Finding Sexual Partners
Not all animals reproduce sexually, but many that do have the same challenge: they need to find a suitable mate and often have to compete with other individuals to obtain one. Significant energy is spent in the process of locating, attracting, and mating with a sex partner.
Types of Mate Selection
Two types of selection that occur during the process of choosing a mate may be involved in the evolution of reproductive traits called secondary sexual characteristics. These types are: intersexual selection (the choice of a mate where individuals of one sex choose mates of the other sex) and intrasexual selection (the competition for mates between species members of the same sex). Intersexual selection is often complex because choosing a mate may be based on a variety of visual, aural, tactile, and chemical cues. An example of intersexual selection is when female peacocks choose to mate with the male with the brightest plumage . This type of selection often leads to traits in the chosen sex that do not enhance survival, but are those traits most attractive to the opposite sex (often at the expense of survival). Intrasexual selection involves mating displays and aggressive mating rituals such as rams butting heads; the winner of these battles is the one that is able to mate. Many of these rituals use up considerable energy, but result in the selection of the healthiest, strongest, and/or most dominant individuals for mating.
Courtship display of the male peacock
This male peacock's courtship display—his intricate, colorful tail feathers—is designed to attract potential mates.
Mating Systems
Three general mating systems, all involving innate as opposed to learned behaviors, are seen in animal populations: monogamous (monogamy), polygynous (polygyny), and polyandrous (polyandry).
In monogamous systems, one male and one female are paired for at least one breeding season. In some animals, such as the gray wolf, these associations can last much longer, even a lifetime. Several explanations have been proposed for this type of mating system. The "mate-guarding hypothesis" states that males stay with the female to prevent other males from mating with her. This behavior is advantageous in such situations where mates are scarce and difficult to find. Another explanation is the "male-assistance hypothesis," where males that remain with a female to help guard and rear their young will have more and healthier offspring. Monogamy is observed in many bird populations where, in addition to the parental care from the female, the male is also a major provider of parental care for the chicks. A third explanation for the evolutionary advantages of monogamy is the "female-enforcement hypothesis." In this scenario, the female ensures that the male does not have other offspring that might compete with her own, so she actively interferes with the male's signaling to attract other mates.
Polygynous mating refers to one male mating with multiple females. In these situations, the female must be responsible for most of the parental care as the single male is not capable of providing care to that many offspring. In resourced-based polygyny, males compete for territories with the best resources. They then mate with females that enter the territory, drawn to its resource richness. The female benefits by mating with a dominant, genetically-fit male; however, it is at the cost of having no male help in caring for the offspring. An example is seen in the yellow-rumped honeyguide, a bird whose males defend beehives because the females feed on the wax. As the females approach, the male defending the nest will mate with them. Harem mating structures are a type of polygynous system where certain males dominate mating while controlling a territory with resources. Elephant seals, where the alpha male dominates the mating within the group, are an example. A third type of polygyny is a lek system. Here there is a communal courting area where several males perform elaborate displays for females; the females choose their mate from this group. This behavior is observed in several bird species.
In polyandrous mating systems, one female mates with many males. These types of systems are much rarer than monogamous and polygynous mating systems. In pipefishes and seahorses, males receive the eggs from the female, fertilize them, protect them within a pouch, and give birth to the offspring . Therefore, the female is able to provide eggs to several males without the burden of carrying the fertilized eggs.
Polyandry
Seahorses are a good example of a polyandrous mating system, in which one female mates with several males. In seahorse reproduction, the male receives the eggs from the female, fertilizes them, protects them within a pouch, and gives birth to the offspring.