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1.0  Introduction

Sex differences in the nervous system are
found all through the animal kingdom (Forger and de Vries, 2010). Sexual dimorphism
can be described as the differences in appearance between males and females of
the same species, which goes beyond the differences in their sexual organs. The
extent of sexual dimorphism found in mammalian species can range from females
being larger than males, to males being larger than females and possessing
striking secondary sexual characteristics which females may be lacking (Ralls,
1977). In this paper, we are interested in the sexual dimorphism of mammal’s
brains and how it can have an effect on certain behaviours displayed by males
and females, specifically behaviours during copulation, maternal and paternal
behaviours, and the hormones that influence these particular behaviours. In the
past, various magnetic resonance
imaging (MRI) studies have addressed the question of certain morphological differences
of the brain of women and men, reporting conflicting results regarding brain
size 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 a
subject of controversy in psychology and society at large (Fine, 2011). To be
able to elaborate on this topic, we first need to understand sexual
differentiation and the process of developing into a male or female, from an
undifferentiated zygote.

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2.0
Sexual Differentiation – Development

Sex
refers to the biological distinction between males and females. Hence, to be
assigned as a female or male several factors are used to determine the
biological sex of an individual;
chromosomes (XX for female, XY for male), gonads, hormones, internal sex organs
and external genitalia (Knox
and Schacht, 2016). It’s stated that, the leading mechanism for sexual
differentiation in mammalian species consists of a specific gene on the Y
chromosome in males that initiate testis development. Hormones that are
produced from the testes, mainly testosterone, flow throughout the body, differentiating
the periphery and brain in a male direction (Forger
and de Vries, 2010). Most testosterone developmental effects on male
brain are actually via oestrogen which is a major driver of male brain
differentiation. In female mammalian species, a lack of
this early exposure to these high levels of testosterone, due to the absence of
testes, allows development of default feminized characteristics (Matsuda, Mori and Kawata, 2012). This
can be described as a primary organisational effect, as generally
organisational effects of hormones occur at development.

 

2.1
Sexual Differentiation – Post-Puberty

Post-puberty,
hormones have activational effects.  They
initiate sex-specific behaviours through actions via male and female body and brain.
It was found that, new cells, including
neurons, arise in many brain regions throughout puberty in both male and female
rats. Sex differences found in pubertal addition of these new cells correspond
with sexual dimorphisms found in adults: “for each region, the sex that gains
more 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 cells
during puberty to maintain and accentuate
sexual dimorphisms in the adult brain (Ahmed
et al., 2008). However, the broad hypothesis for mammalian species
indicates that chromosomal sex genes are responsible for gonadal
differentiation and that gonad specific hormones initiate sex specific brain
organization. In recent years, this principle has been challenged with evidence
suggesting that some sexually dimorphic brain development occurs independently of
peripheral signals (Schlinger,
Soma and London, 2001). Although it is clear that gonadal hormones
can have long-lasting effects on the brain during development, it is noted that
even after puberty and into adulthood gonadal steroids can modify neuronal
structure and even have permanent effects on certain reproductive functions (Gorski, 1986).

2.2
Sexual Differentiation – The Brain

Studies
of brain regulation and human behaviour require measurement of structural
variables, and this has been done predominantly by post-mortem studies (Gur et al., 1991). Sexual
differentiation of the brain can be considered as a process during which
effects of sex steroid hormones secreted during early development is maintained
into adulthood (Matsuda,
Mori and Kawata, 2012). Sexual
dimorphisms between females and males are apparent in several brain areas,
including the preoptic area (POA), bed nucleus of the stria terminalis (BNST) and
hypothalamus (McCarthy
et al., 2009). A report of the
first meta-analysis of typical sex differences on global brain volume found
that, males on average have much larger volumes and higher tissue densities in
the left amygdala, hippocampus, insular cortex, and cerebellum. However, females
typically have higher densities in the left and right frontal poles (Ruigrok et al., 2014). However, these findings
conflict with other articles written, and so the relationship between sex
differences in the brain and human behaviour is a subject of controversy in
psychology and society at large (Fine, 2011).

3.0 Sex Behaviour – Females

The display of copulatory behaviours
usually requires the existence of a mate and is, therefore, preceded by a
search for and approach to a prospective mate of the opposite sex. The
intensity of approach behaviours is determined by a process labelled “sexual
incentive motivation” (Spiteri et al., 2010). Sexual motivation is typically influenced by hormones such
as testosterone
in males, oestrogen
and progesterone in females, and oxytocin in both sexes. In many mammalian species, these sex
hormones control the ability to engage in sexual behaviours, and these
behaviours can be described as sexually dimorphic. Young stated that, most investigations of cyclic
reproductive activity in female mammals have been more interested in the
functional basis of the morphological changes and less interested in an
analysis of the factors underlying the parallel changes in behaviour (Young,
1941). During his studies on mating behaviour in female mammals he found that
with 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 do
not run away from the male, except that following a moment of smelling or
licking by the male, she runs forward a short distance and stops where the
female is then overtaken and caught in the copulatory clasp by the male. When
mounted by the male or fingered on the hindermost part of the back and around
the base of the tail a lordosis is shown (Young, 1941). The female rats control
the pacing of mating through three phases before lordosis; approach,
orientation and runaway. Similar behaviour was recorded for other mammals, from
wild rats, to cows, and further. The females appear to show a more submissive
behavioural attitude when it comes to mating with a male, possibly due to the
different hormones released that influence sexual behaviour. This theory is
backed up as certain findings suggest that the gonadal hormones can influence
submissive behaviour in female Syrian hamsters (Faruzzi
et al., 2005).

3.1
Sex Behaviour – Males

Male
mating competition is largely regarded to account for sexual dimorphisms in
body size (Mitani, Gros-Louis and Richards,
1996).  Previously it was concluded
that, because male mammals often compete more aggressively among themselves for
access to mates than females do, sexual selection is said to be acting against
males much stronger than females (Darwin, 1871). Sexual selection can be
divided into two processes: intrasexual selection, which involves members of the same sex within a particular
species competing with each other in order to gain opportunities to mate with
others, and
intersexual or epigamic selection, in which members of one sex choose to mate with
members of the opposite sex (Ralls, 1977). 
However, most research on sexual selection in mammals has highlighted
the importance of intrasexual selection, for example, “among mammals the
role of aggressive male behaviour tends to be more important than that of
female choice” (Brown 1975). Furthermore, testosterone is the key male gonadal steroid
which influences male mating behaviour. The magnocellular medial preoptic
nucleus (MPN mag), a subdivision of the medial preoptic area (MPOA), plays
a critical role in the regulation of copulation in the male Syrian
hamster; in part by facilitating the effects of gonadal steroids (Brague et al., 2018). It was also found that, raised
levels of gonadal androgens are often required for the expression of male-specific
behavioural and morphological traits in all classes of vertebrates (Golinski et al., 2014).
These behaviours can be described as more masculine, as mounting and, on
occasion, severe aggression can be seen being displayed.

 

4.0
Hormones and Behaviour (Maternal and Paternal)

Parental behaviour is brought about by a
combination of internal processes and external factors that ensure the parents
take 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 the
offspring in some cases (Olazábal et al., 2013). Although studies of mammalian
maternal behaviour are abundant, there have been very few reports on the
assessment of paternal care (Elwood, 1975). 
Testosterone is known to promote an extensive range of behaviours
associated with reproduction in males, including intermale competition, mating
behaviour and courtship behaviour (Adkins-Regan 1998). In a number of mammalian
species, male testosterone levels decline after the birth of offspring (Brown
et al. 1995). These findings would suggest that testosterone has a negative
effect on paternal behaviours, however it previously found that testosterone
promotes paternal behaviour in the California mouse (Peromyscus californi) (Elwood, 1975) as behaviours such as licking
and sniffing the pups were seen, accompanied with an increased testosterone
level. Due to these discrepancies, there is still much we don’t know about the hormones
which regulate and facilitate this paternal behaviour.  However, it was concluded that, the most
consistent evidence for the involvement of hormones in mammalian paternal
behaviour 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 human
fathers (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 the
behavioural endocrinology of male parental behaviour is challenging the
hypothesis that paternal and maternal behaviour are homologous at a neural and
an endocrine level (Wynne-Edwards and Reburn, 2000). If homologous, then the
same hormones would act at the same neural sites to enable the expression of
the same parental behaviours in both males and females (Wynne-Edwards, 2001). This is a valid statement as evidence
for this could be in the fact that males and females have almost all the same
DNA, apart from a number of genes on the Y chromosome, thus, sex differences in behaviour
should come from differential gene expression, rather than structural
dimorphism (Kelley, 1988).

On
the other hand, as previously stated, there are numerous reports and
discussions about mammalian maternal behaviour. In nonhuman primates
and humans, similar to other mammals, hormones are not strictly essential for
the expression of maternal behaviour, but still influence variation in maternal
responsiveness and parental behaviour both within and between individuals (Saltzman and Maestripieri, 2011). When
considering the neuroendocrinology of primate maternal behaviour, initial
evidence indicates that oxytocin and other endogenous opioids affect maternal
attachment to infants, this includes care, contact, grooming, and responses to
separation. Serotonin in the brain affects anxiety and impulsivity, which may
affect maternal behaviours such as infant retrieval or rejection (Saltzman and Maestripieri, 2011).  It is believed that, further studies on maternal and paternal motivation
will continue to add complexity to the system, and contribute to our overall understanding
of the mechanisms that regulate these behaviours, as well as the processes
underlying maladaptive behaviours and psychopathologies (Olazábal et al., 2013).

5.0
Conclusion

To conclude,
although most sexual dimorphism appears after gonadal differentiation, some can
occur at earlier stages in development (Kimura and Matsuyama, 2012). Furthermore, the mammalian
brain is not hugely dimorphic between sexes, although differences clearly exist
for example, in the preoptic area, left amygdala and hippocampus, as well as differences seen
in the size of the brain and the abundance of white and grey matter. However, they
are not extensive in most species. Most differences are seen in the behaviours
displayed between sexes. These behaviours can be described as dimorphic between
sexes and generally play a role in activating, modulating, or inhibiting
certain aspects of maternal or paternal behaviours in mammalian species. Female
lordosis and male mounting or aggression during copulation are also sexually
dimorphic behaviours that can be influenced by gonadal hormones testosterone
and oestrogen. Further studies are however required to gain a more extensive
look at the mammalian brain and neuroendocrinology of both maternal and
paternal behavioural patterns and how other environmental or physiological
factors may affect these.

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