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Sexual selection is a concept first developed by Darwin that explains the evolution of traits. These traits are believed to help an individual outcompete another individual of the same sex. The individual who wins the competition tends to be paternally successful. The process of sexual selection involves mate choice and the competition of males for a female before and after copulation.^[1]

Female choice occurs when a female chooses a male to mate with. The male that is chosen shows phenotypic traits that are attractive to the female. Male to male competition occurs when males compete for access to females. These pre-copulatory processes are exhibited in a wide range of spider species including Stegodyphus lineatus, Argiope aurantia, Schizocosa floridana, Hygrolycosa rubrofasciata, and Schizocosa stridulans.^{[2][3] [4] [5] [6]}

Size is a factor in the success rates between different males who compete for a female. In several species specific spiders such as the Stegodyphus lineatus, A. aurantia and Argyroneta aquatica species, there is male biased sexual dimorphism. This dimorphism is beneficial for larger males but problematic for smaller males. ^[2]The larger males are able to fight off the smaller males by using their large chelicerae and forelegs to their advantage.^[7]

Stegodyphus lineatus, males are a male biased sexually dimorphic species in which larger males are typically more successful in male to male competition. The larger males are observed to be stronger and more aggressive than the smaller males. This strength exhibited by the larger males is detrimental to the smaller males, because the smaller males are unable to win fights with their rivals. This leads to a decrease in the paternal success for smaller males since they are unable to outcompete other males of their species for access to females. ^[8]

Larger spiders from the species A. aurantia, also show that pre-copulatory sexual selection on body size is taking place. Similar to the Stegodyphus lineatus species, large males of the A. Aurantia are more likely to win competition for females. In this species males engage in combat that can result in loss of a leg. This leg loss was seen to be more prevalent in the smaller individuals rather than the larger individuals. This is evidence that larger males were favored in competitive bouts with smaller males. ^[3]

Sexual selection favoring larger bodied males can be seen in aquatic spiders as well. In the water spider Argyroneta aquatica, males and females are found in the water throughout their lifetime.^[9] The males of this species are typically larger than females indicating that there must be sexual selective pressures for large body size on spiders who are aquatic. A possible explanation for the large size comes from the mobility of spiders in the water. The males of larger water spiders are more mobile; therefore more likely to get to a female in aquatic environments than smaller males.

It is beneficial to be a smaller spider under certain conditions. These conditions contribute to possible explanations for the sexual selection of body size in male spiders. In many spider species such as the Misumena Vatia and the Nephila Clavipes species, smaller males climb faster to their mates than larger spiders.^{[10] [11]} This phenomenon is explained by the gravity hypothesis.^[10] This fast climbing to the female shows that smaller males are outcompeting larger males and thus having more reproductive success.^[11]

Smaller males, such as those from the species Misumena vatia, are faster climbers to females than larger males, when females live in high patches of flowers. ^[11] In conditions where females live in low lying areas, larger males are favored.^[3]

Although it seems that the smallest male will have the greatest advantage in male to male competition, there is actually an optimal body size that favors climbing speed. Smaller males will have an advantage over the largest males of the species, however the smallest male will not be the fastest climber. This is because there is an optimal body size for climbing of 7.4 mm.^[11] This optimal body size for climbing is observed in different families of spiders such as Tetragnathidae, Araneidae, Thomisidae and Pholicidae. ^[10]

Males who compete with other males from the same species express phenotypes that are used to fight off competition. These phenotypes (also known as weapons) are parts of the spiders body that can be used to fight off oncoming rivals. The weapons can be in the form of the Spiders chelicerae, teeth or even legs.^[7] In most cases body size correlated with mating success. ^[2] Body size also tends to correlate with the size of chelicerae, typically larger bodied spiders contained larger chelicerae. The size of the chelicerae affects the likelihood of the spider to win competitive battles with other members of the same sex. Evidence that weapons such as foreleg length and chelicerae are sexually selected is observed in the species Lysommanes viridis. Lysommanes viridis, males display weapons that are very pronounced in comparison with females. It is hypothesized that these traits were selected to help males fight off competition.^[7]

The time it takes a spider to develop is crucial to the overall fitness of a spider. It can be regarded that having a larger body size equates to winning more competitive bouts, such as in the example of the Stegodyphus lineatus species.^[2] This idea is true, however does not mean that larger males will always have better fitness. If there is sexual selection based on development time, smaller males are actually favored. In the species Latrodectus hasselti, larger males outcompete smaller males by getting to the females web first. However, these large male spiders have long development times. This means that the larger male will not be able to copulate until he is fully mature. Smaller males tend to have a quick development time which gives them an advantage in mating with a female. This advantage correlates with high paternal success in the species Latrodectus hasselti. Basically there is a tradeoff on sexual selection taking place. Larger males are able to outcompete smaller males, but not able to mate. Smaller males risk getting outcompeted, but are more likely to have paternal success.^[12]

Choosing a mate is a process of pre-copulatory sexual selection where a female or male chooses a mate whose traits they prefer over others. Mate choice is typically displayed by females, but males can be choosy as well. ^[1] Female choice and male to male competition tend to correlate with each other. This is because males compete with each other to get chosen by the female. Due to this correlation it is seen that traits associated with winning competitive bouts are more likely to be chosen by females. Being that body size plays a role in male to male competition, it is likely that females will choose the male with the more efficient body size. One species of Wolf Spiders, Schizocosa floridana, demonstrates sexual selection based on size and condition. In this species, females assess males based on their ability to cope with a changing environment, by observing the way males adapt to differences in food availabilities at different periods. This change in the amount of food across lifecycles is termed resource heterogeneity.^[4] Males who are able to adapt to the changes in food availability are well conditioned and usually show courtship displays such as tapping on their forelegs and waving. females choose the males who express these courtship displays and are larger in size based on predictions of the males foraging past.^[4]

Courtship displays are commonly expressed in individuals of a species. These displays are signals that are used to attract the opposite sex. Examples of courtship displays are degrees of ornamentations, colors and movements. The males of the species, Hygrolycosa rubrofasciata, display certain signals which influence their mating behavior. This activity is referred to as "drumming." Drumming occurs when a male taps his legs on a rough surface such as a leaf to signal he is ready to mate.^[5] The drumming speed has an influence on the females mate choice. Females are more likely to mate with an individual who is drumming at a faster speed. Once the females chooses the male, her body starts to shake. This shaking by the female is a signal to the male that she is ready to mate. It is believed that females choose males who are faster at drumming regardless of size due to viability. Males who exhibit better drumming behavior typically are more viable.^[5] This obeys the good genes hypothesis because females choose the male who possesses a trait that increases his paternity. By choosing this male, the trait can be passed down to the females offspring. ^{[5] [13]}

The males of the species Schizocosa stridulan, contain ornamentation traits in their forelegs that are chosen by females under various conditions. ^[6] These ornamented traits affect the mating success of males when courtship rates are high or low. When courtship rates are high, ornamentation does not increase the reproductive rates of males because of the correlation between the aggressiveness of a spider and the degrees of ornamentation. Due to this correlation it is hypothesized that females choose males without ornamentation to avoid aggression from the males. Females are able to be choosy when courtship rates are high because they do not have to worry about missing out on copulations if there are plenty of male spiders to mate with. When courtship rates are low, males with high degrees of ornamentation are able to get to the female more quickly, thus giving them an advantage over non ornamented males. ^[6]

Sometimes facial color or leg brightness can play a role in mate choice. In several species of jumping spiders, including Habronattus pyrrithrix, and Cosmophasis umbratica, males show different brightness and color of body parts prior to copulation.^[14] These colors can be used to the males advantage in attracting a mate. In the species Habronattus pyrrithrix, the males who have faces that are red and non bright green legs are more likely to attract a mate than males who do not, indicating that females prefer males with those particular traits.^[14]

Although females from the species, Hygrolycosa rubrofasciata, Schizocosa floridana and Schizocosa stridulans tend to be the choosier sex, it is not uncommon to observe males from different spider species such as the Zygiella x-notata and Latrodectus hesperus, to be choosy as well.^{[4] [5] [6] [15] [16]} In the orb weaving spider Zygiella x-notata, reproduction rates are affected by male choice under different conditions. ^[15] These external conditions depend on the amount of competition between males of the species. When competition rates are low, males mate opportunistically with as many females as possible. ^[15] When competition between males is high, larger males choose to mate with a large female as opposed to the smaller males who choose to mate with any female. The belief is that the advantages of larger size in competition, will give the larger males an opportunity to increase their paternal success by allowing them to be more selective of females. ^[15]

Sometimes males choose females who are large and better conditioned to avoid being eaten. Choosing a mal-nourished female can result in a male being cannibalized before copulation. ^[16] Cannibalism by females is often expressed as a way for females to get nutrition from their mates after copulation. ^[17] This cannibalistic behavior by females makes males more selective with whom to mate with. The males from the species Latrodectus hesperus show high mate preference for better conditioned females. By choosing well nourished females, males are able to increase their mating success while limiting their chance of being consumed. This is because well nourished females are less likely to eat their mates than mal-nourished females. ^[16]

Sexual selection occurs after copulation as well as before copulation. ^[18] Post-copulatory sexual selection involves sperm competition and cryptic female choice. Sperm competition occurs when the sperm of more than one male competes to fertilize the egg of the female. Cryptic female choice involves the expelling of a males sperm during or after copulations. ^[19]

Sperm competition occurs in many species of spiders, such as Unicorn catleyi, Nephila Pilipes and A. auranti.^{[20] [21] [22]} To ensure paternal success males will perform behaviors that limit sperm competition. These behaviors consist of guarding the female or inserting parts of the male genitalia into the females reproductive organs. ^[18] One behavior that limits sperm competition is the formation of mating plugs.^[23] These mating plugs are used to prevent the female from mating with other males. The plugs come from parts of the males seminal fluid, genitalia, and body, and are used to prevent the female from copulating with other males.^[24]This process is observed in the species Unicorn catleyi, for example. ^[20] In this species, males plug a females insemination duct with a portion of their palp that contains the ejaculatory duct called the embolus. The embolus that is found in the female's posterior receptaculum suggests that males are trying to limit sperm competition. ^[20]

In some spider species, such as the Nephila pilipes, multiple males try to mate with only one female. This can be harmful to the female, because it forces her to participate in energy costly matings. In response to this polyandry, the female produces mating plugs of her own to prevent too many males from copulating with her. ^[23]

The mating plugs transferred to females by the males are believed to be a possible cause of monogyny. ^[22] For example, in the spider species A. aurantia, males will sometimes plug a female with both pedipalps to prevent sperm competition. When this occurs, the male loses his ability to mate with more than one female. ^[22]

Cryptic female choice is a post-copulatory process of mate choice. ^[19] This process is observed in numerous spider species such as, Physocyclus globosus and Argiope bruennichi. ^{[19] [25]} For example in the Argiope bruennichi species, males produce energetic courtship displays prior to copulation. Regardless of the displays, females are observed to mate with multiple males. Once copulation is over the offspring of the female is more likely to have the courtship display phenotype than not. The females of this species must be cryptically discarding sperm from the non courtship males while keeping the other males sperm for copulation. ^[25] This allows females to mate with as many males as she wants prior to copulation, while being more choosy of males after copulation. Discarding the sperm of a male who does not perform courtship displays indicates that females feel that males who perform courtship displays have the greatest fitness. ^[25]