Why do men live shorter? Because they have a Y chromosome!

[Jokodo]

Sometimes, the simplest explanation really is the best...

In humans and other mammals, males have significantly lower life expectancy than females. Various prima facie plausible adaptationist explanations could be, and have been, proposed: Males live shorter because of fiercer competition (sexual selection), or because their premature death has less impact on offspring's survival (natural selection). The actual explanation (or a large part of it), may however be much simpler: Males only have one X chromosome, so copying errors accumulating during aging have more severe effects as there is no backup, so to speak.

An interesting piece of evidence for this non-functionalist explanation comes from a new comparative study: The sex with the reduced sex chromosome dies earlier: a comparison across the tree of life (Biology Letters). The authors report that not only is the mammalian pattern reversed in birds (where males are the homogametic sex with two "Z" sex chromosomes, while females have one "W" and one "Z" chromosome), but similar patterns are repeated across other branches of the tree of life (amphibians, reptilians, fish, crustaceans): in clades where males have two of the same sex chromosomes, they tend to live longer, where males have two different (or in some cases only one), females live longer.

 

[fromderinside]

Inferior genes covers it I believe.

 

[Jokodo]

Inferior genes covers it I believe.

Not "inferior".

Just not enough of them.

 

[Bronzeage]

I've never heard of a coroner listing cause of death as "Y chromosome". I waded through the article, even though it had more Xs and Ys than a 10th grade algebra final exam. Other than speculating about species where the competition for a mate is taxing, there's no talk about how the Y chromosome manages to cut short the lives of men.

 

[DBT]

This article may be of interest;

INTRODUCTION

''The vast majority of animal species have two sexes and those sexes often differ in many aspects of their biology. Most obviously, males range from a tiny fraction of the size of females to considerably larger and live considerably shorter to substantially longer lives (Austad, 2006; Finch, 1990). Sex differences in longevity can potentially be exploited to help understand mechanisms underlying variation in longevity within a species. Yet these differences remain little studied despite considerable variation among different genotypes of commonly used laboratory species such as C. elegans (McCulloch and Gems, 2003), Drosophila melanogaster (Malick and Kidwell, 1966), and Mus musculus (Austad, 2011).

Surprisingly, much more has been reported about sex differences in longevity among wild populations than among captive populations because numerous long-term field studies have tried to understand the evolutionary forces underlying sex differences in behavior and a host of life history traits such as rates of development, mating systems, and reproductive patterns. Thus we know that female short-finned pilot whales live nearly twice as long as males (Kasuya and Marsh, 1984); and that female African lions, red deer, black-tailed prairie dogs, numerous monkeys and apes are also longer-lived than males, although these differences are not as extreme as in the pilot whales (Bronikowski et al., 2011; Clutton-Brock (ed), 2009; Clutton-Brock and Isvaran, 2007). The reputed general pattern among mammals in nature is for females to be longer-lived, although this is known for certain in relatively few species. We know there are a number of mammals such as Japanese macaques, savannah baboons, and bannertail kangaroo rats where there is virtually no sex difference (Clutton-Brock and Isvaran, 2007; Waser, 1991). Rarely, male mammals even appear to be the longer-lived sex (see below).''

 

[bigfield]

I've never heard of a coroner listing cause of death as "Y chromosome". I waded through the article, even though it had more Xs and Ys than a 10th grade algebra final exam. Other than speculating about species where the competition for a mate is taxing, there's no talk about how the Y chromosome manages to cut short the lives of men.

I'm not sure what explanation you were looking for, but the paper does explain that a Y/W chromosome does not do as good a job of masking harmful mutations on the other sex chromosome.

The unguarded X hypothesis [7] suggests that the reduced or absent second sex chromosome in the heterogametic sex (e.g. the Y chromosome in mammals or the W chromosome in birds) might lead to heterogametic organisms being more likely to express undesirable morphological and physiological characteristics. This prediction is based on the fact that any recessive deleterious mutation on the X or Z chromosome is likely to be expressed in the heterogametic sex, while these mutations will generally be masked by the second copy of the X or Z chromosome in the homogametic sex [7–11]

The study doesn't explain exactly what mutations are being expressed; it just shows that the unguarded X hypothesis makes an accurate prediction that heterogametic (XY, ZW, X0, Z0) organisms don't live as long as homogametic (XX, ZZ) organisms of the same species.

---

I also found this interesting: The paper also presents the finding that the difference in lifespan is greater when the male is the heterogametic sex:

Our second major finding was that when males are the heterogametic sex, they die 20.9% earlier than their female counterparts, but when females are the heterogametic sex, they die only 7.1% earlier than their male counterparts. Three possible explanations for this surprising trend include: (1) the degree of degradation of the Y chromosome, (2) telomere dynamics, and (3) side effects of sexual selection.

 

[Jokodo]

I've never heard of a coroner listing cause of death as "Y chromosome".

I never heard of a coroner listing "sexual selection gone wrong" or "he's done his share of insemination, we don't need him no more" either.

Of course the proximate causes of death among both males and females are varied and overlap to a great extent in any species. Nonetheless there is a pattern in that one sex (in mammals: almost always the males) is more susceptible to a broad range of age related ailments, or develops them earlier on average. The study discusses the distal causes of that pattern more than the proximal causes of any individual's death.

A common prejudice (shared by posters on this forum but unfortunately also hinted at by a number of popularisations of the theory of evolution) is that every biological trait, and especially every sex difference, is best explained as the product of differential selection. Such functionalist explanations for sexual dimorphism in longevity come in various flavours, but they mostly boil down to "females have been under stronger selective pressure to extend their lifespans"; sometimes they border on the teleological, when no attempts are made to explain how selection of longer lived females wouldn't also benefit their male offspring. This study hints the actual explanation may have more to do with biochemistry than with ancestral sex roles.

I waded through the article, even though it had more Xs and Ys than a 10th grade algebra final exam. Other than speculating about species where the competition for a mate is taxing, there's no talk about how the Y chromosome manages to cut short the lives of men.

I believe there is: having only one copy of X chromosome makes mammalian males more susceptible to the effects recessive mutations and age related degradation of the genome.

 

[Jokodo]

As a side note, had the comparison only included mammals and birds, the functionalist explanation could still be viable. Because paternal care is much more common in birds, one could argue that longevity in the avian male is selected for the same reasons as in the mammalian female, to better ensure the survival of immature offspring. The inclusion of cold blooded vertebrates and arthropods, and the finding that similar patterns repeat there, makes this alternative much less plausible.

Arguably, though this is admittedly speculative, an inverse causality might hold instead: it is because the avian male tends to live relatively longer than his mammalian counterpart for independent reasons that extensive male care for offspring is a viable evolutionary strategy and has emerged time and again among birds but not so much amongst mammals.