Researchers at the University of Sheffield have discovered that a twin brother's testosterone in the uterus can reduce his female twin´s chances of marrying and having children.
Testosterone and oestrogen can cross membrane barriers in the uterus. A female twin foetus is therefore exposed to a brother's testosterone and a male twin foetus to a sister's oestrogen. However, male and female foetuses have similar oestrogen levels, so a female is more likely to be affected.
Certain characteristics, including facial features and fertility, can be changed by exposure to opposite sex foetuses. But until now little was known about how this effected future reproduction.
Dr Virpi Lummaa and colleagues from the University´s Department of Animal and Plant Sciences used Finnish church records spanning 1734 to 1888 as part of their research. Of 754 twins from five distinct Finnish populations in the records, females who survived to adulthood were 25% less likely to have children if their twin was a male. Those who did have offspring gave birth, on average, to two fewer babies than women who had a twin sister. Women who had a male twin were also 15% less likely to get married.
The researchers believe that this is due to a number of factors. Firstly, females exposed to a male twin can have masculine traits, attitudes and behaviours, therefore affecting their decision to get married or a male´s attractiveness towards them. Secondly, exposure to elevated levels of testosterone during development can promote the onset of diseases that compromise fertility, such as reproductive cancers.
Dr Lummaa said: "The study used data from humans living in a pre industrial era so as to obtain results that are not affected by advanced health care and contraception. They showed that as a consequence of a male twin´s influence on a female´s fertility, mothers who produce opposite sex twins have fewer grandchildren and hence lower evolutionary fitness."
"These results provide the first evidence that sex ratio can influence offspring fitness and have significant implications for understanding the adaptive nature of sex ratios in humans."
The research has been published in the Proceedings of the National Academy of Sciences (PNAS) journal.
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