Around the world, women outlive men by an average of around 5.4 years. A distinction in life expectancy between sexes is surprisingly common among other mammals , too, though it's not fully understood why.

An international research team has now uncovered the evolutionary roots of this phenomenon, with a detailed study of sex differences in the lifespans of mammals and birds.

Biology defines sex in many different ways – and the human concept of gender adds even more nuance to these terms. In this study, sex was defined by the presence of specific chromosomes.

Among mammals, females are defined as having two X chromosomes, while males have only one X and one Y. Male mammals are therefore heterogametic: each of their chromosomes is a different shape, rather than two of the same.

In bird

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