Our species, like most animals on earth, has adopted a mechanism of sexual reproduction for the propagation of our species. In evolutionary terms, this has been an enormously successful strategy. We’ve populated all continents and there are currently 7 billion of us on the planet. The advantages of sexual reproduction are manifest; removal of potentially harmful mutations from the genome, genetic diversity in the offspring, resistance to harmful pathogens, culminating in survival benefits for the population as a whole. Sexual reproduction involves the union of two haploid gametes, one produced by the male (spermatozoan); and one produced by the female (ovum). Following sexual intercourse, the successful fusion of the gametes at fertilisation in the female uterine tube produces the diploid zygote, signifying the earliest stage of a new life. Implantation of the zygote in the female uterus initiates early pregnancy. From pregnancy to birth, maternal hormones are involved at all stages of development, and the fetal hormones also play a significant role after the first trimester, with the fetal-placental unit cooperating to fulfil the endocrine requirements of pregnancy. After birth, pronounced adaptations in the maternal neuroendocrine system ensure the mother is able to appropriately care for, protect and nurse her newborn.
Getting it together
The newborn baby is the joyful outcome of a loving partnership. But in humans, the commonly perceived ideal is the long-term monogamous relationship between man and woman, ensuring a stable, loving and nurturing environment for the offspring to thrive. However, many children are brought up in monogamous same-sex relationships, thus our understanding and appreciation of monogamy has evolved. Monogamy, or the exclusive sexual relationship between two people, is dependent on formation of a social bond which is then reinforced by the action of hormones, acting centrally in the brain. Hormones, such as the peptide hormone, oxytocin, have been implicated in trust and social bonding in humans, especially women. However the role of oxytocin per se in human sexual behaviour remains controversial. Interest in oxytocin emerged following studies in mammalian voles, which display species-dependent partner preference behaviour. Monogamy in prairie voles has been linked to elevated levels of peptide function in the brain, specifically oxytocin in females and vasopressin in males. These hormones are released in the brain during mating, and it is thought that they are essential for formation of partner preference. In female prairie voles, in particular, oxytocin appears to augment dopamine function in the brain’s reward pathway, facilitating positive reinforcement and the establishment of monogamous attachment.
Not now dear, I’ve got a head ache
Of course sex is not just a means for procreation, it is a pleasurable activity in its own right. One’s desire for sex is referred to as ‘libido’ and this trait varies greatly between individuals, being largely determined by genes and hormonal interactions. Illness and chronic stress can all negatively impact on our normal sex behaviour and indeed, decline in frequency of sex may be a barometer for mental health. Androgens are the steroid hormones responsible for libido in both sexes. In men, testicular androgens like testosterone control sex behaviour, and in females, ovarian and adrenal androgens are responsible. These sex hormones, together with oestrogens, act in the brain to regulate all aspects of sex behaviour. Specific regions of the brain for sexual drives include the ventromedial hypothalamus and amygdala. Actions of gonadal hormones in the hypothalamus are important for coordination of all aspects of sexual function, including sexual motivation, sexual arousal and sexual behaviour itself. Indeed individuals with damage to the hypothalamus generally show reduced desire for sex. In contrast, testosterone may be a treatment for low libido and sexual dysfunction in both men and women. Better understanding of sexual behaviour is therefore an important aim of reproductive neuroendocrine research.