Issue 4: October 2013

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).

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Brain & BEHAVIOuR

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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?

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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.

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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?

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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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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).

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

“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

(BBC News, 2013)

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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 6

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.