Tying it all together: telomeres, sexual size dimorphism and the gender gap in life expectancy
Introduction
The worldwide phenomenon of higher life expectancy for women, although generally acknowledged, is still an unsolved puzzle. In 1998, according to the US Census Bureau, the average gap in life expectancy at birth between the sexes was 7 years in developed countries and 3 years in developing countries [1]. Even though more boys than girls are born in all countries, middle-aged women start to outnumber men and the female advantage increases with age.
While risky behavior, unhealthy lifestyle (alcohol, tobacco) and hormonal differences might have contributed to higher male mortality rates, they cannot entirely explain the phenomenon. Risky behavior in males for example cannot account for the discrepancy of mortality rates beyond the age of 60, but at that point men are twice as likely to die as women [2], mainly due to cardiovascular disease and cancer [1]. In addition, women have been increasingly adopting the ‘unhealthy’ lifestyle of men in the last 40 years or so, but in most developed countries the gender gap has still widened [1]. This leaves us with the hormonal differences. In 1969, Hamilton published data about the median life span of castrated men in comparison to intact men (although mentally ill), who were inmates at the same hospital. He found a 13.6 year difference in lifespan [3]. Paradoxically, median lifespan in the control group was very low. A median life span of 55.7 years, as reported in this paper, would match the known data of that time, if it were life span at birth, but Hamilton collected his data from a preselected group. After all the participants must have reached a certain age to become inmates at a hospital for mentally retarded. Since infant and early childhood mortality was still incredibly high during that time, median life span is expected to be much higher in this preselected group. So the only thing he proved was that testicular hormones are particularly harmful to mentally retarded men incarcerated in US state mental hospitals in the early 20th century. Nevertheless, testosterone has been regarded as a ‘killer’ since. Although, a study on biographical data of castrated singers compared to intact male singers born between 1581 and 1858 did not reveal any significant difference in mean life span. These data published in Nature in 1993 show that prepubertal removal of the testes had no influence on the longevity of men [4].
In contrast to the allegedly unhealthy testosterone, estrogen is generally considered as life prolonging. But even if estrogen would have a positive impact on life expectancy, estrogen levels in postmenopausal women are on a record low, comparable to estrogen levels in males. It is hard to imagine that a hormone increases life expectancy, if it is virtually absent during the last 30 years of a woman’s lifetime.
Another theory on the cause of the gender gap is based on the random inactivation of one X chromosome in female cells [5]. For X-linked diseases it has been known that having two X chromosomes provides a health advantage [5]. Since it is very unlikely that mutations in genes on the X chromosome are involved in all age-related diseases and that mutated versions of these genes occur in all men, this model might be of academic value only.
Section snippets
The hypothesis
Clearly, to solve the puzzle, we have to look for other gender-specific differences. Like the gender gap, sexual size dimorphism (SSD) with men being the larger sex is a worldwide phenomenon. In mammals, a strong positive correlation between SSD and male-biased mortality has been reported [6]. The authors of a research article, recently published in Science [7], linked large body size to a higher burden of parasitism, resulting in increased mortality rates due to infectious diseases. However,
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