Dads are important. Pardon the motherhood statement about... fatherhood. Just sometimes, even self-evident things need to be said. All else being equal, fathers who are involved in their children's upbringing directly improve those children's survival, health, social and educational development.
Now for the second self-evident proposition: some dads are better than others. A great many dads lavish as much love and care on their children as those kids' mothers do. And many dads do more. Fathers make all sorts of deep, selfless, sacrifices to meet their children's needs.
But fathers vary enormously in how involved they are an in the ways they care. According to anthropologist Sarah Blaffer Hrdy, the contributions that fathers make to child caring and to the family vary dramatically among and within cultures. Far more dramatically, in fact, than the contributions mothers make.
What makes the difference, then, between a superdad and a low-investing or even absentee dad? What distinguishes a guy who raps lovingly about "the child he had with the child from Destiny's Child" from the papa who was a rolling stone?
Tempted as some might be to vilify low-investing men, evolutionary analysis usually seeks first to understand behaviour rather than to judge it. Evolutionary theory certainly can illuminate why fathers don't all invest equally. And an intriguing study, published this month, supports the idea of a biological continuum between high and low-investing fathers.
The key comes from life-history theory, which concerns itself with the economics of how organisms acquire resources and spend them on growth, reproduction, survival and other vital but costly activities. For example, just as one cannot spend the same dollar on rent and food, so one cannot spend the same kilojoule of energy on making sperm and on fighting off an infection. Different adaptive priorities trade-off with one another, just as different spending priorities trade-off within a household budget.
Most men lack the time, money and energy to be both sexually prolific and high-investing fathers at the same time. For some time, life-history theorists have postulated a trade-off between parental investment and mating effort -- the investment a man makes in finding, courting and mating with new partners.
No surprises there. Men who spend all their time, money and effort chasing new women are more likely to neglect or even abandon the children they already have. But how does this trade-off arise? Variation in testosterone among men, seems to be right in the middle of this issue:
- Men with naturally-occurring high testosterone (T) have more sexual partners.
- Men with low T suffer low libido.
- Couples in which the man has high T report lower relationship quality and are more likely to divorce.
- Becoming a father leads directly to a drop in a man's T.
- Low-T fathers are more involved in caring for their children.
On top of all this correlative evidence, experimental manipulation in a bird (the Lapland Longspur) reveals that testosterone supplementation directly increases a male's investment in courtship singing. This comes at the expense, however, of his effort attending the chicks.
The size of a male's testes also correlates with investment in mating. Species in which females tend only to mate with one male in a given breeding cycle tend to have smaller testes than those in which sperm has to compete with that from other males for the chance to fertilise the same egg. Big testes equals more sperm equals more tickets in the great sperm lottery.
Within species, some evidence suggests that males with larger testes mate with more females and do so more often than less testicularly spectacular males. Large testes take energy to maintain. They also present a vulnerable liability, favouring compactness. Perhaps men whose bodies are biologically geared to invest in courting and mating with new mates might make the risky investment in larger testes, but those aiming for caring monogamy act to minimise their testicular liability?
In a 1995 book, Mark Bellis claimed that testes size was associated with sexual strategies in men. However, Leigh Simmons and colleagues at UWA found no such evidence in a 2004 paper. With this in mind, Jennifer S. Mascaro, Patrick D. Hackett and James K. Rilling from Emory University set out to test the relationship between testicular size, testosterone and men's parenting effort.
The Paternal Brain
Mascaro and her collaborators recruited 70 fathers, each with a child aged one or two, and each of whom were living with the child and the child's mother. The easy part was measuring male testes and plasma testosterone levels. They also administered comprehensive questionnaires to establish which parenting tasks each father performed, from the routine (bathing baby, attend child during the night) to the less common (taking the child to medical appointments).
Anyone who has parented alongside another knows that people don't always accurately estimate their contribution to the work of caring. So the researchers asked the mens' partners about who-did-what in the child-care department. Mums and dads actually agreed reasonably well on the division of labour.
And in a welcome development, the team used Functional Magnetic Resonance Imaging (fMRI) to explore brain activity in order to refine the picture of male parental investment. They observed how men responded to pictures of their own child, an unknown child and an unknown adult. And they used a variety of images, capturing a neutral expression, a happy or a sad face for each subject.
High testosterone levels and large testes were each, independently, associated with lower paternal caregiving. The authors interpret this as showing that high T and big testes result in lower paternal engagement. Especially since neither the number of hours men worked nor the amount they earned were correlated with testes size or with testosterone. It's not as if men switched from one form of investment to another.
I'm no neuroscientist, so I won't judge the quality of the evidence from that part of the study, but the claims are certainly promising. Fathers with small testes displayed more of the brain activity typically associated with nurturing when viewing pictures of their own child. Especially when the picture showed their child with a happy or sad, rather than neutral, expression. These same fathers were also better dads; at least they were more involved in caring for their infants.
Dads: born and made
Nowhere in Mascaro's paper, in this column, or in the surrounding discussion I have seen, has anybody suggested the links are deterministic. Testosterone and testicle volume together only explained 21 percent of the variation in paternal caring. If you've got big (but healthy) testicles, that doesn't make you a bad dad. If you're low-T you may or may not be a good father, super responsive to your child's emotional state. So, fathers, I shouldn't have to say this, but don't take today's news personally.
The important point is that we're starting to come to terms with the complex interplay between biology and social behaviour involved in the all-important business of being a good father.
Collectively, these results provide the most direct support to date that the biology of human males reflects a trade-off between mating and parenting effort.
I wouldn't disagree with this conclusion. I'm amazed at how neatly the results uphold the prediction and the consistency of four lines of evidence: anatomic, hormonal, neuroscientific and social. This paper gives quite a firm idea of the way in which the trade-off works. And it begins to bridge several traditions of study to build a useful model of how biology and social factors interact to shape behaviour.
But why do men vary so much? I expect an avalanche of interesting research on this question. We do know that human life-histories vary from "fast" to "slow". Living fast involves early puberty and becoming a parent relatively young. It can be triggered by poverty, inequality, childhood neglect and, interestingly, father absence. Which can create a self-reinforcing cycle. Breaking that cycle involves lots of parental investment, both in nurturing and providing for material needs.
One obvious place in which to begin involves longitudinal studies to explore how the associations form between parental care, testicle size and testosterone. There are some exciting hints that low T men become better fathers, but do better fathers also experience a bigger drop in T? And how do testes wax and wane when men have kids? We don't really know much at all about whether and how testes size changes over men's lifetimes.
These many related areas of science remain quite some way off tying the complex mish-mash of factors that shape life-histories to the mating-parenting trade-off. But I believe research in this area has immense capacity to improve the lives of everybody involved.
I've enjoyed assembling videos of songs associated with fatherhood. Of course there are many, many more. Please Tweet me @Brooks_Rob with any good suggestions you want to share. Use #DadSongs.
Here are a few extras.
And talking about testicles:
Rob Brooks does not work for, consult to, own shares in or receive funding from any company or organisation that would benefit from this article, and has no relevant affiliations.
This article was originally published at The Conversation.
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