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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: 
v Robots are a dog’s best friend: Dogs ability to pick up robot social 
cues. 
v Zebra fish versus Lion fish: A day in the life under the sea
Brain Lorem & Ipsum 
behaviour 2 
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).
3 
3 
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?
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.
5 
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)
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.
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.

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Brain and behaviour newspaper article: Ethar Bashir

  • 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: v Robots are a dog’s best friend: Dogs ability to pick up robot social cues. v Zebra fish versus Lion fish: A day in the life under the sea
  • 2. Brain Lorem & Ipsum behaviour 2 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).
  • 3. 3 3 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?
  • 4. 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.
  • 5. 5 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)
  • 6. 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.