Courtship display
A courtship display is a set of display behaviors in which an animal, usually a male, attempts to attract a mate; the mate exercises choice, so sexual selection acts on the display. These behaviors often include ritualized movement ("dances"), vocalizations, mechanical sound production, or displays of beauty, strength, or agonistic ability.[1][2][3][4][5][6]
Female display[edit]
Female courtship display is less common in nature as a female would have to invest a lot of energy into both exaggerated traits and in their energetically expensive gametes.[17] However, situations in which males are the sexually selective sex in a species do occur in nature. Male choice in reproduction can arise if males are the sex in a species that are in short supply, for example, if there is a female bias in the operational sex ratio.[18] This could arise in mating systems where reproducing comes at an energy cost to males.[19][20] Such energy costs can include the effort associated in obtaining nuptial gifts for the female[21] or performing long courtship or copulatory behaviors.[22] An added cost from these time and energy investments may come in the form of increased male mortality rates, putting further strain on males attempting to reproduce.
In pipefish (Syngnathus typhle), females use a temporary ornament, a striped pattern, to both attract males and intimidate rival females. In this case, the female of a species developed a sexually selected signal which serves a dual function of being both attractive to mates and deterring rivals.[23]
Sexual ornaments[edit]
Sexual ornaments can serve to increase attractiveness[39] and indicate good genes and higher levels of fitness.[40] When exposed to exaggerated male traits, some females may respond by increasing maternal investments. For example, female canaries have been shown to produce larger and denser eggs in response to male supranormal song production.[41]
Sexual conflict is the phenomenon in which the interests of males and females in reproduction are not the same: they are often quite different:[42]
This has many consequences. Courtship displays allow the mate performing the selection to have a means on which to base the copulatory decision. If a female chooses more than one male, then sperm competition comes into play. This is competition between sperm to fertilize an egg, which is very competitive as only a single sperm will achieve union.[43] In some insects, the male injects a cocktail of chemicals in seminal fluid together with sperm. The chemicals kill off older sperm from any previous mates, up-regulates the female's egg-laying rate, and reduces her desire to re-mate with another male. The cocktail also shortens the female's lifespan, also reducing her likelihood of mating with other males.[44] Also, some females can get rid of the previous male's sperm.[45]
After mating has taken place, males perform various actions to prevent females from mating again. What action is performed depends on the animal. In some species, the male produces a mating plug after insemination. In some hymenoptera, the male provides a huge quantity of sperm, enough to last the female's entire life. In some birds and mammals, the male may participate in agonistic behaviors with other candidate males.[42][46][47]
Agonistic behavior and courtship[edit]
Although rare, agonistic behavior between males and females during courtship displays is seen in nature. Intraspecific agonistic behavior that results in the death of a combatant is rare because of the associated risk of death or injury. However, agonistic behavior that turns dangerous does occur.
In some species, physical traits that are sexually selected for in male courtship displays may also be used in agonistic behavior between two males for a mate. In fiddler crabs (genus Uca), males have been sexually selected to have one enlarged claw, which can take up anywhere from a third to a half of their total body mass, and one regular claw. Although the enlarged claw is believed to have developed for use in combat for territorial defense, it is not uncommon for males to employ this claw in battle for a mate.[48] Even though this claw developed as a weapon, it is also closely linked with the crabs' courtship display: it is waved in a certain pattern to attract females for mating.[49]
Agonistic behavior in courtship displays is not limited to male-male interactions. In many primate species, males direct agonistic behavior toward females prior to courtship behaviors. Such behavior can include aggressive vocalizations, displays, and physical aggression.[50] In the western gorilla (Gorilla gorilla), dominant males exhibit agonistic behavior toward female gorillas at very high rates, with the majority of those interactions being courtship-related. Most documented cases of male gorilla aggression toward females is courtship related and is used primarily as a strategy to prevent females from migrating to another male.[51]
In many cases, male courtship displays will cause forms of contest competition to develop. This is often seen within lek mating systems. For example, males will seek to obtain a certain spot or position to perform their courtship display. The best spots are regions of high contention as many males want them for themselves. Because of this direct conflict, agonistic encounters between males are fairly common.
Extended courtship period[edit]
Mating is preceded by a courtship/pairing period in many animal mating systems. It is during this period that sexually mature animals select their partners for reproduction.[52] This courtship period, which involves displays to attract a mate by a member of a species, is usually short, lasting anywhere from 15 minutes to a few days. However, certain animals may undergo an extended courtship period, lasting as long as two months.[53]
One such exception is the emperor penguin (Aptenodytes forsteri). Emperor penguins engage in an extended courtship period that can last up to two months, the longest of any Arctic seabird. Their courtship period accounts for 16% of the total time they spend breeding, whereas in their closest relatives, the king penguin (Aptenodytes patagonicus), the courtship period takes up just three per cent of their breeding cycle.[53][54]
Environmental factors[edit]
Various environmental factors, such as temperature, photoperiod, resource and light availability, have an effect on the timing and effectiveness of courtship displays in certain species of animals.[53]
In guppies (Poecilia reticulata), variation in the light environment plays a huge role in their ability to attract mates.[61] Guppy males alter both their 'courtship mode', whether they perform a full courtship display or try to 'engage' in sneak copulations, and distance from females as light intensity changes.[62] Courtship mode also varies with light spectrum and relates to predation risk.[63] On average, male guppies seek out and spend more time in the environment in which their color pattern is the most visible. Males, in the light environment that made them most visible, copulated with the most females.[61]
In emperor penguins (Aptenodytes forsteri), resource availability determines when male emperor penguins will be able to return to their breeding grounds to initiate their courtship rituals.[53] The greater the concentration of resources in their feeding ground, the quicker they will be able to restore their body reserves for winter, and the sooner they will be able to return to their breeding grounds. An early return to their breeding grounds comes with an increased likelihood of finding a mate.[64]
The effectiveness of Hirtodrosophila mycetophaga mating displays is influenced by the color of the bracket fungus that it mates and courts upon;[65] these flies choose brackets that are lighter, making their displays more visible to the opposite sex.[65]