9.0 Introduction
Understanding sex ratios
In Australia, there are approximately 100 males for every 100 females. In Bahrain country, there are 153 males for every 100 females (1.53 M:F). And in El Salvador, there are 92 males for every 100 females (0.92). A quick look at the map clarifies that, in human populations, sex ratios vary globally. Do you note any patterns in the image below?
![](https://open.lib.umn.edu/app/uploads/sites/206/2017/08/Check-Yourself-Icon-01-186x300.png)
Check Yourself
Interesting sex ratios are not unique to humans, and in some non-human organisms differential sex allocation leads to dramatically skewed sex ratios. For example, in some population of the bluestreak cleaner wrasse, there is only one male for every 6-8 females.
![](https://open.lib.umn.edu/app/uploads/sites/206/2020/09/Gewoehnlicher-putzerfisch.jpg)
In jacanas, males may outnumber females by more than 2:1.
![](https://open.lib.umn.edu/app/uploads/sites/206/2020/09/Irediparra_gallinacea_-_Comb-crested_Jacana-1024x682.jpg)
How can we understand some of these wildly different sex ratios, in both human and non-human populations?