Sperm and egg formation

Sperm cells are made in the testes of males. Through a process of cellular division called meiosis, sperm-producing cells with regular genetic material (diploid cells, meaning they posses two copies of each chromosome) undergo division with the end product being two cells each with only one copy of each chromosome (haploid, half of the parent cell). Because sperm production is optimal at temperatures slightly colder than average body temperature, testes are housed in a pouch

These giraffes illustrate flehmen behavior through which the male assesses the sexual receptivity of the female by sniffing her urine and allowing it to run over the Jacobson's organ in the roof of the mouth. (Photo by Rudi van Aarde. Reproduced by permission.)

of skin just outside the body, the scrotum. But not all species have scrotal testes year-round, and some species such as bats have testes that are kept internal for most of the year and become external prior to breeding. Other species such as aquatic cetaceans (whales and dolphins) or sirenians (dugong and manatees), as well as terrestrial armadillos, elephants, and sloths retain internal testes even during reproductive season; interestingly, sperm production readily occurs at normal body temperatures in these species.

Once formed, the sperm cells are stored in the epididymis where they remain until sperm is ejaculated. Not surprisingly, the mating system of species is often correlated with the size of testes, and many species with promiscuous mating systems will have larger testes simply because copulations are more frequent and males require more sperm, for example lions. Sperm production in humans is constant throughout reproductive life, but in seasonally breeding animals it only occurs immediately prior to the reproduction period as sperm made too far in advance degrades with age. Because millions of sperm are released with each ejaculate, the number of sperm produced by a male during a lifetime is astronomically high.

In females, the process is much different. At birth, females already possess in their ovaries all the eggs they will ever produce. Eggs are stored as follicles, and they will start develop ing into fully functional eggs once sexual maturity is reached. As follicles mature, they expand in size on the surface of the ovary until ovulation is triggered by release of luteinizing hormone. Peaks of this hormone occur either as part of the es-trous cycle or are triggered by physical stimuli in induced ovulators such as cats (Felidae) and rabbits (lagomorphs).

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