Animals that reproduce sexually have evolved a wide variety of different systems for maximizing the number of young that can be raised. In the simplest system, each female is partnered by a male, and the partnership lasts for life. In more complex systems, the fittest adults have many partners while others have none at all.
In polygynous breeding systems, successful males mate with more than one female. Polygyny is common in birds, particularly in species where the males establish breeding territories that provide access to food. A male with a good territory may attract several mates, while one with an inferior territory may attract few or none. Polygyny can also be seen in some mammals and is taken to extremes in species such as elephant seals. The largest and most powerful male elephant seals, weighing up to four times as much as the females, clash viciously for dominance on a breeding beach. A successful male can assemble a harem of over twenty females, but weaker males are excluded from breeding altogether.
In polyandrous breeding systems, one female mates with several males. This kind of breeding system is rare and usually occurs in species where the males take on the work of raising the young. An example of a polyandrous bird is the North American spotted sandpiper. In this species, females compete for males. A single female can lay up to five sets, or clutches, of eggs, and each clutch is incubated by a different partner.
The most specialized mating systems of all occur in animals that form permanent family groups. In social insects, which include many bees and wasps and all ants and termites, each group or colony is founded by a single female or queen. The queen is the only individual in the colony to reproduce. Her offspring, which can number more than a million, forage for food, maintain the nest, and care for the young.