HOW THE BRAIN CONTROLS SEXUAL BEHAVIOUR

HOW THE BRAIN CONTROLS SEXUAL BEHAVIOUR
We can be pretty sure that, in males, the preoptic area is involved in the control of sexual behaviour because:
1. lesions of this region permanently abolish male sexual behaviour;
2. electrical stimulation of this area can elicit copulatory activity;
3. neuronal and metabolic activity is induced in this area during copulation; and
4. small implants of the male hormone testosterone into this area restore sexual behaviour in castrated rats.

[David Buss (1953– ), a professor in the Evolutionary Psychology Research Lab, University of Texas at Austin, has pioneered the use of modern evolutionary thinking in the psychology of human behaviour and emotion. His primary research has focused on human mating strategies and conflict between the sexes. He has championed the idea that men and women have different long-term and short-term mating strategies, and that monogamous and promiscuous mating strategies may coexist. Some interesting extensions to his work include references to sexual jealously and coercion, homicide, battery and stalking. In an effort to find empirical rather than circumstantial evidence to show that human psychological preferences have evolved and are not only learned, Buss has performed many cross-cultural studies containing up to 10,000 participants from many countries around the globe. Overall, his evolutionary psychology has highlighted the dynamic and contextsensitive nature of evolved psychological mechanisms.]

In females, the preoptic area is involved in the control of reproductive cycles, and is probably directly involved in controlling sexual behaviour too. The ventromedial nucleus of the hypothalamus (VMH) is also involved in sexual behaviour. Outputs from the VMH project to the periaqueductal gray of the midbrain, and this region is also necessary for female sexual behaviour, including lordosis (the position adopted by a female to accept a male) in rodents. This behaviour can be reinstated in ovariectomized female rats by injections of the female hormones oestradiol and progesterone into the VMH brain region. Can the brain help us to understand sexual arousal at the sight and smell of someone to whom we are sexually attracted? By receiving inputs from the amygdala and orbitofrontal cortex, the preoptic area receives information from the inferior temporal visual cortex (including information about facial identity and expression), the superior temporal auditory association cortex, the olfactory system and the somatosensory system. It is presumably by these neural circuits that the primary rewards relevant to sexual behaviour (such as touch and perhaps smell) and the learned stimuli that act as rewards in connection with sexual behaviour (such as the sight of a partner) reach the preoptic area. And it is likely that, in the preoptic area, the reward value of these sensory stimuli is modulated by hormonal state, perhaps (in females) related to the stage of the menstrual cycle – women are more receptive to these sensory stimuli when they are at their most fertile. The neural control of sexual behaviour may therefore be organized in a similar way to the neural controls of motivational behaviour for food. In both systems, external sensory stimuli are needed to provide the reward, and the extent to which they do this depends on the organism’s internal state, mediated by plasma glucose concentration for hunger and hormonal status for sexual behaviour. For sexual behaviour, the internal signal that controls the motivational state and the reward value of appropriate sensory stimuli alters relatively slowly. It may change, for example, over four days in the rat oestrus cycle, or over weeks or even months in the case of many animals that only breed during certain seasons of the year.

The outputs of the preoptic area include connections to the tegmental area in the midbrain. This region contains neurons that are responsive during male sexual behaviour (Shimura & Shimokochi, 1990). But it is likely that only some outputs of the orbitofrontal cortex and amygdala that control sexual behaviour act through the preoptic area. The preoptic area route may be necessary for some aspects of sexual behaviour, such as copulation in males, but the attractive effect of sexual stimuli may survive damage to the preoptic . Research findings suggest that, as for feeding, outputs of the amygdala and orbitofrontal cortex can also influence behaviour through the basal ganglia. Much research remains to be carried out into how the amygdala, orbitofrontal cortex, preoptic area and hypothalamus represent the motivational rewards underlying sexual behaviour. For instance, it has recently been found that the pleasantness of touch is represented in the human orbitofrontal cortex (Francis et al. 1999). Findings such as these can enhance our understanding of sexuality in a wider context.

SOCIOBIOLOGY AND SEXUAL BEHAVIOUR

SOCIOBIOLOGY AND SEXUAL BEHAVIOUR
Sperm warfare
Monogamous primates (those with a single mate) living in scattered family units, such as some baboons, have small testes. Polygamous primates (those with many mates) living in groups, such as chimpanzees, have large testes and copulate frequently. This may be related to what sociobiologists call ‘sperm warfare’. In order to pass his genes on to the next generation, a male in a polygamous society needs to increase his probability of fertilizing a female. The best way to do this is to copulate often and ejaculate a large quantity of sperm, increasing the chances that his sperm will reach the egg and fertilize it. So, in polygamous groups, the argument is that males have large testes to produce large numbers of sperm. In monogamous societies, with less competition between sperm, the assumption is that the male just picks a good partner and produces only enough sperm to fertilize an egg without the need to compete with others’ sperm. He also stays with his partner to bring up the offspring in which he has a genet ic investment, and to guard them (Ridley, 1993).
What about humans? Despite widespread cultural pressure in favour of monogamy or restricted polygamy, humans are intermediate in testis size (and penis size) – bigger than might be expected for a monogamous species. But remember that although humans usually do pair, and are apparently monogamous, we also live in groups, or colonies. Perhaps we can find clues about human sexuality from other animals that are paired but also live in colonies. A problem with this type of comparison, though, is that for most primates (and indeed most mammals) it is the female who makes the main parental investment – not only by producing the egg and carrying the foetus, but also by feeding the baby until it becomes independent. In these species, the male apparently does not have to invest behaviourally in his offspring for them to have a reasonable chance of surviving. So the typical pattern in mammals is for the female to be ‘choosy’ in order to obtain a healthy male, and for the males to compete for females. But, because of its large size, the human brain is not fully developed at birth, so infants need to be looked after, fed, protected and helped for a considerable period while their brain develops and they reach independence. So in humans there is an advantage to paternal investment in helping to bring up the children, because the paternal resources (e.g. food, shelter and protection) can increase the chances of the father’s genes surviving into the next generation to reproduce again. In humans, this therefore favours more complete pair bonding between the parents.

Couples in colonies
It is perhaps more useful to compare humans with birds that live in colonies, in which the male and female pair up and both invest in bringing up the offspring – taking turns, for example, to bring back food to the nest. Interestingly, tests on swallows using DNA techniques for determining paternity have revealed that approximately one third of a pair’s young are not sired by the male of the pair (Ridley, 1993). So the female is sometimes mating with other males – what we might call committing adultery! These males will probably not be chosen at random: she may choose an ‘attractive’ male by responding to indicators of health, strength and fitness. One such indicator in birds is the gaudy tail of the male peacock. It has been suggested that, given that the tail handicaps movement, any male that can survive with such a large tail must be very healthy or fit. Another theory is that a female would choose a male with an attractive tail so that her sons would be attractive too and also chosen by females. (This is an example of the intentional stance, since clearly the peahen is incapable of any real propositional thought; but it has also been criticized as representing a somewhat circular line of argument.) Choosing a male with an attractive tail may also benefit female offspring, so the argument goes, because of the implied health/fitness of the fathering peacock. In a social system such as the swallows’, the ‘wife’ needs a reliable ‘husband’ to help provide resources for ‘their’ offspring. A nest must be built, the eggs must be incubated, and the hungry young must be well fed to help them become fit offspring (‘fit’ here means capable of ‘successfully passing on genes into the next generation’). The male must in some sense ‘believe’ that the offspring are his – and, for the system to be stable, some of them must actually belong to him. But the female also benefits by obtaining genes that will produce offspring of optimal fitness – and she does this by sometimes ‘cheating’ on her ‘husband’. To ensure that the male does not find out and therefore leave her and stop caring for her young, she ‘deceives’ him by ‘committing adultery’ secretly, perhaps hiding behind a bush to mate with her ‘lover’. So the ‘wife’ maximizes care for her children by ‘exploiting’ her ‘husband’, and maximizes her genetic potential by finding a ‘lover’ with better genes that are subsequently likely to make her offspring more attractive to future potential mates.
The implication is that genes may influence our motivational behaviour in ways that increase their subsequent success.

[Richard Dawkins (1941– ), Professor for the Public Understanding of Science at the University of Oxford, in his book The Selfish Gene (1976), highlighted the way in which natural selection operates at the level of genes rather than individuals or species. W.D. Hamilton, also of the University of Oxford, provided some of the theoretical foundations for this approach (described in The Narrow Roads of Gene Land, 2001). ‘Selfish gene’ theory provides potential explanations for a number of aspects of animal and human behaviour that are otherwise difficult to explain. For example, it explains how the likelihood that an individual will display altruistic behaviour towards another depends on how closely the two are related genetically. This approach has also been used to understand the phenomenon of sperm competition, and the effects that this has on sexual behaviour. This approach is now thought of as a modern version of Darwinian theory, and has set a new paradigm for many disciplines including biology, zoology, psychology and anthropology.]

Are humans like swallows?
Again, how might this relate to human behaviour? Though it is not clear how important they are, there is some evidence to suggest that such factors could play some part in human sexual behaviour. One potentially relevant piece of evidence in humans concerns the relatively large testis and penis size of men. The general argument in sociobiology is that a large penis could be adaptive in sperm competition, by ensuring that the sperm are placed as close as possible to where they have a good chance of reaching an egg, and so displacing other sperm, thereby winning the ‘fertilization race’. A second line of evidence is that studies in humans of paternity using modern DNA tests suggest that husbands are not the biological fathers to about 14 per cent of children (Baker & Bellis, 1995; see Ridley, 1993). So it is possible that the following factors have shaped human sexual behaviour in evolution: women might choose a partner likely to provide reliability, stability, provision of a home, and help with bringing up her children; women might also be attracted to men who are perhaps successful and powerful, increasing the likelihood of producing genetically fit children, especially sons who can themselves potentially have many children; men might engage in (and be selected for) behaviours such as guarding the partner from the attentions of other men, to increase the likelihood that the children in which he invests are his; and men might be attracted to other women for their childbearing potential, especially younger women. Much of the research on the sociobiological background of human sexual behaviour is quite new and speculative, and many of the hypotheses have still to be fully tested and accepted. But this research does have interesting implications for understanding some of the factors that may influence human.

SEXUAL BEHAVIOUR

SEXUAL BEHAVIOUR
Just as we need to eat to keep ourselves alive, working to obtain rewards such as food, so we need to have sex and reproduce in order to keep our genes alive. In this part of the chapter, we will look at the following two questions: How can a socio-biological approach (that is, an approach which seeks to reconcile our biological heritage as a species with our highly social organization) help us to understand the different mating and child-rearing practices of particular animal species? How does the human brain control sexual behaviour?