by Charles Q. Choi, Live Science Contributor | October 07, 2015 03:13pm ET
Like an awkward junior-high birthday party, some animal species tend to have many more males than females or vice versa, and scientists have long wondered why. Now, they've figured out a key culprit: sex chromosomes.
An animal's sex is often determined by the sex chromosomes it inherits. The new research reveals that species with X and Y sex chromosomes, including mammals, generally have female-skewed populations, whereas species with the less familiar Z and W sex chromosomes have a sex ratio tilted toward males.
The proportion of adult males to adult females in a species, known as its adult sex ratio, can vary widely in nature. For example, scientists have known that among tetrapods — that is, four-limbed animals such as mammals, birds, reptiles and amphibians — birds possess male-skewed adult sex ratios, and mammals are usually female-skewed. Extreme ratios are seen in some marsupial species, in which the males die after the mating season, sometimes leaving populations made up entirely of pregnant females.
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