Contrary to what, in general, happens in mammals, where females tend to live longer than males, in the world of birds they have shorter lives than males.
The conclusion comes from a study that analyzed longevity data for 92 bird species from the orders Passeriformes and Piciformes (which includes woodpeckers), collected between 1992 and 2018 in the United States and Canada.
In a paper published in the journal Proceedings of the Royal Society B, the scientists say they have found that, on average, females of 35 species show, after reaching adulthood, a survival rate 12.4% lower than that of the males of the same species.
“What I found interesting was the fact that this pattern was consistent across many different types of birds, from flycatchers and blackcaps to cardinals and tanagers,” says Joanna Wu, from the University of California, Los Angeles (UCLA) and the first author of the study.
Adult survival is strongly influenced by factors such as energy expenditure and other biological costs associated with reproduction, movement and competition for resources. Migrations demand a lot in terms of energy and expose the birds to predators and the risk of injuries. When they are not on the move, birds must defend their territory and their resources.
For females, the added costs associated with egg production and laying are another pressure, even though offspring care may, depending on the species, be shared by both parents.
Looking across decades of data, the researchers found that males have higher survival rates in both migratory and non-migratory species, although they are larger in long-distance migrants. Moreover, it was noted that in species with sharper wings, males also tend to live longer than females, something that was also observed in males with higher levels of body fat.
The team acknowledges that, beyond these aspects, they have not yet been able to find a clear explanation for the differences between males and females in terms of longevity. Still, there are several hypotheses on the table, and one of them concerns genetics.
The Chromosomal Hypothesis
In mammals, females have two copies of the same sex chromosome – XX – and males have two different sex chromosomes – XY. As the researchers explain, this means that if one of the chromosomes carries a defective gene, females have a “backup,” but males do not.
In human males (that is, men), problems with the sex chromosomes and the lack of a “Plan B” are associated with greater susceptibility, for example to heart diseases and cancer as aging leads to degeneration of the Y chromosome.
Therefore, there are scientists who consider a relationship between longevity and the pair of sex chromosomes. However, in birds these chromosomes are inverted: females have XY and males XX.
“In birds, unlike mammals, it is the females who carry two different chromosomes, while the males carry two of the same,” says Wu. “This could be more evidence linking the sex chromosome to lower longevity in female birds, but that is a question we have not yet directly investigated,” she adds.
The researcher says that future investigations should also try to determine whether there is any relationship between the reduced longevity of female birds and the pronounced decline of several bird populations occurring worldwide.
“If the females have already reproduced successfully before dying, the fact that their lives are shorter does not make much difference. But we do not yet know if that is the case or not,” she explains.
“We know that habitat loss and climate change are the main culprits, but we want to understand whether these factors may be making survival even harder for females than for males in a world that humans are rapidly transforming.”
