Sexual dimorphism refers to the sex differences among the same species of organism. This can be seen through out the animal kingdom. Humans also have varying degrees of sexual dimorphism. There are two sets of characteristics that the human sexes have. The primary sex characteristics are related to organs of sexual reproduction. Secondary sexual characteristics are physical attributes that develop after puberty. Sexual dimorphism is the result of millions of years of human evolution from the first australopiths to the homo sapiens . There are billions of people on Earth and humanity continues to be a successful species.
Puberty marks the biological transition to adulthood. Girls will become women and boys will become men with the ability to sexually reproduce. Growth hormone will be released by the pituitary gland causing the body to increase in size (around ages 14 to 16). Girls develop their growth spurt sooner than boys. Although boys are slower to grow they will eventually catch up and surpass girls in height ( men continue to grow until their 20s). Girls develop their menstrual period ( the menarche happens around ages 12 to 13). Boys testes will enlarge and will produce more testosterone. Gonadotropic-releasing hormone coordinates the regulation of testosterone in conjunction with the anterior pituitary gland and the hypothalamus. Women will produce more estrogen, instead of androgens. Girls will also produce progesterone The hypothalamus acts as a director in the changes in the body. Children approaching the adolescent life cycle stage will change physiologically. Pubic hair appears on young men and women. Both sexes gain under arm hair. Men, not women gain facial hair and their voice will deepen. Boys will have their penis grow and women will develop ovaries ,breasts. and see changes in the uterus ( including the vagina). When this process is over adulthood becomes the next stage in the human life cycle.
Puberty marks the biological transition to adulthood. Girls will become women and boys will become men with the ability to sexually reproduce. Growth hormone will be released by the pituitary gland causing the body to increase in size (around ages 14 to 16). Girls develop their growth spurt sooner than boys. Although boys are slower to grow they will eventually catch up and surpass girls in height ( men continue to grow until their 20s). Girls develop their menstrual period ( the menarche happens around ages 12 to 13). Boys testes will enlarge and will produce more testosterone. Gonadotropic-releasing hormone coordinates the regulation of testosterone in conjunction with the anterior pituitary gland and the hypothalamus. Women will produce more estrogen, instead of androgens. Girls will also produce progesterone The hypothalamus acts as a director in the changes in the body. Children approaching the adolescent life cycle stage will change physiologically. Pubic hair appears on young men and women. Both sexes gain under arm hair. Men, not women gain facial hair and their voice will deepen. Boys will have their penis grow and women will develop ovaries ,breasts. and see changes in the uterus ( including the vagina). When this process is over adulthood becomes the next stage in the human life cycle.
Here are some of the basic secondary sex characteristics. Men have broader shoulders, while women have wider hips. Men have a larger laryngeal prominence and women have mammary glands.
The primary sex characteristics are the genitals. Both men and women contain gonads that aid in the reproduction of off spring. The gonads are responsible for producing gametes. Haploid cells are formed from gametes. Men produce spermatozoon in the testes and women ovum. The sperm and egg will combine to form a zygote which will become an embryo and a new life will develop. The male reproductive system contains various parts. The testes produce sperm and sex hormones. Males produce more androgens than women. Men have testosterone close to a range of 0.95-4.3 pg/dl. The testes are protected by the scrotum, which is a sac like structure. Internally, testes contain the seminiferous tubules. These structures contain cells that are in the process of spermatogenesis. The epididymis transfers sperm to the urethra (for ejaculation). The vasa deferentia acts a storage area for sperm ( just like the epididymus). The bulbourethral gland ( commonly called the cowper's gland) contributes fluid to semen. The prostate gland secretes fluid. When the man becomes aroused the penis will become erect and release semen.
The anatomical structure of the male reproductive system
The female reproductive system has different anatomical structures. These structures include the uterus,vulva, vagina, fallopian tubes, and ovaries. The clitoris is similar to the penis in terms of being an organ of copulation. It has twice as many nerve endings when compared to the penis. The uterus of the female body is responsible for containing the developing embryo and fetus. The ovaries will produce eggs and sex hormones. The oviducts are the location of fertilization of eggs and direction of them. The vagina during copulation will facilitate the entry of the penis. Hormone production functions in a similar manner between the hypothalamus and the anterior pituitary gland. Feedback control manages the amount of estrogen and progesterone. Just like males hormone levels are controlled by a feedback loop. The hypothalamus will produce gonadotropic hormone and then directs the anterior pituitary gland to form follicle producing hormone and luteinizing hormone. FHS will produce estrogen and LH will form progesterone. This process happens in males except testosterone and other androgens are produced. The corpus lutem produces progesterone and the follicle is responsible for estrogen.
The anatomical structure of the female reproductive system
The secondary sex characteristics are attributes not related to the reproductive system. Phenotypic traits vary among men and women. Men because of the production of testosterone have more muscle mass and larger bone structure (as well as denser ligaments and tendons). Women's production of estrogen allows for the storage of fat (mostly on the posterior, thighs,and hips). This means on average men would be stronger than women, however this follows a bell curve in graphical analysis. It is not impossible for a woman to be stronger than a man. Men have a vast majority of their strength concentrated in the upper body, while women are closer to men in the lower body. Males have more type II muscle fibers and greater capacity for muscular hypertrophy. At the cellular level there is no difference between male and female muscle. The skeleton differs between men and women. Women have wider hips and a pelvis that is shaped to accommodate child birth. The skull has more mass in males. Seeing as women have lower bone mass, this makes them more vulnerable to osteoporosis. Men and women's faces also differ. A male face will be square like in the jaw and a female face will be rounder. Genetically sex can be determined by certain chromosomes. Males contain the XY chromosomes and females contain XX. The SRY gene regulates sexual differentiation of the Y chromosome.
The Y chromosome
Brain size does differ among males and females. This does not cause a difference in intelligence, which varies from person to person. The reason for the difference in brain size is due to body size. Other organs of the body do differ physiologically. Men have a high lung capacity and higher red blood cell count. Women have a higher white blood cell count and as a result would be better at fighting off infectious diseases. Women will on average live longer than men. However, there are men who do reach advanced age. The human population varies in a multitude of ways and biology is not the only factor. Organisms are products of both biology and environment.
The reason sexual dimorphism exists is because of natural selection and mate choice in the animal kingdom. Natural selection will favor particular traits to be inherited in an animal population. Producing offspring is critical to survival of a species. Not only do animals fight to survive, but they also compete for mates. The earliest hominids did this. Humans to day with advanced civilization still in a different manner compete for mates ( we call it courtship or dating). Certain attributes could indicate which organisms have a better chance of producing offspring.Women's wide hips and rounder appearance could indicate to males that she is fertile. Men's stronger looking stature could be a signal to women that he is a better provider of offspring. Mate choice can be more complex than just these two factors. Sex differences are the product of human evolution and environmental factors .
Further Reading
Mader, Sylvia. Biology. New York: McGraw-Hill,2010.
Barnes-Svarney, Patricia. Science Desk Reference. New York : The Stonesong Press Inc, 1995.
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