In the wild, some species tend to have more males than females, while some extreme populations might be composed of adult females exclusively. According to new findings published in Nature last week, the proportion of males to females – called the adult sex ratio – in many animal populations is based on the sex chromosomes. And that ratio can be predicted too.

Researchers have known for a while that the adult sex ratio varies a lot among different species of four-legged tetrapods – that is, all mammals, birds, reptiles, and amphibians (even those that have lost their limbs like snakes). Bird populations tend to have more males, mammals skew towards females, and with some marsupials, the males die after mating season, leaving entire populations of just pregnant females. These variations influence many important social behaviors, including mate choice and parental care. Yet the causes of these variations have remained unclear. One factor that could help explain adult sex ratios is the genetic sex determination system. 

In humans and other mammals, females have XX sex chromosomes, while males have XY. Birds, on the other hand, have a ZW genetic sex determination system in which females are ZW and males are ZZ. So while male mammals have two different sex chromosomes, in birds, it’s flipped: Female birds have two different sex chromosomes, which is known as heterogamety. Meanwhile, reptiles and amphibians have shifted between XY and ZW sex determination systems many times throughout their history.

After studying sex ratios in 344 species from 117 tetrapod families, András Liker from the University of Pannonia and colleagues discovered that the heterogametic sex tends to be “underrepresented” within a population. When the female has two different sex chromosomes (like with birds), the population skews towards males. When the male has two different sex chromosomes (like with mammals), this tends to result in a female-biased sex ratio. 

The genetic sex determination system, the team found, can explain 24% of sex ratio variation between species of amphibians and 36% for reptiles. However, despite studying different genetic factors that might explain this pattern – such as sex-linked mutations – the researchers aren’t sure what the underlying mechanisms are, and they plan to investigate the link further.