Most female mammals are only fertile during certain periods during their estrous cycle, at which point they are ready to mate.Individual male and female mammals meet and carry out copulation.
After fertilization, and the formation of a zygote, and varying degrees of development, in many species the eggs are deposited outside the female; while in others, they develop further within the female and are born live.
There are three extant kinds of mammals: monotremes, placentals and marsupials, all with internal fertilization.
In the first stage of sexual reproduction, "meiosis", the number of chromosomes is reduced from a diploid number (2n) to a haploid number (n).
During "fertilization", haploid gametes come together to form a diploid zygote and the original number of chromosomes is restored.
Biologists studying evolution propose several explanations for why sexual reproduction developed and why it is maintained.
These reasons include reducing the likelihood of the accumulation of deleterious mutations, increasing rate of adaptation to changing environments, All of these ideas about why sexual reproduction has been maintained are generally supported, but ultimately the size of the population determines if sexual reproduction is entirely beneficial.Dimorphism is found in both sex organs and in secondary sex characteristics, body size, physical strength and morphology, biological ornamentation, behavior and other bodily traits.However, sexual selection is only implied over an extended period of time leading to sexual dimorphism.Generally in animals mate choice is made by females while males compete to be chosen.This can lead organisms to extreme efforts in order to reproduce, such as combat and display, or produce extreme features caused by a positive feedback known as a Fisherian runaway.For instance, in human reproduction each human cell contains 46 chromosomes, 23 pairs, except gamete cells, which only contain 23 chromosomes, so the child will have 23 chromosomes from each parent genetically recombined into 23 pairs.