Asked by: Sanchit Sachdev


Female Surinam toad with embedded, fully formed froglets on the back via wikicommons

Answer: For most creatures, the primary purpose of life is to propagate the species—to pass on one’s genes to the next generation. In other words, for a majority of the inhabitants of Earth, life is all about sex and reproduction. Unsurprisingly, most species are well adapted to this purpose and have developed a number of habits and traits that allow them to protect their offspring. For example, the female surinam toad has her eggs embedded into her back by the male in order to protect them from predators, and there is a frog native to South America that carries its tadpoles in its mouth (the male can carry about three dozen tadpoles in his vocal pouch).


But of course, these are some of the most obvious ways that creatures have evolved to ensure the survival of their species. Ultimately, there are more intrinsic factors that relate to species’ survival—factors that frequently get over looked or taken for granted: sex ratio.


Although there are several species that are able to reproduce asexually (like bacteria, unicellular fungi, some annelid worms, and sea stars), most species require a male and a female to reproduce. If you look at any given species, you will notice that the ratio of males to females is relatively equal i.e., there are about as many men as there are women. This may seem like a happy accident, but it is more than random luck. It’s science.


In most cases, a creature’s sex is determined by its chromosomes. If a being has two X chromosomes, then it will become a female. If it has a Y and an X chromosome, it will become a male. Of course, there are some hermaphrodites (individuals that have reproductive organs normally associated with both male and female sexes), but such instances are generally the exception, as opposed to the rule. Also, I should note that there are other things that determine sex besides chromosomes, but chromosomes are the primary factor.


Now then, during reproduction, the male contributes one chromosome and the female contributes one chromosome. Since females generally have XX chromosomes, the females egg cell will contribute one X chromosome. The male’s individual sperm may contain either an X chromosome or a Y chromosome. If the fertilizing sperm cell contains an X chromosome, the egg will develop into a female (as it will have XX chromosomes). If it contains a Y chromosome, it will develop into a male (as a result of the XY). As a result, it is the male who ultimately determines the sex of the child.


To answer the initial question, since the sperm essentially has an equal chance of delivering an X or Y chromosome, the sex ratio of a species generally works out to be about even. This is similar to the chance of getting heads or tails when you flip a coin. Statically, if you flip a coin 100 times, you should get heads about 50 times and tails about 50 times. The same is true of sex, about 50% of people will be male and 50% will be female.


In essence:

–There are equal numbers of X and Y chromosomes in mammalian sperms

–X and Y stand equal chance of achieving conception

–Therefore, an equal number of male and female zygotes are formed.

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