Have taller women evolved to have more babies?

Behind the Headlines

Monday April 29 2013

Researchers looked into women's fertility rates, height and BMI

“Taller, skinnier women have evolved to have more babies than their shorter counterparts,” the Mail Online website has claimed.

It reports on research examining the characteristics of women in two villages in the west African nation of The Gambia over more than 50 years.

Researchers were interested in whether recent trends for decreased mortality and fertility rates in human populations over time may influence natural selection on other traits. They analysed records of just under 3,000 women between 1956 and 2010 to find their body mass index (BMI) and number of births.

Initially, women who were shorter and with higher BMIs were more likely to reproduce successfully, but over time the reverse became true. The study did not investigate the reasons for this, but the researchers suggest that improvements in healthcare are changing the relationship between height, BMI and health in The Gambia.

Other factors could also play a role, including cultural changes (such as men’s changing preferences for sexual partners). Due to the highly specific sample population in the study, we can't say whether these trends in height, BMI and adult fertility would be found in UK women.

The wider implications of the study are that it suggests that evolution, driven by natural selection, is not just something that happened to our ancestors. It can still have a significant influence on the human population.

However, due to the highly specific sample population in the study, it is difficult to assess whether the findings would relate to women in the UK. Analysis of similar data would be required to determine whether this was the case.

 

Natural selection

Natural selection is key to evolutionary theory. It means that traits that increase the probability of organisms (including humans) living in a particular environment reproducing, will, over time, become more common in the population in that environment.

 

For example, if you grow up in a culture where having blue eyes is seen as being sexually attractive, over time, the amount of people with blue eyes in that environment will tend to increase.

Where did the story come from?

The study was carried out by researchers from various research centres in Germany, the UK, The Gambia, and the US.

The collection of the data analysed in the study was funded by the UK Medical Research Council, and the researchers were funded by various bodies, including the Wellcome Trust and the European Research Council.

The study was published in the peer-reviewed journal Current Biology.

Despite this being a study conducted in The Gambia, the Mail Online illustrated the story with a picture of German model Heidi Klum (who has four children).

And the Mail Online's headline, “taller, skinnier women have evolved to have more babies than their shorter counterparts”, is not strictly correct. The study did not find that being taller and having a lower body mass index are evolutionary adaptations that enable women to have more children. The fact that over time taller women with lower BMIs in the Gambia had a reproductive advantage over shorter women with higher BMIs is likely to relate to other factors such as what women’s height and BMI say about their health.

 

What kind of research was this?

This was a longitudinal study looking at the evolutionary consequences of changes in the characteristics in a population over time, in this case in The Gambia.

The researchers say that human populations have recently shown declines in both mortality (death) and fertility rates and that the evolutionary consequences of this have not been extensively investigated. In particular, they looked specifically at how the changes influenced variation in the population in ‘relative fitness’ in evolutionary terms (essentially how able individuals are to reproduce successfully).

The researchers looked at how height and BMI might have influenced this ability to reproduce successfully.

 

What did the research involve?

The researchers used data that had been collected from women in two rural villages in one district in The Gambia between 1956 and 2010. They collected data for 2,818 women, who together provided a total of 51,909 years of follow-up in total.

The women’s heights and weights had been recorded, and their BMIs calculated. Researchers used methods in their analyses that allowed them to take into account the fact that women’s measurements had not all been taken at the same age, and some women provided more than one measurement at different ages.

Births and deaths were also recorded.

The researchers used an annual measure of ‘fitness’ in the population that assessed how many babies the women had each year. They also assessed how BMI and height related to ‘fitness’, and whether this relationship changed over time.

 

What were the basic results?

The researchers found that over time, variation in ‘relative fitness’ in the population declined. This was largely as a result of reduction in the variation in survival in early life – with a reduction in deaths among girls before they reached adulthood and had a chance to reproduce. As with most developing countries, child mortality was very high in The Gambia for much of the 20th century – a trend that gradually improved over time.

Survival among girls aged under 15 increased over time and variation in relative adult fertility increased at the same time.

There was a change in how height and BMI related to adult fertility in the Gambian population. Taller women initially had lower adult fertility, but over time they showed higher adult fertility. Women with a higher BMI initially had higher adult fertility, but by the end of the study period they showed lower adult fertility. So initially, up to 1974, women who were shorter and with higher BMIs (height less than 157cm and BMI greater than 21) reproduced more, after 1975 women who were taller and with lower BMIs (height greater than 158cm and BMI less than 21) reproduced more. The researchers’ analyses suggested that the relationship may have been influenced by healthcare improvements that affected how health related to height and BMI.

 

How did the researchers interpret the results?

The researchers concluded that their findings show the changes in selective pressures on humans over time. They say that the findings suggest that changes in the characteristics of human populations and social, culture, medical and economic environment are likely to modify but not remove natural selection in humans. They say that this is likely to be increasingly driven by changes in culture – particularly in medical practice and public health measures.

 

Conclusion

This study provides insight into changes in how height and BMI have related to female reproductive fitness in The Gambia over a long period of time. While the general evolutionary principles identified in this study may apply to populations worldwide, the specific findings as they relate to height, BMI and reproductive fitness may not. Analysis of similar data from other populations would help to determine whether this was the case.

A key limitation of this research is that the women’s heights and BMIs were not all measured at the same age or on a regular basis. The researchers note that if they had annual measurements of the women’s heights and BMIs this would have allowed a more detailed look at the relationship between these factors and reproductive fitness.

Overall, the study provides interesting insight into how selection in humans changes as population characteristics and our complex social, culture, medical and economic environment changes. However, these findings are likely to be of more interest from an evolutionary perspective than a medical one. Shorter women with higher BMI should not be unduly alarmed by this news.

However, being underweight or overweight can affect your chances if you are trying to conceive.

Find out more about how to protect your fertility.

Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter.

Links to the headlines

Taller, skinnier women have evolved to have more babies than their shorter counterparts. Mail Online, April 26 2013

 

Links to the science

Courtiol A, Rickard IJ, Lummaa V, et al. The Demographic Transition Influences Variance in Fitness and Selection on Height and BMI in Rural Gambia. Current Biology. Published online April 25 2013

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Analysis by Bazian

Edited by NHS Choices

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