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Sex ratios

What might be the effects of a surplus of males (male-biased sex ratio), or of females (female-biased)? For starters, the gender that is abundant will (all else being equal) compete against each other for access to the less abundant gender. The less abundant gender, in turn, will be able to take advantage of the abundance of the other gender by being choosy in whom they mate with. So, the abundant ones will compete, and the less abundant ones will be choosy.

Females, by definition, are the gender that produce large gametes, and because they have so much invested in each egg, they often spend a lot of time and resources on those eggs, or the offspring that live-bearing animals carry within their bodies. They are also the gender that is more likely to bestow parental care on the offspring. Females often spend extra time and effort in creating and birthing and caring for their offspring. While they are engaged in these activities, they are not interested in mating; they are either unreceptive, or actually not fertile. Therefore, in many cases, there are fewer sexually fertile and receptive females than males. For this reason, the most common setup in nature is that females are choosy, and males compete against each other.