In mammals, the sex of a baby is determined by the chromosomes it inherits from its parents. But, the combination an animal inherits can be quite different.
What are the odds of giving birth to a boy? Learn more on this Moment of Science.
In the case of the African stalk-eyed fly, where the males are like hammer head sharks with their eyes extended on long stalks, sometimes measuring one and half times the length of their bodies, long stalks are an indicator of better genes.