1.0 is a major driver of male


0  IntroductionSex differences in the nervous system arefound all through the animal kingdom (Forger and de Vries, 2010). Sexual dimorphismcan be described as the differences in appearance between males and females ofthe same species, which goes beyond the differences in their sexual organs. Theextent of sexual dimorphism found in mammalian species can range from femalesbeing larger than males, to males being larger than females and possessingstriking secondary sexual characteristics which females may be lacking (Ralls,1977). In this paper, we are interested in the sexual dimorphism of mammal’sbrains and how it can have an effect on certain behaviours displayed by malesand females, specifically behaviours during copulation, maternal and paternalbehaviours, and the hormones that influence these particular behaviours. In thepast, various magnetic resonanceimaging (MRI) studies have addressed the question of certain morphological differencesof the brain of women and men, reporting conflicting results regarding brainsize and the ratio of white and grey matter (Menzler et al., 2011). Thus,the relationship between sex differences in the brain and human behaviour is asubject of controversy in psychology and society at large (Fine, 2011). To beable to elaborate on this topic, we first need to understand sexualdifferentiation and the process of developing into a male or female, from anundifferentiated zygote.

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2.0Sexual Differentiation – DevelopmentSexrefers to the biological distinction between males and females. Hence, to beassigned as a female or male several factors are used to determine thebiological sex of an individual;chromosomes (XX for female, XY for male), gonads, hormones, internal sex organsand external genitalia (Knoxand Schacht, 2016).

It’s stated that, the leading mechanism for sexualdifferentiation in mammalian species consists of a specific gene on the Ychromosome in males that initiate testis development. Hormones that areproduced from the testes, mainly testosterone, flow throughout the body, differentiatingthe periphery and brain in a male direction (Forgerand de Vries, 2010). Most testosterone developmental effects on malebrain are actually via oestrogen which is a major driver of male braindifferentiation.

In female mammalian species, a lack ofthis early exposure to these high levels of testosterone, due to the absence oftestes, allows development of default feminized characteristics (Matsuda, Mori and Kawata, 2012). Thiscan be described as a primary organisational effect, as generallyorganisational effects of hormones occur at development.  2.

1Sexual Differentiation – Post-PubertyPost-puberty,hormones have activational effects.  Theyinitiate sex-specific behaviours through actions via male and female body and brain.It was found that, new cells, includingneurons, arise in many brain regions throughout puberty in both male and femalerats. Sex differences found in pubertal addition of these new cells correspondwith sexual dimorphisms found in adults: “for each region, the sex that gainsmore cells during puberty has a larger volume in adulthood” (Ahmed et al., 2008).Eradicating gonadal hormones before puberty eliminates these sex differences,indicating that gonadal steroids are the cause for the addition of new cellsduring puberty to maintain and accentuatesexual dimorphisms in the adult brain (Ahmedet al., 2008). However, the broad hypothesis for mammalian speciesindicates that chromosomal sex genes are responsible for gonadaldifferentiation and that gonad specific hormones initiate sex specific brainorganization.

In recent years, this principle has been challenged with evidencesuggesting that some sexually dimorphic brain development occurs independently ofperipheral signals (Schlinger,Soma and London, 2001). Although it is clear that gonadal hormonescan have long-lasting effects on the brain during development, it is noted thateven after puberty and into adulthood gonadal steroids can modify neuronalstructure and even have permanent effects on certain reproductive functions (Gorski, 1986).2.2Sexual Differentiation – The BrainStudiesof brain regulation and human behaviour require measurement of structuralvariables, and this has been done predominantly by post-mortem studies (Gur et al., 1991). Sexualdifferentiation of the brain can be considered as a process during whicheffects of sex steroid hormones secreted during early development is maintainedinto adulthood (Matsuda,Mori and Kawata, 2012).

Sexualdimorphisms between females and males are apparent in several brain areas,including the preoptic area (POA), bed nucleus of the stria terminalis (BNST) andhypothalamus (McCarthyet al., 2009). A report of thefirst meta-analysis of typical sex differences on global brain volume foundthat, males on average have much larger volumes and higher tissue densities inthe left amygdala, hippocampus, insular cortex, and cerebellum. However, femalestypically have higher densities in the left and right frontal poles (Ruigrok et al., 2014). However, these findingsconflict with other articles written, and so the relationship between sexdifferences in the brain and human behaviour is a subject of controversy inpsychology and society at large (Fine, 2011).

3.0 Sex Behaviour – FemalesThe display of copulatory behavioursusually requires the existence of a mate and is, therefore, preceded by asearch for and approach to a prospective mate of the opposite sex. Theintensity of approach behaviours is determined by a process labelled “sexualincentive motivation” (Spiteri et al., 2010). Sexual motivation is typically influenced by hormones suchas testosteronein males, oestrogenand progesterone in females, and oxytocin in both sexes. In many mammalian species, these sexhormones control the ability to engage in sexual behaviours, and thesebehaviours can be described as sexually dimorphic.

Young stated that, most investigations of cyclicreproductive activity in female mammals have been more interested in thefunctional basis of the morphological changes and less interested in ananalysis of the factors underlying the parallel changes in behaviour (Young,1941). During his studies on mating behaviour in female mammals he found thatwith white rats, movements during copulation are often fast-paced and darting,with hops accompanied by a shaking of the ears or the entire body. Females donot run away from the male, except that following a moment of smelling orlicking by the male, she runs forward a short distance and stops where thefemale is then overtaken and caught in the copulatory clasp by the male. Whenmounted by the male or fingered on the hindermost part of the back and aroundthe base of the tail a lordosis is shown (Young, 1941).

The female rats controlthe pacing of mating through three phases before lordosis; approach,orientation and runaway. Similar behaviour was recorded for other mammals, fromwild rats, to cows, and further. The females appear to show a more submissivebehavioural attitude when it comes to mating with a male, possibly due to thedifferent hormones released that influence sexual behaviour. This theory isbacked up as certain findings suggest that the gonadal hormones can influencesubmissive behaviour in female Syrian hamsters (Faruzziet al., 2005). 3.

1Sex Behaviour – MalesMalemating competition is largely regarded to account for sexual dimorphisms inbody size (Mitani, Gros-Louis and Richards,1996).  Previously it was concludedthat, because male mammals often compete more aggressively among themselves foraccess to mates than females do, sexual selection is said to be acting againstmales much stronger than females (Darwin, 1871). Sexual selection can bedivided into two processes: intrasexual selection, which involves members of the same sex within a particularspecies competing with each other in order to gain opportunities to mate withothers, andintersexual or epigamic selection, in which members of one sex choose to mate withmembers of the opposite sex (Ralls, 1977). However, most research on sexual selection in mammals has highlightedthe importance of intrasexual selection, for example, “among mammals therole of aggressive male behaviour tends to be more important than that offemale choice” (Brown 1975). Furthermore, testosterone is the key male gonadal steroidwhich influences male mating behaviour. The magnocellular medial preopticnucleus (MPN mag), a subdivision of the medial preoptic area (MPOA), playsa critical role in the regulation of copulation in the male Syrianhamster; in part by facilitating the effects of gonadal steroids (Brague et al., 2018). It was also found that, raisedlevels of gonadal androgens are often required for the expression of male-specificbehavioural and morphological traits in all classes of vertebrates (Golinski et al.

, 2014).These behaviours can be described as more masculine, as mounting and, onoccasion, severe aggression can be seen being displayed. 4.0Hormones and Behaviour (Maternal and Paternal)Parental behaviour is brought about by acombination of internal processes and external factors that ensure the parentstake care of the young, contributing to their survival by providing food,shelter, warmth, protection, and appropriate stimulation. In mammalian species,lactating females are mostly responsible for providing all the care, however,males and other members of the family can contribute to the care of theoffspring in some cases (Olazábal et al., 2013). Although studies of mammalianmaternal behaviour are abundant, there have been very few reports on theassessment of paternal care (Elwood, 1975).

 Testosterone is known to promote an extensive range of behavioursassociated with reproduction in males, including intermale competition, matingbehaviour and courtship behaviour (Adkins-Regan 1998). In a number of mammalianspecies, male testosterone levels decline after the birth of offspring (Brownet al. 1995).

These findings would suggest that testosterone has a negativeeffect on paternal behaviours, however it previously found that testosteronepromotes paternal behaviour in the California mouse (Peromyscus californi) (Elwood, 1975) as behaviours such as lickingand sniffing the pups were seen, accompanied with an increased testosteronelevel. Due to these discrepancies, there is still much we don’t know about the hormoneswhich regulate and facilitate this paternal behaviour.  However, it was concluded that, the mostconsistent evidence for the involvement of hormones in mammalian paternalbehaviour is for prolactin, which was found in species such as the golden hamster(Mesocricetus auratus), mouse (Mus musculus), and rabbit (New Zealand White)(Wynne-Edwards, 2001). This can also be seen in humanfathers (Fig.1). Figure 1.

Mean (± SE) levels of Oxytocin and Prolactin in first-time human fathers in the second and six month following birth of the child. (Sourced from Gordon et al., 2010)         Furthermore, recent research into thebehavioural endocrinology of male parental behaviour is challenging thehypothesis that paternal and maternal behaviour are homologous at a neural andan endocrine level (Wynne-Edwards and Reburn, 2000).

If homologous, then thesame hormones would act at the same neural sites to enable the expression ofthe same parental behaviours in both males and females (Wynne-Edwards, 2001). This is a valid statement as evidencefor this could be in the fact that males and females have almost all the sameDNA, apart from a number of genes on the Y chromosome, thus, sex differences in behaviourshould come from differential gene expression, rather than structuraldimorphism (Kelley, 1988). Onthe other hand, as previously stated, there are numerous reports anddiscussions about mammalian maternal behaviour. In nonhuman primatesand humans, similar to other mammals, hormones are not strictly essential forthe expression of maternal behaviour, but still influence variation in maternalresponsiveness and parental behaviour both within and between individuals (Saltzman and Maestripieri, 2011). Whenconsidering the neuroendocrinology of primate maternal behaviour, initialevidence indicates that oxytocin and other endogenous opioids affect maternalattachment to infants, this includes care, contact, grooming, and responses toseparation. Serotonin in the brain affects anxiety and impulsivity, which mayaffect maternal behaviours such as infant retrieval or rejection (Saltzman and Maestripieri, 2011).  It is believed that, further studies on maternal and paternal motivationwill continue to add complexity to the system, and contribute to our overall understandingof the mechanisms that regulate these behaviours, as well as the processesunderlying maladaptive behaviours and psychopathologies (Olazábal et al.

, 2013).5.0ConclusionTo conclude,although most sexual dimorphism appears after gonadal differentiation, some canoccur at earlier stages in development (Kimura and Matsuyama, 2012). Furthermore, the mammalianbrain is not hugely dimorphic between sexes, although differences clearly existfor example, in the preoptic area, left amygdala and hippocampus, as well as differences seenin the size of the brain and the abundance of white and grey matter. However, theyare not extensive in most species. Most differences are seen in the behavioursdisplayed between sexes.

These behaviours can be described as dimorphic betweensexes and generally play a role in activating, modulating, or inhibitingcertain aspects of maternal or paternal behaviours in mammalian species. Femalelordosis and male mounting or aggression during copulation are also sexuallydimorphic behaviours that can be influenced by gonadal hormones testosteroneand oestrogen. Further studies are however required to gain a more extensivelook at the mammalian brain and neuroendocrinology of both maternal andpaternal behavioural patterns and how other environmental or physiologicalfactors may affect these.


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