From humans to black-tailed prairie dogs, female mammals often outlive males – but for birds, the reverse is true.

Now researchers say they have cracked the mystery, revealing that having two copies of the same sex chromosome is associated with having a longer lifespan, suggesting 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 write. “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 say they have found the trend is widespread. Writing in the journal Biology Letters, the team report that they gathered data on sex chromosomes and lifespan 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 reveal that individuals with two of the same sex chromosomes live 17.6% longer, on average, than those with either two different sex chromosomes or just one sex chromosome.

The team say 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 will 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.

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The team found that in species where males have two of the same sex chromosomes, these males live on average 7.1% longer than females. However, in species where the sex chromosome pattern is the other way around, such as humans, females live 20.9% longer on average than males.

The researchers say the extent of the longevity gap may 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 co-author of the research from the University of New South Wales.

But there are also other possibilities as to why the longevity gaps differ in size, including that oestrogen appears to protect the ends of chromosomes from being damaged – a process linked to ageing.

“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 behaviours, establishing territories and access to quality nutrition,” said Xirocostas.

Prof Steven Austad, an expert on ageing at the University of Alabama at Birmingham, who was not involved in the study, said the theory that having two of the same sex chromosomes is beneficial for longevity was appealing.

“I would think that it plays some role in understanding sex differences in longevity but it certainly isn’t the only factor,” he said, noting that both risk-taking behaviour and parental roles also appeared to be important. “For instance, owl monkey males live longer than females and the males play a big role in infant care in that species,” he said, noting such males have two different sex chromosomes.

Austad said the upshot was that longevity is not only about sex chromosomes. “There is a general trend, but with numerous exceptions,” he said.