From humans to black-tailed prairie dogs, female mammals often outlive males — but for birds, the reverse is true.
Now researchers have said that they have cracked the mystery, revealing that having two copies of the same sex chromosome is associated with having a longer lifespan, suggesting that the second copy offers a protective effect.
“These findings are a crucial step in uncovering the underlying mechanisms affecting longevity, which could point to pathways for extending life,” the authors wrote. “We can only hope that more answers are found in our lifetime.”
The idea that a second copy of the same sex chromosome is protective has been around for a while, supported by the observation that in mammals — where females have two of the same sex chromosomes — males tend to have shorter lifespans.
In birds, males live longer on average and have two Z chromosomes, while females have one Z and one W chromosome.
Scientists said that they have they have found the trend is widespread.
Writing in the journal Biology Letters, the team said that they gathered data on sex chromosomes and lifespans across 229 animal species, from insects to fish and mammals.
Hermaphroditic species and those whose sex is influenced by environmental conditions — such as green turtles — were not included.
The results revealed that individuals with two of the same sex chromosomes live 17.6 percent longer, on average, than those with either two different sex chromosomes or just one sex chromosome.
The team said that the findings back a theory known as the “unguarded X hypothesis.”
In human cells, sex chromosome combinations are generally either XY (male) or XX (female). In females only one X chromosome is activated at random in each cell.
As a result, a harmful mutation in one of the female’s X chromosomes would not affect all cells, and hence its impact can be masked. By contrast, as males only have one X chromosome, any harmful mutations it contains are far more likely to be exposed.
The team found that in species where males have two of the same sex chromosomes, males live on average 7.1 percent longer than females.
However, in species where the sex chromosome pattern is the other way around, such as humans, females live 20.9 percent longer on average than males.
The team said that the extent of the longevity gap might reflect other factors at play, including that males tend to take more risks when it comes to securing a sexual partner, including fighting.
“These pressures to travel far to find a mate, establish a territory and compete with other members of your sex are not seen often in females,” said Zoe Xirocostas, a coauthor of the research from the University of New South Wales.
However, there are other possibilities as to why the longevity gaps differ, including that estrogen appears to protect the ends of chromosomes from being damaged — a process linked to aging.
“Our study suggests that the unguarded X is an underlying genetic factor that can influence lifespan, but many external factors can influence longevity in different ways, such as predation, risky behaviors, establishing territories and access to quality nutrition,” Xirocostas said.
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