Nonetheless, there are a few key distinctions in our physiques. There is an argument that this pattern was driven by the physical constraints on the mechanisms by which two gametes get together as required for sexual reproduction. Men, free from the requirements of childbirth, benefit from being as strong and lithe as possible, both in their search for food, and when in competition with other men. Breasts also help men figure out who to pursue to achieve reproductive success. A distinguishing characteristic of the class Mammalia is the presence of mammary glands. During reproduction , the male contributes either an X sperm or a Y sperm, while the female always contributes an X egg.
Women store the extra fat in their breasts, hips, buttocks, and as subcutaneous fat in the bottom layer of their skin giving a woman's skin its softer, plumper feel. These derivations have been traced by Renkama  who illustrated how Greek letters can be transformed into the graphic male and female symbols still recognised today. Along with chiseled jaws, higher testosterone has been shown to correlate with muscle strength and aggression in men, as well as with genetic vigor. Members of Hymenoptera , such as ants and bees, are determined by haplodiploidy , where most males are haploid and females and some sterile males are diploid. Male and female bodies are well-designed for each gender's role in a primitive society. Breasts also help men figure out who to pursue to achieve reproductive success. They also tend to rate men with masculine features as more attractive, especially when they themselves are ovulating and, thus at least subconsciously on the lookout for a male sex partner who'll produce fit offspring. Perhaps for this reason, studies have shown that women judge men with more angular features as likely to be dominant over men with rounder, more effeminate faces. This is because the genes that code for nipple development switch on in utero, and at a very early embryonic stage even before the genes gear up that turn us into males or females. Alternative theories exist, but most scientists think breasts are an evolutionary trick for snagging men; though they're actually filled with fat, not milk, they signal a woman's bountiful ability to feed her children. To compensate for the difference in size, one of the female's X chromosomes is randomly inactivated in each cell of placental mammals while the paternally derived X is inactivated in marsupials. They're looking for a mate with whom to produce healthy offspring. In the forward of a book on the subject, "Fair Women, Dark Men" by Peter Frost, University of Washington sociologist Pierre van den Berghe wrote, "Although virtually all cultures express a marked preference for fair female skin, even those with little or no exposure to European imperialism, and even those whose members are heavily pigmented, many are indifferent to male pigmentation or even prefer men to be darker. This is because sex hormones called androgens stimulate hair growth, and men have more of those hormones. Women are the only primates who are busty all the time, even when they aren't nursing. Unlike those animals, men and women are more physically similar than we are different. The mammary glands are modified sweat glands that produce milk, which is used to feed the young for some time after birth. Mammary glands are present in all mammals, although they are seldom used by the males of the species. In short, sex hormones control the divergence of male and female facial features. Sex-determination system The sex of a particular organism may be determined by a number of factors. The ZW sex-determination system , where males have ZZ as opposed to ZW sex chromosomes, is found in birds, reptiles and some insects and other organisms. A full, buoyant bosom can therefore demonstrate fertility. Women are built for carrying and birthing children, and must have wider hips and keep extra fat in store for the ordeal of pregnancy. There is no single genetic mechanism behind sex differences in different species and the existence of two sexes seems to have evolved multiple times independently in different evolutionary lineages. The answer is that the pitch of a man's voice correlates with the amount of the male sex hormone testosterone he has, and his testosterone level is itself indicative of his genetic quality and sexual fitness. However, these conditions frequently result in sterility. These may be genetic or environmental, or may naturally change during the course of an organism's life.
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