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 behaviors 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 but instead increase the fitness of the offspring. Since the offspring of a female will inherit half of the genetic information from the male counterpart, those traits she saw as attractive will be passed on, producing an offspring that is potentially 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.
In some species, males initiate courtship rituals only after mounting the female. Courtship may even continue after copulation has been completed. In this systems, the ability of the female to choose their mates is limited. This process, known as copulatory courtship, is prevalent in many insect species.
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. 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. This could arise in mating systems where reproducing comes at an energy cost to males. Such energy costs can include the effort associated in obtaining nuptial gifts for the female or performing long courtship or copulatory behaviors. An added cost from these time and energy investment 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.
Multi-Modal Signal Processing
Many species of animals engage in some type of courtship display to attract a mate, such as dancing, the creation of sounds and physical displays. However, many species are not limited to just one of these behaviors. It has been show that the males of a multitude of species ranging many taxa create complex multi-component signals that have an effect on more than one sensory modality, also known as multi-modal signals. There are two leading hypothesis on the adaptive significance of multi-modal signal processing. The multiple message hypothesis states that each signal that a male exhibits will contribute to a possible mate's perception of the male. The redundant signal hypothesis states that the male exhibits multiple signals that portray the same 'message' to the female, with each extra signal acting as a fall-back plan for the male should there be a signaling error. The choosy sex may only evaluates one, or a couple, traits at a given time when interpreting complex signals from the opposite sex. Alternatively, the choosy sex may attempt to process all of the signals at once to facilitate the evaluation of the opposite sex.
The process of multimodal signaling is believed to help facilitate the courtship process in many species. One such species in which multimodal signaling is seen to improve mating success is in the green tree frog (Hyla cinerea). Many anuran amphibians, such as the green tree frog, may use visual cues as well as auditory signals to increase their chances of impressing a mate (6). When the calls of the tree frogs were held equal, it was determined that females tended to overlook an auditory-only stimulus in favor of males who combined auditory/visual multimodal signal. It was seen that female green tree frogs preferred when males coupled the visual display with the auditory communication, concluding that male green tree that are visually accessible can increase their probability of mating success.
Peacock spiders (Maratus voltans), are exceptionally sexually dimorphic in appearance and signaling behavior. During courtship, male peacock spiders compete using both visual displays as well as using vibratory signals for intersexual communication. Because of the intense sexual selection on male peacock spiders, reproductive success of an individual relies heavily on a male spider's ability to combine visual and vibratory displays during courtship. The combination of these displays in courtship offers support both to the redundant signal and multiple messages hypotheses for the evolution of multi-modal signaling in species.
Multi-modal signaling is not limited to just males. Females in certain species have more than one trait or characteristic that is used in a courtship display to attract mates. In dance flies (Rhamphomyia longicauda), females have two ornaments, inflatable abdominal sacs and pinnate tibial scales, that they use as courtship displays in mating swarms. Intermediate variations of such female-specific ornaments are sexually selected for by male dance flies in wild populations. These ornaments may also be a signal of high fecundity in females.
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.
- 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 behaviors with other candidate males.
Agonistic Behavior and Courtship
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. 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. Even though this claw developed as a weapon, it is also closely linked with the crabs' courtship display and is waved in a certain pattern to attract females for mating.
Agonistic behavior is not limited to male-male interactions in courtship displays. In many primate species, males direct agonistic behavior towards females prior to courtship behaviors. Such behavior can include aggressive vocalizations, displays, and physical aggression. In the western gorilla (Gorilla gorilla), dominant males exhibit agonistic behavior towards female gorillas at very high rates, with the majority of the interactions being courtship related. Most documented cases of male gorilla aggression towards females is courtship related, and is used primarily as a strategy to prevent females from migrating to another male.
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
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. 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 2 months.
One such exception is the emperor penguin (Aptenodytes forsteri). The emperor penguin engage in an extended courtship period which can last up to 2 months, the longest of any other arctic seabirds. Their courtship period accounts for 16% of the total time they spend breeding, whereas in their closest relatives, the king penguin, the courtship period takes up just 3% of their breeding cycle.
Effect on Environment
Various environmental factors, such as temperature, photoperiod, resource availability and light availability, have an effect on the timing and effectiveness of courtship displays in certain species of animals.
In guppies (Poecilia reticulata), variation in light environment play huge rolls in their ability to attract mates. Guppy males alter both their 'courtship mode', whether they full courtship display or try to 'engage' in sneak copulations, and distance from females as light intensity changes. Courtship mode also varies with light spectrum and relates to predation risk. On average, male guppies seek out and spend more time in the environment in which their color pattern was the most visible. Males, in the light environment that made them most visible, copulated with the most females.
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. 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.
There are multiple hypotheses as to how courtship displays may have evolved in animals, including the Fisherian runaway model and the good genes hypothesis.
As explained by the Fisherian runaway model, sexually dimorphic males with exaggerated ornamentation may have been sexually selected for in species with female choice. Fitness of these males would increase, resulting in the proliferation males with such ornamentation over time. 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.
The “good genes” hypothesis proposes that female selection of a mate is dependent on whether or not the male has genes that would increase the quality of the offspring of the female. In some cases, exaggerated male ornamentation may be indicative to a choosing female that a male who is able to place such a large investment in a somewhat counterintuitive trait to survival would carry good genes. 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.
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