Manipulating sex ratios for more grandchildren

In most mammal species, males have a much more variable number of offspring than females do. Unlike females, males are not limited by their biology and can have many times more babies. The flip side is that males are also much more likely than females to end up with zero offspring. This means that an animal who produces a daughter will probably end up with some grandchildren. Having a son is a greater gamble. The animal may end up with far more or far fewer grandchildren. I’m sure I don’t need to explain that these predictions do not apply to humans.

All of this means that if an animal can skew the number of sons or daughters it has, it may be able to increase the number of grandchildren it has. When things seem auspicious for sons to mate with many females, having more sons would be the way to go. If, on the other hand, events conspire against males, the safer bet would be to invest in daughters. 

There is evidence that in some species, high status females produce more male offspring and low status females produce more female offspring. This suggests that animals are manipulating the sex ratios of their offspring to optimize their chances of having grandchildren. After all, sons are likely to be particularly successful at finding and keeping mates if they have a high status whereas daughters are likely to reproduce regardless of status. However, it’s been impossible to count up all the grandchildren resulting from each offspring, and without that data we couldn’t determine whether skewing sex ratios is a successful strategy.

Joseph Gardner of Stanford University and his colleagues have now filled that void. They collected 90 years worth of captive breeding records from 198 mammalian species to generate a three-generational data pool. They could determine whether any individual tended to have more or fewer sons, and how this affected the eventual number of grandchildren the animal had.

The researchers found that individuals that produced more sons than average got up to one quarter more grandchildren. Individuals that produced more daughters than chance would dictate also did better in the grandchild lottery, though not by as much. This means that individual mammals did gain an advantage by skewing the sex ratio of their offspring. Depending on their circumstances, the animals might have done better with either male or female offspring.

As a reminder, this data was collected from captive breeding populations. The authors are the first to admit that this means human control could have played a role in manipulating the sex ratios. However, they point out that if anything, curators would have done their best to neutralize sex differences rather than make them more extreme. Since we already know that many mammals do have different numbers of sons and daughters depending on their status, it’s not surprising to learn that this strategy does maximize the number of grandchildren.

Collette M. Thogerson, Colleen M. Brady, Richard D. Howard, Georgia J. Mason, Edmond A. Pajor, Greg A. Vicino, & Joseph P. Garner (2013). Winning the Genetic Lottery: Biasing Birth Sex Ratio Results in More Grandchildren PLoS ONE, 8 (7) DOI: 10.1371/journal.pone.0067867.