There is a benefit in looking like dad

MEN are doomed to uncertainty. Women know who their children are, but the ubiquity of sexual cheating makes it difficult for males of many species, humans included, to be sure which youngsters actually belong to them. If a male’s reproductive strategy amounts to little more than “Wham, bam, thank-you ma’am”, this may not matter to him much. But if, as in the human case, he takes an interest in his offspring, it matters a lot. There are few more foolishactions, from an evolutionary point of view, than raising another male’s progeny.

This line of reasoning led Alexandra Alvergne and her colleagues at the University of Montpellier, in France, to wonder if human fathers recognize features of children that might give away whose offspring they really are, and use those to guide the amount of attention doled out to each putative son and daughter. To find out, they established an experiment among villagers in the Sine Saloum region of Senegal, where polygynous marriages (i.e., men with multiple wives) are common. In such societies the incentives for unmarried men and the opportunities for neglected women to engage in what zoologists who study other harem-forming species refer to as “sneaking” are particularly high. It gives the men a chance to reproduce and the women a chance to spread their bets.

Thirty families with at least two children aged between two and seven agreed to participate in a two-part study, in return for gifts of farming equipment and school supplies that were given to the head of their village for appropriate distribution. In the first part, photographs were taken of children and their “fathers”. Over 100 judges, selected from distant villages, were then given images of individual children along with images of three adult men. These judges were asked to

decide which man was each child’s father.

In a second part of the study, the researchers collected T-shirts worn by fathers and their children while sleeping. These T-shirts were presented to 300 judges from other villages who had not participated in the first part of the study. Previous work has shown that parents and offspring can often be paired from the smell of such T-shirts, so the judges were asked to compare the scent of each child’s T-shirt with that of two T-shirts worn by men, and identify the shirt that had been worn by the child’s father.

During interviews conducted individually with mothers and fathers, Dr Alvergne and her team asked questions about how many hours a day fathers spent with their children. They also asked indirect questions, like whether the father gave the mother money for the children and, if so, how much. Answers were scored numerically, with responses like “he gives me a lot of money each week” getting four points and “he gives me no money” getting zero. Questions associated with marital conflict, drug use and jealousy could generate negative points.

The scores were combined to create a paternal-investment index which ranged from a high of +4 to a low of -4. The nutritional condition of all the children in the study was also assessed, in the form of their body-mass indices (BMI), a measure of how much fat their bodies contained.

The result, published in the latest edition of Animal Behaviour, is that children who looked and smelled like their fathers did indeed enjoy more paternal care than those who did not. They averaged +1 on the paternal investment index. The “non-resemblers”, those adjudged by outsiders not to smell or look like their fathers, averaged -1. This mattered, for there was also a strong connection between paternal investment and a child’s nutritional state. Children whose index was +2 had an average BMI of 16.5. Those whose index was -2 averaged a BMI of 15.

A BMI of 15 is not unhealthy (it is only slightly below the average for a five-year-old American boy). But 16.5 is nicely near the top of the range for a five-year-old, without him tipping into being overweight. Family resemblances, then, do matter. Looks may not kill, but they can leave you feeling peckish.

(The Economist Jun 18th 2009)

 

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