A courtship display is a set of display behaviors in which an animal attempts to attract a mate and exhibit their desire to copulate. These behaviours
often include ritualized movement ("dances"), vocalizations, mechanical sound production, or displays of beauty, strength, or agonistic ability
In most species, the male is the sex that initiates courtship displays in pre-copulatory sexual selection. Performing a display allows the male to present his traits or abilities to a female. Mate choice, in this context, is driven by females. Direct or indirect benefits are often key deciding factors in which males get to copulate and which don't.
Direct benefits can be seen due to the expression of preference. Females can raise their own fitness if they prefer to respond to particular types of signals, independent of costs and certain benefits associated with mating. For example, choosing to mate with males that produce more localized signals would incur less of an energetic investment for a female as she searches for a mate. On the other hand, females can put in more energy towards this process and still attain a higher fitness if they mate with only particular types of males. With this, the males being chosen may impose lower costs on the female or even provide more in terms of material or offspring contributions.
Indirect benefits are benefits that may not directly affect the parents' fitness. As such, indirect benefits increase the fitness of the offspring. For example, offspring of a female will inherit half of the genetic information from the male counterpart. This means that those traits she saw as attractive will be passed on, producing an offspring that is more fit.
The male Six-Plumed bird-of-paradise, Parotia lawesii, exemplifies this idea of male courtship display with its ritualized "ballerina dance" and unique occipital and breast feathers that serve to stimulate the female visual system. This stimulation, along with many other factors, results in subsequent copulation or rejection.
Elaborate courtship displays can also be seen in many other animal species. For example, the male peacock spider, Maratus volans, uses vibratory signals concurrently with visual displays to court females. The adult male Eastern fence lizard, Sceloporus undulatus, possesses blue and black ventral patches that serve the purpose of sex recognition and intrasexual social signaling. Males will show off these patches to females in an attempt to convince her to copulate.
Male jumping spiders conduct complex, visual courtship displays using both movements and physical bodily attributes. Males differ than females as they possess plumose hairs, colored or iridescent hairs, front leg fringes, structures on other legs, and other, often bizarre, modifications. These characteristics are used in a courtship "dance" in which the colored or iridescent parts of the body are displayed.
Another example is present in hummingbirds. Male hummingbirds participate in complex aerial maneuvers that attract females. Smaller males can conserve more energy, perform more courtship displays, and increase their fitness.
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.
Sex-role reversal with courtship displays is far less common in the animal kingdom. In this situation, males are the ones driving sexual selection and putting pressure on the females to attract a mate for copulation. Females experiencing the selective pressure of males will be the ones who develop and possess sexually dimorphic traits. This may include ornamentation, coloration, or development of specific behaviors. This effect can be seen with Long-Tailed Dance Flies, Rhamphomyia longicauda, displaying inflatable abdominal sacs and pinnate tibial scales that are female-specific. Generally, females must invest more energy into the development of gametes and reproduction compared to males. Because of this, this sex-role reversal is a rare phenomenon. Not only must females invest energy into the production of more costly gametes, but they must also put energy towards the development of sexually dimorphic characteristics or performances.
Often, males and females will perform synchronized or responsive courtship displays in a mutual fashion. With many socially monogamous species such as birds, this duet facilitates pre-copulatory reassurance of pair bonding and strengthens post-copulatory dedication to the development of offspring (e.g. Great Crested Grebe, Podiceps cristatus). For example, male and female Crested Auklets, Aethia cristatella, will cackle at one another as a vocal form of mutual display which serves to strengthen a bond between the two. In some cases, males may pair up to perform mutual, cooperative displays in order to increase courtship success and attract females. This phenomenon can be seen with Long-tailed Manakins, Chiroxiphia linearis.
Sexual conflict is the phenomenon in which the interests of males and females in reproduction are not the same: they are often quite different:
- Males: their interest is to mate with a large number of completely faithful females, thus spreading their genes widely throughout a population.
- Females: their interest is to mate with a large number of fit males, thus producing a large quantity of fit and varied offspring.
This has many consequences. Courtship displays allow the mate performing the selection to have a means by which they can base their 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. 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 females' egg-laying rate and reduces her desire to re-mate with another male. The cocktail also shortens the females' lifespan, also reducing her likelihood of mating with other males. Also, some females can get rid of the previous male's sperm.
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 plugs the female copulatory duct after insemination. In some hymenoptera, the male provides a huge quantity of sperm, enough to last the females' entire life. In some birds and mammals, the male may participate in agonistic behaviours with other candidate males.
Within a Fisherian runaway model, males with certain exaggerated, sexually dimorphic traits may be more fit than those who do not possess them. This means that a gene or set of genes will be favored by female choice over time. This would explain why and how such elaborate traits develop within certain species. However, as time goes on and generations pass, the survival advantage associated with one trait may dissipate due to extreme exaggeration to the point that it decreases fitness.
Based upon the good genes hypothesis, certain exaggerated traits may be supported by other factors that allow for extreme development. For example, the costs associated with bright, and complex plumage can be high. Only males with good genes are able to support a large investment into the development of these traits, which in turn, displays their high fitness.
Both of these hypotheses have valuable input when associated with courtship displays. The development of certain behaviors and sexually dimorphic traits is a central characteristic of this phenomenon. Because both serve to explain the development of elaborate traits it is best to view the two as interacting mechanisms that serve to reinforce one another. Many courtship behaviors are also genetically linked. Individuals that possess genes associated with successful courtship displays will pass this genetic information down from generation to generation. Continued development of these characteristics over time may have led to the unique and brilliant courtship displays we see today.