09 October 2014

Evolution of sexual dimorphism in human lifespan (3)

Part 3. Why are men the short-lived sex?
Eunuchs and antagonistic pleiotropyThe beginning of the article is here.

According to the patriarchal hypothesis discussed in the previous part of the article, the longevity of women is the result of inheritance of autosomal (non-sexual) genetic determinants of longevity, the selection of which occurs in the chromosomes of men. However, this raises the question [17]: if longevity is the result of the evolution of men, why do women live longer than men? In this case, the pronounced sexual difference in human reproductive models provides a plausible explanation: a large variability in the number of offspring in men. The number of children of one woman is limited by the number of successful pregnancies, whereas the number of children of one man is limited by the number of his sexual partners. Moreover, polygyny implies the absence of children in some men [17]. This creates a strong selective pressure underlying the selection of traits that contribute to the reproductive success of men, for example, developed musculature [14]. At least some of these characteristics relate to secondary sexual characteristics determined by male endocrine factors, including androgenic hormones such as testosterone and its more active derivative dihydrotestosterone.

Such a pronounced selection based on the success of male reproduction could well lead to the evolution of a compromise between the characteristics that make it possible to create a patriarchal family already in youth, and the health-damaging effects of male endocrine features that manifest themselves later in life [18]. From this point of view, the modern model of male aging is the result of the action of antagonistic forces of selection. On the one hand, the selection taking place in old age, due to the protected patriarchal position, status and access to young wives, increases life expectancy. On the other hand, selection at an early age based on traits that increase the likelihood of becoming a patriarch primarily creates an endocrine status, ultimately having a detrimental effect on health and reducing life expectancy. This is a clear example of antagonistic pleiotropy (the influence of one gene on several phenotypic traits) [14, 35].

According to this scenario, removal of the seminal glands (castration) can increase a man's life expectancy. This assumption is confirmed by the results of a study involving mentally ill men who were "lucky" to live in the United States in the early-mid 20th century. In those years, the popular eugenics movement gave rise to the widespread practice of sterilization of the "genetically objectionable" [36]. An analysis of the mortality data of 297 castrated men and 735 age-appropriate men of the control group revealed a significant increase in the life expectancy of the former (70.7 years compared to 64.7 years, p < 0.001) [37]. When only men castrated in early youth were included in the analysis, the effect of this procedure on life expectancy became even more pronounced: their average life expectancy was 11.6 years longer compared to the control group.

Despite the fact that the results of the study conducted by Hamilton and Mestler support the idea of the life-shortening effect of the seminal glands, it is possible that in this case the positive effect of castration is due to the maintenance of mentally ill men in specialized institutions. It should also be noted that the average life expectancy of the control group was significantly lower than the general population [37]. Moreover, retrospective studies have not revealed a clear relationship between testosterone levels and life expectancy in men with seminal glands. One of the studies involving former US military men over the age of 40, on the contrary, demonstrated the relationship between low testosterone levels and increased mortality from any cause, which persisted even after adjustments for age, mortality for medical reasons and body mass index [38].

This issue has also been studied in the framework of studies involving eunuchs, who have historically been present in a number of countries, starting from Yugoslavia and pumping China [39, 40]. In Western Europe, eunuchs were singers castrated in the prepubescent period to preserve the high timbre of the voice (soprano and mezzo-soprano). In one study, the life expectancy of 50 Italian castrated singers was compared with the life expectancy of non-castrated singers who lived in the same time period. The revealed difference in life expectancy was only 1.2 years and was not statistically significant (65.5±13.8 years compared to 64.3±14.1 years) [41]. However, in this case, the lack of statistical significance could be due to the small sample size [42]. Another explanation is that some of the alleged castrati may actually have been men who retained a high timbre of voice after reaching puberty.

A plausible explanation for this long-standing mystery was given in 2012 as a result of a study that analyzed the life expectancy of eunuchs who lived at the court of the Joseon imperial dynasty in Korea [43]. These eunuchs could hold high official positions, which ensured the safety of their genealogical data, including dates of birth and death. As a result of the analysis of these data dated 1556-1861, 81 eunuchs were identified with an average life expectancy of 70.0±1.76 years (range 27-109 years). For comparison, the average life expectancy of men with comparable social status varied in the range of 50.9-55.6 years, that is, eunuchs lived 14.4-19.1 years longer. Moreover, three eunuchs out of 81 survived to the age of 100, 101 and 109 years. Thus, the frequency of longevity in this population is at least 130 times higher than in the upper strata of society of the corresponding time interval [43]. The results of this study confirm the conclusions made by Hamilton and Mestler in 1969, according to which functioning seminal glands significantly shorten men's lives. These data also agree well with the idea that the seminal glands are the determining factor of the sex difference in life expectancy.

Ending: Hormones and the health of older men

Portal "Eternal youth" http://vechnayamolodost.ru09.10.2014

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