In females, breasts are a manifestation of higher levels of estrogen; estrogen also widens the pelvis and increases the amount of body fat in hips, thighs, buttocks, and breasts. Estrogen also induces growth of the uterus, proliferation of the endometrium, and menses.
I wonder what our bodies would look like if they'd evolved only in necessary ways.....
Are baby cows born with udders?
In: Cattle Reproduction [Edit categories]
No. The udder develops during the time the heifer, after being bred at 15 months of age (breeding age), is pregnant with calf. The udder doesn't start developing until the third trimester, when it starts filling up with milk, and is ready until the calf is born and gets up and starts suckling.
Since all mammals have breasts to nurse their young, it might seem odd to search the unique evolutionary past of humans for an explanation of their existence. Yet, there is a reason to do so. There is something different about human female breasts, leading scientists to develop theories about them: after the age of puberty, the breasts of a human female become “permanently enlarged,” although this is not the case in other primates, our closest evolutionary relatives.
If you were to look at a chimpanzee or a gorilla, or any other ape or monkey for that matter, the breasts of a non-pregnant, non-nursing female would not be enlarged. Although the breast tissue of a female ape or monkey swells when she is pregnant and nurses her young, once she is done lactating, it typically recedes back to a flattened form. Most of the time, you would not be able to distinguish a male from a female by the size and shape of breasts alone; breasts in non-human primates are not what scientists call “a sexually dimorphic trait,” one that exists in two different forms in the males and females of the same species.
Examples of sexually dimorphic traits in non-human primates include the large canine teeth of male, but not female, baboons, and the large size of male gorillas in comparison to females.
Darwin proposed that natural selection might not be the only evolutionary force at work. Instead, he claimed sexually dimorphic traits arose because the individuals who possessed them had an increased reproductive advantage stemming from another source: the advantage it conferred on them in terms of increased access to mates. Darwin argued that many sexually dimorphic traits were not advantageous to an individual in the struggle for survival but in the struggle over mating opportunities. Darwin called this differential access to mates “sexual selection,” and outlined how it worked in animals, including humans, in his book The Descent of Man (1871).
Darwin focused on two types of traits that increased access to mates: the weapons and other characteristics used and displayed by males when fighting among themselves over access to females, such as the large canines of baboons, and those traits that make males more attractive to females, such as peacock plumage. Interestingly, Darwin posited that females choose males, not that males choose females, suggesting sexual selection operated on males alone. This may be because he recognized that from a species standpoint, it makes good sense for all females to mate, while this may not be equally true of males: theoretically, it takes only one “good” male to impregnate a large number of females. Whatever the reasons for Darwin’s view that sexual selection did not directly modify females, it is clear he did not think that male choice among more or less desirable females was an important evolutionary force.
Yet, this is exactly what contemporary scientists argue in their attempts to explain the difference between the breast of humans and other primates. They claim that human female breasts became sexual signals to attract males. Females with this trait would have increased mating opportunities, thereby passing the genes for permanently enlarged breasts on to subsequent generations. A prime example of this way of thinking is the following statement by anthropologist, Bernard Campbell:
Since in the primates, and particularly among men, the choice of partners lies with the male, it follows that only the physical characteristics of the female are subject to sexual selection of this kind… breasts have always been attractive to men and have no doubt been subject to sexual selection (1970:304).
But why should this particular body trait have become the object of sexual desire?
Desmond Morris, a curator of mammals at the London Zoo, was among the first to offer an explanation. In his widely read book, The Naked Ape (1967), he suggested that permanently enlarged breasts in human females resulted from hominid bipedalism. It is widely believed among scientists that our earliest ancestors survived by becoming bipedal, that is, by walking on two feet in an erect position. Although there are a number of specific explanations for why this trait might have conferred an adaptive advantage on some ancient hominid population, scientists agree that bipedalism is the defining characteristic of the human species. Indeed, when a fossil primate is unearthed, the only sure way to determine whether it is a hominid is to figure out whether the individual in question walked upright.
The link between bipedalism and permanent breast enlargement, according to Morris, has to do with the erotic nature of breasts. He argues that as early humans (hominids) began walking upright, face-to-face encounters between the sexes became the norm, affecting the position used in sexual intercourse: males would no longer mount females from behind as they do among non-human primates. In the non-human primate position, presentation of the female buttocks to the male is an erotic display that stimulates male interest and excitement. With the advent of bipedalism, Morris argues, if females were to be successful in shifting male interest around to the front, evolution would have to do something to make the female frontal region more stimulating to males. This was accomplished, Morris says, through self-mimicry in which female breasts came to look like rounded buttocks: female breasts became mimics of “the ancient genital display of [the] hemispherical buttocks” (1967:75). Szalay and Costello (1991) have continued this line of thinking, but argue that permanently enlarged breasts sexually arouse males not because they look like buttocks, but because they mimic the appearance of female genitalia.
For sociobiologists and evolutionary psychologists, enlarged breasts are just another weapon in a female’s arsenal for attracting a man and keeping him around. For example, J. Cant (1981) suggests that breasts became signals alerting a male to which female would be more likely to raise his offspring successfully. Cant argues that females better able to build up fat and maintain it would have more reserves to convert to parental investment during pregnancy and lactation. Since breasts are primarily composed of fat, Cant argues that they became an indicator of the nutritional status of the female.
Roger Short (1976) also argues that females with shapely breasts were more likely to attract a male and keep him around, ensuring that he would take care of her and their offspring. Owen Lovejoy (1981) has come to a similar conclusion: he argues that females who could use the permanent features of her body to attract a male would have a distinct advantage over those women who relied on behavioral strategies alone.
These theorists stress how keeping a man around saved the human species from extinction. Short, for instance, argues that females became dependent on males for food in the far reaches of the human evolutionary past because females became restricted in their movements due to the increased dependency of infants on her care. Unable to support herself and her children alone, a woman would need male parental investment in children became increasingly necessary for survival. According to him, breasts became objects of attraction that ensured the pair bonding between a male and a female, and thus the survival of children.
Lovejoy not only focuses on the role male provisioning of dependent females played in the development of permanent breast enlargement, he also puts it at the very center of his explanation of bipedalism and the evolution of humans. Cant thinks bipedalism arose to solve what he calls “the demographic dilemma of apes.” This dilemma resulted from the reproductive strategy used by apes, one referred to as K-selection. As paleoanthropologist, Donald Johanson and science writer, Maitland Edey explain:
There are two fundamentally different ways in which an animal can function sexually: It can produce a great many eggs with an investment of very little energy in any one egg, or it can produce very few eggs with a large investment in each. These are known as the “r” strategy and the “K” strategy respectively . . . “K” is obviously far more efficient than “r,” but it too has its limits. Accidents, predation, seasonal food failure, illness-all take their toll on animals. Losing an infant to one of these hazards after an investment of five or six years is hideously costly compared with the loss of an egg by [an r strategy animal]. (1981:46)