Mating success and mating system

There are three general mating systems based on innate behaviour (i.e., biological rather than learned, social behaviour): monogamy, polygyny, and polyandry.

1. In monogamous systems, one male and one female are paired for at least one breeding season (possibly for longer or even for life). The males share substantially in parental care. This occurs when males and females have equal success in terms of intersexual selection. Put another way: male mating success is similar to female mating success.
2. Polygynous mating refers to one male mating with multiple females. The female generally becomes responsible for most of the parental care. This occurs when variance in male mating success is higher than variance in female mating success: one male dominates the mating within a group, so he mates with multiple females.
3. In a polyandrous mating system, one female mates with many males. This is a less common system, with examples being pipefishes and seahorses. In these cases, the males carry the embryo and give birth to offspring. This happens when variance in female mating success is high -> one female is dominant over others in mating -> one female mates with multiple males.

This is related to Bateman’s principle, which states that in most animals, variance in mating success is greater in males than in females, which is why most animals have a polygynous mating system.