For species that reproduce sexually, only those organisms that successfully mate with a partner will pass on their genetic traits to the next generation. And so evolutionary pressure will maintain a population of highly active breeders.
Early evolutionary theories about the sexual behavior of animals suggested that males generally evolve to become aggressively competitive, often fighting among themselves for access to females or carrying out ornamental displays in order to attract them.
Sperm is easy to produce, and after fertilizing a female, the males of many species have little or no further parental responsibility. They have little to lose by fertilizing as many females as they can. Males that fertilize more females will contribute more to the genetic makeup of the next generation, and so males will generally evolve to become competitively promiscuous.
Early evolutionary theories also suggested that females generally evolve to be more selective about their sex partners. Eggs are large and biologically expensive to produce, pregnancy can be an enormous burden, and the females of some species continue to care for their young until they become self-sufficient. With such a large investment in reproduction, females will evolve to focus more on males with genetic traits that when passed on their young, will improve their chances of surviving and reproducing.
Variations in sexual behavior
Of course, these male and female stereotypes are far from universal in the animal world. In many species, males also play a role in raising the young. The females of some species are more aggressive than the males. Some species have more than two genders. In some species, males often turn into females or females turn into males.
In some species, males have to pay for the opportunity to have sex by offering food to the female. Females that invite more sex get paid more food and so they can biologically afford to produce more young. In this way, they contribute more to the genetic makeup of the next generation, and so it is the females that evolve to become promiscuous.
Sexual promiscuity can help to overcome the problem of genetic incompatibility. In populations where there is a high degree of genetic diversity, some genetic combinations might not be able to produce offspring, or the offspring they produce may not be fit for survival. It is not that either partner is sterile or unfit, it is just that they would both do better with more genetically compatible partners. In this way, evolution often favors those who mate with more than one partner.
The various ways in which different animal species reproduce is largely the result of chance and circumstance, as each species randomly mutates and selectively adapts to take advantage of whatever opportunities are present in the surrounding environment.
Dominant male behavior
The sexual behavior of insects is the result of an accumulation of beneficial mutations to nerve cells over the course of evolutionary history which cause the insect to react in instinctual ways when stimulated by certain sensations.
In contrast, the sexual behavior of mammals, with their relatively large brains, is governed by a combination of biological instinct, learned behavior, habit, and opportunity.
For many species of mammal, the strongest and most physically aggressive male will dominate a group and exclusively mate with all of the females. Dominant males usually continue to dominate until they are defeated in combat by a more powerful rival. Often the dominant male will kill any babies that are not his own.
The dominant male strategy is only really possible when animals live in settled groups, where hierarchies of power and domination can easily arise. It becomes impossible when populations are scattered and power monopolies are difficult to maintain because individuals can easily walk away, or when a group is too large for any single individual to control.
Some species are more likely to survive if the male chooses to stay with the female and help raise the young. In such cases, females who choose males that stay, and those males that do stay will outbreed those that do not, and so evolutionary pressure may favor monogamy. Some species may even evolve instincts which incline them to stay with one partner for life.
Even in monogamous relationships though, desire and opportunity occasionally lead one of the partners to stray outside of the relationship. In some monogamous species, females will reject philandering males, and males will only stay at home if they believe that the babies they are helping to raise are their own.
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