1.0 found throughout the animal kingdom (Forger and de

1.0  Introduction

Sex differences in the nervous system are
found throughout 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
degree of sexual dimorphism found in mammals ranges from species in which
females are larger than males, to those in which males are much larger than
females and possess striking secondary sexual characteristics which females
lack (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, numerous magnetic resonance imaging (MRI) studies have addressed the
question of morphological differences of the brain of men and women, reporting
conflicting results regarding brain size and the ratio of grey and white 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 that triggers testis development in males. Hormones produced from
the testes, mainly testosterone, then circulate 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 the 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 during 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”. Removing gonadal
hormones before puberty eradicates these sex differences, indicating that
gonadal steroids are the reason 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 dramatic 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 males and females 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 assumed to be a far
stronger force among males, 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 family members can also 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 array 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, current research into the
behavioural endocrinology of male parental behaviour is testing 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 necessary for
the expression of maternal behaviour, but nevertheless 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 brain size and the abundance of grey and white 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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