1.
Issue 4: October 2013
Brain & BEHAVIOuR
The jewels of fatherhood
Over the past two decades there has been
an immense increase in absentee or
uninvolved fathers, with figures reaching
a staggering 30% of all households
(Healthline News, 2013).
However, there are many loving fathers
making profound, selfless sacrifices in
order to satisfy their children’s needs.
With countless advantages related to
paternal involvement such as improved
psychological, educational and social
outcomes, it raises the question:
What distinguishes an absentee father
from a super dad?
Testosterone
Testosterone is a sex hormone that holds
the significant role in regulating both
paternal investment and mating effort,
which is fundamentally essential in
understanding the dynamics of
fatherhood.
It has been identified that testosterone
tremendously influences behaviour by
promoting the competitive behaviours
between males that relate to reproductive
success (Eisenegger, Haushofer & Fehr,
2011).
INSIDE THIS ISSUE:
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cues.
v Zebra fish versus Lion fish: A day in the life under the sea

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Specifically, testosterone
physiologically stimulates the
maintenance and development
of behaviours and traits
associated with mating effort in
males, including musculature
(i.e. upper body muscle mass),
libido, aggression and courtship
(Gray et al., 2002)
So, how does testosterone
influence fatherhood?
1
According to Wingfield et
al., (2001), the physiological
mechanism that underlies
male’s mating/parenting
evolutionary tradeoff is in
fact facilitated by
testosterone, and the term:
The Challenge Hypothesis
was introduced.
The challenge hypothesis
explains that men with
elevated levels of
testosterone engage in
various reproductive
behaviours such as seeking
new mates, risk taking,
sexual activity and
competitive behaviours,
which in turn increases their
mating opportunities
(Archer, 2006). After
succeeding in securing a
mate and/or fathering
offspring testosterone levels
will then be decreased,
promoted by the parenting
effort strategy.
2
The challenge hypothesis
stems from supporting
research evidence on
monogamous birds
(Wingfield et al., 2001).
Testosterone levels are high
at the beginning of breeding
season, which support
reproductive behaviour and
physiology. During male-male
reproduction
competition, levels of
testosterone are raised
further. Testosterone levels
are then decreased, when
paternal care is expressed by
males towards their young.
Polygynous birds, however,
display high levels of
testosterone that throughout
the breeding season remain
constant, which is associated
with the decrease in parental
care.
Evolutionary Tradeoff
To fully address this question,
one must first understand the
branch of evolution called: Life
History Theory. This theory
indicates that an organism’s
energy is finite when it comes
to reproduction, and
investment of this energy goes
to either parenting or mating
strategies (Gettler, 2011). In the
male life history, it details the
tradeoff between the
individual’s allocation of time
and energy to mate attraction
and male-male competition
(mating effort) and the
allocation of time and energy
caring for their children and
mates (parenting effort).

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This is due to the
testosterone levels being
close to a maximum, which
allows for the lack of
response to the physiological
changes of the challenge
hypothesis (Archer, 2006).
Could this be the answer
that distinguishes an
absentee father to a
super dad?
4
Unfortunately, some men
prefer the second life history
strategy identified as being
accompanied by high mating
effort, no paternal care and
lesser commitment towards
one mate.
This preference as explained
by Storey et al., (2000),
shows that high levels of
testosterone was found
among men that had high
interests in erotica as well as
a high number of
heterosexual mates. These
men also expressed various
behaviours associated with
mating effort. This study
also highlights the vital
point that males with high
testosterone thought of
friends more than family or
mates, displaying a
significantly wide contrast
with the males of the study
that had low testosterone.
These findings directly show
that high testosterone men
invest more time and energy
to mating effort and low
testosterone men devote
more time and energy to
parental effort (Wingfield et
al., 2001).
Which raises my next
question…
Does testosterone decrease
with more time and energy
invested in childcare?
Paternal childcare
The answer is yes! A study
carried out aimed to measure
the testosterone levels of
fathers, found that males who
were highly involved in daily
childcare had lower levels of
testosterone in comparison to
those fathers who did not
participate in childcare
(Fleming, Corter, Stallings, &
Steiner, 2002).
Additional longitudinal
evidence (research over a long
period), goes even further by
indicating that testosterone
levels was the lowest among
fathers reporting more hours of
paternal childcare investment
(Gettler, 2011). Specifically,
those fathers who put in at least
three hours of paternal
childcare expressed lower levels
of testosterone.
All of the mating strategies
facilitated by testosterone
such as male-male
competition, aggression and
libido are unnecessarily
incompatible with long-term
commitment and effective
parenting, therefore natural
selection decreases
testosterone levels, which
goes hand in hand with
increased parental
investment.
Why do some men
choose not to make
their parental
investment?

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Brain Lorem & Ipsum
behaviour 4
Testicle size
The size and function of
testicles are both related to
mating strategies (Mascaro,
Hackett & Rilling, 2013).
Among monogamous
primates, testes size is
smaller than breeding
systems with more than one
mate. And it is claimed that
mating strategies in humans
is predicted by testes size.
They aimed to investigate if
the life history theory
reflecting the
mating/parenting tradeoff
can be explained through
human anatomy – male’s
testicles.
Although testosterone is
produced in the testes, testes
size reflects the variation in
sperm production more than
hormone production
(Harcourt, Purvis & Liles,
1995).
70-80% of testes volume is of
seminiferous tubules (tubes
producing and carrying
semen) indicating testes size
is more highly linked to
sperm count and quality in
comparison to testosterone
levels.
Mascaro, Hackett & Rilling
(2013) explain the
importance of this fact,
explaining that testes size is
associated with parenting
effort, which is reflected by
the tradeoff between
spermatogenesis
(categorized as a type of
mating effort), and parental
care.
This study brought to light,
that the fathers who
possessed smaller testicles
were related to greater
parental investment and
caregiving than those with
larger testicles.
Lower testosterone levels
were also seen in numerous
studies of fathers, when
presented with infant cries
and odors (Storey et al.,
2000). In comparison to non-fathers,
fathers felt more
sympathetic, greater
alertness and a greater need
to respond to infant cries,
which was due to lower
testosterone levels both
during and prior to the cries.
To sum this up, greater
paternal responsiveness is
linked to lower testosterone.
With vast research and
evidence strongly
linking testosterone to
fatherhood, there is
however another vital
factor that has recently
received attention– and
that is testicle size.

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Paternal Brain Evidence
If fathers differ in their preference to
the mating/parenting tradeoff, this
will be illustrated in their brain
function (Mascaro, Hackett & Rilling,
2013). The study used brain scans
(fMRI) to see if there would be a link
between testes size, brain activity
and parenting effort. This was done
through observations of men’s
responses to pictures of unknown
children as well as their own child
and child auditory (cry).
“Men with smaller testes had a stronger brain
response to viewing pictures of their own children
within an area of the brain that is involved in both
reward and parental motivation,” says Prof. Rilling
among those with high
paternal investment,
testosterone was not linked
to the nurturing related
brain activity. This VTA
activity was related to
testicular volume.
This strong evidence shows
that the differences in male
anatomy prove that
competing evolutionary
strategies significantly
reduce mating effort to allow
for effective parenting
investment.
This allows increased chances
of animal’s offspring to
continue their lineage (BBC
News, 2013).
Does this then mean
women should look for
potential fathers based on
their testicle size?
This area within the brain is
called the ventral tegmental
area (VTA) that is associated
with the brains reward
system. It is this brain
region that is mostly
associated with nurturing
that showed an increased
neural activation of fathers
with smaller testes in
comparison to those fathers
with bigger testes (Mascaro,
Hackett & Rilling, 2013).
Another important aspect of
this study showed that
although low levels of
testosterone was seen
(BBC News, 2013)

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Brain Lorem & Ipsum
behaviour 6
Jennifer Mascaro, one of the authors of the
study, explained that while the study
discovered the relationship between testes size
and parenting - its still unclear if the life
strategy is a cause or consequence of testes size.
She explains further, that they assume testes
size influences the drive of childcare, but could
also be that testes shrink when fathers indulge
in childcare such as when testosterone levels
decrease in invested fathers.
"We are not saying you can determine a
man's parenting aptitude based on their
individual biology. But it does suggest that
some men may be wired to participate in
childcare more than others. They may take to
it more readily," Mascaro explained
(The Guardian, 2013).
Could the environment affect
testes size?
Early life experiences such as stress
and unpredictability and a childhood
where a father was absent was highly
correlated to more promiscuous
sexual activity at a relatively earlier
stage, which in turn influenced their
life strategy.
It is then assumed that fathers at
childhood who experienced more
unpredictability and stress leaned
more towards mating effort at the
expense of parenting effort, and boys
without a father present may react to
their absence by adopting the mating
effort strategy (The Guardian, 2013).
While this article explores the strong evidence linking testosterone and testicle size to
fatherhood, this is just the beginning of better understanding parental behaviour and the
biological aspects of fathering.
And being “ballsy” has become more than a compliment.

7.
Brain & behaviour ISSUE 4: OCTOBER 2013
References
Archer, J. (2006). Testosterone and human aggression: an evaluation of the
challenge hypothesis. Neuroscience and Behavioral Reviews, 30 (2), pp. 319-345.
BBC News. (2013). Testicle size 'link to father role'. Retrieved 2 October, 2013,
from http://www.bbc.co.uk/news/health-24016988
Eisenegger, C., Haushofer, J., & Fehr, E. (2011). The role of testosterone in social
interaction. Trends in Cognitive Sciences, 15 (6), pp. 263-271.
Fleming, A.S., Corter, C., Stallings, J., & Steiner, M. (2002). Testosterone and
prolactin are associated with emotional responses to infant cries in new fathers. Hormones
and Behavior, 42 (1), pp. 399-413.
Gettler, L.T., McDade, T.W., Feranil, A.B., & Kuzawa, C.W. (2011). Longitudinal
evidence that fatherhood decreases testosterone in human males. PNAS, 108 (39), pp.
16194-16199.
Gray, P.B., Sonya, M.K., Barrett, E.S., Lipson, S.F., & Ellison, P.T. (2002).
Marriage and fatherhood are associated with lower testosterone in males. Evolution and
Human Behavior, 23 (1), pp. 193-201.
Healthline News. (2013). Testosterone and Fatherhood: Do 'Manlier' Men Make
Worse Dads? Retrieved 28 September, 2013, from http://www.healthline.com/health-news/
men-do-manly-men-make-worse-parents-090913
Mascaro, J.S., Hackett, P.D., & Rilling, J.K. (2013). Testicular volume is inversely
correlated with nurturing-related brain activity in human fathers. PNAS, 110 (39), pp.
15746-15751.
Storey, A.E., Walsh, C.J., Quinton, R.L., & Wynne, K.E. (2000). Hormonal
correlates of paternal responsiveness in new and expectant fathers. Evolution and Human
Behavior, 21 (3), pp. 79-95.
The Guardian (2013). Testicle size may indicate men’s childcare aptitude.
Retrieved 28 September, 2013, from
http://www.theguardian.com/lifeandstyle/2013/sep/09/testicle-size-men-childcare-aptitude-
parenting-us
Wingfield, J.C., Lynn, S.E., & Soma, K.K. (2001). Avoiding the ‘costs’ of
testosterone: ecological bases of hormone-behavior interactions. Brain Behavior Evolution,
57 (1), pp. 239-251.