New research shows that the sex of the offspring matters for wild sheep – male lambs induce more rapid aging in mothers than females. We don't yet know if the same is true in humans, although there's evidence that it is in certain cultures.
It takes a lot of energy to produce offspring, and for mammalian species the mother's effort isn't over at birth. Milk production, protecting young from predators, and ensuring they don't do anything stupid can be tiring work.
Many species are sexually dimorphic, with one sex considerably larger than the other. The most extreme size differences usually involve much larger females, as in spiders or black dragonfish, but among mammals males are usually larger. Some studies have shown that where this is true, there is a short-term cost to having male offspring, measured by lower chances of reproducing again, and even surviving, the following year.
Dr Mathieu Douhard of the Université de Sherbrooke, Canada, has investigated the much harder question of whether there are long-term consequences in the sex of a child using bighorn sheep at the appropriately named Ram Mountain, Alberta. Not only are male bighorns considerably larger than the females, but this is true from birth.
In Proceedings of the National Academy of Sciences, Douhard reveals that mothers of rams do indeed pay a price. Once ewes get past 8 years old, their chance of having new lambs survive the winter decreases, but the rate of decrease is much sharper for those who weaned mostly male offspring earlier in life. Apparently, females who had a lot of sons when young were too worn out by the experience to care for their younger children successfully. However, the difference was only seen where male lambs survived to weaning – giving birth to males had no noticeable effect. As the paper puts it: “These results support the contention that costs of gestation are lower than those of lactation.”
However important the results might be for animal breeders, particularly sheep farmers, it's the implications for humans that are of more widespread interest. We need to be wary of jumping to the simplistic conclusions that too often get drawn from other species. For one thing, although male humans are usually larger than females, there is a lot more overlap than for bighorn sheep. Moreover, for humans the difference generally doesn't show up until puberty.
Social factors are also more likely to affect you than it would for ewes. A study among the Sami people found that having more sons reduced women's life expectancy, while having daughters increased it, but research in other cultures has not replicated the results.
