90 percent born at 26 to 27 weeks, survive. It is rare for a baby weighing less than 500 g (17.6 ounces) to survive. The chances of a fetus surviving increase 3-4% per day between 23 and 24 weeks of gestation and about 2-3% per day between 24 and 26 weeks of gestation. After 26 weeks the rate of survival increases at a much slower rate because survival is high already.
   In the case of mammals it has been suggested that large adult body sizes favor production of large, precocious young, which develop with a long gestation period. Large young may be associated with long lifespan, extended reproductive period, and reduced litter size. It has been suggested that altricial strategies in mammals may be favoured if there is a selective advantage to mothers that are capable of resorbing embryos in early stages of development. Some ecological niches require young to be precocial for survival, such as cetaceans: restricted to water, and immobile, their helpless young would quickly drown.
  
  Section::::Terminology.
   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.
   BULLET::::- Infants who are carried are generally calmer, as more of their primal/survival needs are met. The caregiver can be seen, heard, smelled, touched, tasted, provide feeding and the motion necessary for continuing neural development, gastrointestinal and respiratory health and to establish balance (inner ear development) and muscle tone is constant.
  
  BULLET::::- Parental rhythms (walking, heartbeat, etc.) can have balancing and soothing effects on infants.
  
  BULLET::::- Infants are "humanized" earlier by developing socially. Babies are closer to people and can study facial expressions, learn languages faster and be familiar with body language.
  
  BULLET::::- Independence is established earlier.
   In order to reduce the amount of time that infants are particularly vulnerable to infanticide, females have been shown to wean infants earlier when risk of infanticide is high. For example, female white-headed leaf monkeys were observed to wean their infants significantly more quickly during male takeovers as compared to socially stable periods. Females with infants too young to be weaned left with the old males and returned after their offspring had fully weaned, again after a significantly shorter weaning period than during stable times.
  
  Section::::See also.
  
  BULLET::::- Infanticide
  
  BULLET::::- Infanticide (zoology)
  
  BULLET::::- Infanticide in carnivores
  
  BULLET::::- Infanticide in rodents
   Gestation length in humans is believed to be shorter than most other primates of comparable size. The gestation length for humans is 266 days, or eight days short of nine months, which is counted from the first day of the woman's last menstrual period. During gestation, mothers must support the metabolic cost of tissue growth, both of the fetus and the mother, as well as the ever-increasing metabolic rate of the growing fetus. Comparative data from across mammals and primates suggest that there is a metabolic constraint on how large and energetically expensive a fetus can grow before it must leave the mother's body. It is thought that this shorter gestation period is an adaptation to ensure the survival of mother and child because it leads to altriciality. Neonatal brain and body size have increased in the hominin lineage, and human maternal investment is greater than expected for a primate of our body mass. The obstetrical dilemma hypothesis suggests that in order to successfully undergo childbirth, the infant must be born earlier and earlier, thereby making the child increasingly developmentally premature. The concept of the infant being born underdeveloped is called altriciality. Humans are born with an underdeveloped brain; only 25% of their brains fully developed at birth as opposed to non-human primates where the infant is born with 45–50% brain development. Scientists have believed that the shorter gestation period can be attributed to the narrower pelvis, as the baby must be born before its head reaches a volume that cannot be accommodated by the obstetric canal.
   BULLET::::- Pregnancy is very short, typically four to five weeks. The embryo is born at a very early stage of development, and is usually less than long at birth. It has been suggested that the short pregnancy is necessary to reduce the risk that the mother's immune system will attack the embryo.
   In primates, early skin-to-skin contact is part of a universal reproductive behaviour, and early separation is used as a research modality to test the harmful effects on early development. Research suggests that for all mammals, the maternal environment (or place of care) is the primary requirement for regulation of all physiological needs (homeostasis), maternal absence leads to dysregulation and adaptation to adversity.
   A randomized controlled trial published in 2004 reports that babies born between 1200 and 2200g became physiologically stable in skin-to-skin contact starting from birth, compared to similar babies in incubators. In another randomized controlled trial conducted in Ethiopia, survival improved when skin-to-skin contact was started before 6 hours of age.
   BULLET::::- Constant carrying or physical contact with other people (usually their mothers or fathers) in the several months after birth, as these adults go about their day-to-day business (during which the infants observe and thus learn, but also nurse, or sleep); this forms a strong basis of personal security for infants, according to Liedloff, from which they will begin developing a healthy drive for independent exploration by eventually starting to naturally creep, and then crawl, usually at six to eight months; She calls this the "In-Arms" phase.
   According to the Swiss biologist Adolf Portmann, it is a characteristic feature of humans that due to their early birth, many developmental processes do not happen in isolation, but embedded in a sociocultural environment. Due to their complete dependence, humans are particularly amenable for social interactions and their environmental condition, which, according to Portmann, is a precondition for cultural and intellectual learning.
   Pregnancy varies person-to-person and across cultural and socioeconomic lines. Human gestation is between 30 and 40 weeks long. The dynamic between the mother and the fetus is one of conflict: it is in the best interest of the fetus to gestate as long as possible to continue receiving the nutritional and developmental benefits of being physically attached to the mother. For the mother, however, pregnancy is a highly demanding and risky time. Earlier births avoid complications in the birth of a too-large infant. The length of the pregnancy is a compromise between these two demands, and is influenced by factors such as socioeconomic status, health, and fetal development. Women of lower socioeconomic status have been shown to deliver their babies earlier on average than women of higher socioeconomic status. Research has also shown that stress, especially during early pregnancy, can cause shorter gestation length and increase premature births.
   The parts of the fetal brain that control movement will not fully form until late in the second trimester, and the first part of the third trimester. Control of movement is limited at birth, and purposeful voluntary movements develop during the long period up until puberty. According to an overview produced by the Royal College of Physicians of Edinburgh, purposive movement begins at about 18 weeks, gradually replacing reflex movements, and purposeful voluntary movements then develop further after birth.
   Different animals employ different precocial and altricial strategies; there is no clear distinction between the two states, and a wide range of intermediate states. The ability of the parents to obtain nutrition and contribute to the pre-natal and post-natal development of their young appears to be associated.
   Infant mortality in the wild is at its highest during weeks 5–15 of their lives, when they begin to move around and explore on their own, this is because one of the greatest threats to infant survival is falling from the canopy.
  
  Section::::Distribution.
   There is no sharp limit of development, gestational age, or weight at which a human fetus automatically becomes viable. According to studies between 2003 and 2005, 20 to 35 percent of babies born at 23 weeks of gestation survive, while 50 to 70 percent of babies born at 24 to 25 weeks, and more than 90 percent born at 26 to 27 weeks, survive. It is rare for a baby weighing less than 500 g (17.6 ounces) to survive. A baby's chances for survival increases 3-4% per day between 23 and 24 weeks of gestation and about 2-3% per day between 24 and 26 weeks of gestation. After 26 weeks the rate of survival increases at a much slower rate because survival is high already. Prognosis depends also on medical protocols on whether to resuscitate and aggressively treat a very premature newborn, or whether to provide only palliative care, in view of the high risk of severe disability of very preterm babies.
   Placentals' best-known feature is their method of reproduction:
  
  BULLET::::- The embryo attaches itself to the uterus via a large placenta via which the mother supplies food and oxygen and removes waste products.
  
  BULLET::::- Pregnancy is relatively long and the young are fairly well-developed at birth. In some species (especially herbivores living on plains) the young can walk and even run within an hour of birth.
  
  It has been suggested that the evolution of placental reproduction was made possible by retroviruses that:
   BULLET::::- Initially babies cry briefly – a very distinctive birth cry
  
  BULLET::::- Then they will enter a stage of relaxation, recovering from the birth
  
  BULLET::::- Then the baby will start to wake up
  
  BULLET::::- Then begin to move, initially little movements, perhaps of the arms, shoulders and head
  
  BULLET::::- As these movements increase the baby will actually start to crawl towards the breast
  
  BULLET::::- Once the baby has found the breast and therefore the food source, there is a period of rest. Often this can be mistaken as the baby is not hungry or wanting to feed
   The obstetrical dilemma hypothesis has had several challenges to it, as more data is collected and analyzed. Several different fields of study have taken an interest in understanding more about the human birth process and that of human ancestor species.
  
  Section::::Challenges to the obstetrical dilemma hypothesis.:Early brain growth rates.
   Section::::Challenges to the obstetrical dilemma hypothesis.:Energetics of gestation and growth hypothesis.
   The term "high-risk newborns" refers to neonates with a significant chance of mortality or morbidity, especially within the first month of being born. High-risk newborns will often show abnormal responses of primitive reflexes, or lack a response entirely. Performance of primitive reflexes in high-risk newborns will often vary in response depending on the reflex (e.g., normal Moro reflex may be present, while the walking reflex is absent or abnormal). Normal performance of primitive reflexes in newborns can be linked to a greater likelihood of having higher Apgar scores, higher birth weight, shorter hospitalization time after birth, and a better overall mental state.
  
presuppositions: Animals and babies are born at a similar stage of development.
corrections: Due to the small size of a woman's pelvis, human babies are born less developed than animal offspring are.
raw_presuppositions: Animals and babies are born at a similar stage of development. Animals and babies are born at a similar stage of development.
raw_corrections: Due to the small size of a woman's pelvis, human babies are born less developed than animal offspring are. Due to the small size of a woman's pelvis, human babies are born less developed than animal offspring are.