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Sexual conflict or sexual antagonism occurs when the two have conflicting optimal fitness strategies concerning reproduction, particularly over the mode and frequency of mating, potentially leading to an evolutionary arms race between and . In one example, males may benefit from multiple matings, while multiple matings may harm or endanger females due to the anatomical differences of that species. Sexual conflict underlies the evolutionary distinction between male and female.
The development of an evolutionary arms race can also be seen in the chase-away sexual selection model, which places inter-sexual conflicts in the context of secondary sexual characteristic evolution, sensory exploitation, and female resistance. (2025). 9780199206292, Oxford University Press. ISBN 9780199206292 According to chase-away selection, continuous sexual conflict creates an environment in which mating frequency and male secondary sexual trait development are somewhat in step with the female's degree of resistance. It has primarily been studied in , though it can in principle apply to any sexually reproducing organism, such as and fungus. There is some evidence for sexual conflict in plants.
Sexual conflict takes two major forms:
Sexual conflict may lead to antagonistic co-evolution, in which one sex (usually male) evolves a favorable trait that is offset by a countering trait in the other sex. Similarly, interlocus sexual conflict can be the result of what is called a perpetual cycle. The perpetual cycle begins with the traits that favor male reproductive competition, which eventually manifests into male persistence. These favorable traits will cause a reduction in the fitness of females due to their persistence. Following this event, females may develop a counter-adaptation, that is, a favorable trait that reduces the direct costs implemented by males. This is known as female resistance. After this event, females' fitness depression decreases, and the cycle starts again. Interlocus sexual conflict reflects interactions among mates to achieve their optimal fitness strategies and can be explained through evolutionary concepts.
Sensory exploitation by males is one mechanism that involves males attempting to overcome female reluctance. It can result in chase-away selection, which then leads to a co-evolutionary arms race. There are also other mechanisms involved in sexual conflict such as traumatic insemination, forced copulation, penis fencing, love darts and others.
Female resistance traditionally includes reducing negative effects to mechanisms implemented by males, but outside the norm may include sexual cannibalism, Haplodiploidy on offspring and increased aggression to males.
Some regard sexual conflict as a subset of sexual selection (which was traditionally regarded as mutualistic), while others suggest it is a separate phenomenon.T Lodé "la guerre des sexes chez les animaux" Eds O Jacob, Paris, 2006,
Males and females differ in the following general components of fitness, thus leading to sexual conflict. Refer to the accompanying figure in this section.
Sex-biased genes could either be male- or female-biased and sequence analysis of these protein coding genes have revealed their faster rate of evolution which has been attributed to their positive selection vs. reduced selective constraint. Apart from sex specific natural selection and sexual selection that includes both intersexual and intrasexual selection, a third phenomenon also explains the differences in gene expressions between two sexes – sexual antagonism. Sexual antagonism represents an evolutionary conflict at a single or multiple locus that contribute differentially to the male and female fitness. The conflict occurs as the spread of an allele at one locus in either male or female that lowers the fitness of the other sex. This gives rise to different selection pressure on males and females. Since the allele is beneficial for one sex and detrimental to the other, counter adaptations in the form of suppressor alleles at different genetic loci can develop that reduce the effects of deleterious allele, giving rise to differences in gene expression. Selection on such traits in males would select for suppressor alleles in females thus increasing the chances of retaining the deleterious allele in the population in interlocus sexual conflict.
The retention of such antagonistic alleles in a population could also be explained in terms of increase in the net fitness of the maternal line, for example, the locus for male sexual orientation in humans was identified on subtelomeric regions of X chromosomes after studies conducted on 114 families of homosexual men. Same sex orientation was found to be higher in maternal uncles and male cousins of the gay subjects. An evolutionary model explained this finding in terms of increased fertility of the females in maternal lines, hence adding to net fitness gain.
According to Darwin (1859), sexual selection occurs when some individuals are favored over others of the same sex in the context of reproduction. Sexual selection and sexual conflict are related because males usually mate with multiple females while females typically mate with fewer males. It is hypothesized that both chase away selection and sexual conflict may be the result of males' use of sensory exploitation. Males are able to exploit females' sensory biases due to the existence of female choice. For example, females may behave in ways that are considerably biased towards mating and fertilization success due to the attractiveness of males who exhibit a deceptive or exaggerated secondary sex characteristic. Since some male traits are detrimental to females, the female becomes insensitive to these traits. Sexually antagonistic co-evolution entails the cyclic process between the exaggerated (persistent) traits and the resistant traits by the sexes. If male traits that decrease female fitness spread, then female preference will change.
Male water striders exhibit forced copulation on the female. As a result, the female will struggle with the male to reduce the detrimental effects. Female struggle is a by-product of female resistance.
The population of pygmy fish Xiphophorus pygmaeus or pygmy sword-tail fish initially consisted of small males. A study tested female choice using large hetero-specific males. They found that the female pygmy swordtail fish favored larger sized males, indicating that females changed their preference from small males to large males. This pattern of female preference for larger male body size disappeared in populations consisting of smaller males. The study concluded that this behavior is caused by female resistance and not due to a general preference for larger body size males.
Sexual conflict is exhibited when males target other males through sperm competition. For example, Iberian rock lizard ( Lacerta monticola) males create hard mating plugs. These mating plugs are placed within the female cloaca instantly after copulation, which was hypothesized to function as a "chastity belt." However, the study found no evidence to support the hypothesis, as males were able to displace the mating plugs of other males. There is no direct conflict between males and females, but males may evolve manipulative traits to counter the removal of their mating plugs.
Males also develop different behaviors for paternity assurance. A study of sperm competition revealed that there was a positive relationship between testis size and levels of sperm competition within groups. Higher levels of sperm competition were correlated to larger accessory reproductive glands, seminal vesicles, and interior prostates. Larger mating plugs were less likely to be removed.
Also the earthworm Lumbricus terrestris show behavior where both parts try to make sure as much sperm as possible is absorbed by their partner. To do this they use 40 to 44 copulatory setae to pierce into the partner's skin, causing substantial damage. Piercing the partner's skin influences sperm uptake in the earthworm Lumbricus terrestris - The Koene group
There are cases where hermaphrodites can fertilize their own eggs, but this is usually rare. Most hermaphrodites take on the role of a male or female to reproduce. Sexual conflict over mating can cause hermaphrodites to either cooperate or display aggressive behavior in the context of gender choice.
Infanticide has been extensively studied in vertebrates such as , big cats, and mice. However, this behavior also occurs in the . For example, in the spider Stegodyphus lineatus, males invade female nests and toss out their egg sacs. Females only have one clutch in their lifetime, and experience reduced reproductive success if the clutch is lost. This results in vicious battles where injury and even death can occur. Jacana jacana, a tropical wading bird, provides an example of infanticide by the female sex. Females guard a territory while males care for their young. As males are a limited resource, other females will commonly displace or kill their young. Males can then mate again and care for the young of the new female.
This behavior is costly to both sides, and counter-adaptations have evolved in the affected sex ranging from cooperative defense of their young to loss minimization strategies such as aborting existing offspring upon the arrival of a new male (the Bruce effect).
In bed bugs Cimex lectularius, for example, males initiate mating by climbing onto the female and piercing her abdomen. The male will then directly inject his sperm along with the accessory gland fluids into the female's blood. As a result, the female will have a distinct melanized scar in the region the male pierced. It was observed that males not only pierce females but also other males and nymphs. The females may suffer detrimental effects which can include blood leaking, wounds, the risk of infection, and the immune system having difficulty fighting off sperm in the blood.
A study focused on the mating effects of bed bugs of other species such as female Hesperocimex sonorensis and a male Hesperocimex cochimiensis. It was observed that H. sonorensis females died in a period of 24 to 48 hours after mating with H. cochimiensis males. When examining the females, it was evident that their abdomens were blackened and swollen due to an enormous number of immunoreactions. There is a direct relationship between the increase of mating and the decrease in female's lifespan.
Female bed bug mortality rate due to traumatic insemination could be related more to STDs rather than just the open wound. The same environmental microbes that were found on the male's genital were also found within the female. A study found a total of nine microbes, with five microbes actually causing mortality of females during copulation.
African bat bugs Afrocimex constrictus also perform extra-genitalic traumatic insemination. Males will puncture the female outside her genitals and ultimately inseminate them. It was observed that both males and females suffer from traumatic insemination. Males suffer from traumatic insemination because they expressed female like genitals, and were often at times mistaken for females. Females also displayed polymorphism because some females had distinct "female-like" genitals while others had a "male-like" appearance. The results showed that males along with females who had "male-like" genitals suffer less traumatic insemination compared to the distinct females. Female polymorphism could in fact be a result of evolution due to sexual conflict.
Male spiders Harpactea sadistica perform extra-genitalic traumatic insemination with their needle-like intromittent organs that puncture the female's wall, resulting in direct insemination. Males also puncture females with their cheliceral fangs during courtship. Females have atrophied spermathecae (sperm-storage organs). The sperm storage organ removes sperm from males who mate later, which reflects cryptic female choice. Cryptic female choice refers to a female's opportunity to choose with which sperm to fertilize her eggs. It has been suggested that males may have developed this aggressive mate tactic as a result of the female sperm storage organ.
Frequent mating in D. melanogaster is associated with a reduction in female lifespan. This cost of mating in D. melanogaster females is not due to receipt of sperm but is instead mediated by accessory gland proteins (Acps). Acps are found in male seminal fluid. The toxic effects of Acps on females may have evolved as a side effect of the other functions of Acps (e.g. male-male competition or increased egg production). Drosophila males may benefit from transferring toxic semen but it is not likely that their main reproductive benefit is directly from reducing female lifespan.
After Acps are transferred to the female, they cause various changes in her behavior and physiology. Studies have revealed that females who received Acps from males suffered decreased lifespan and fitness. Currently it has been estimated that there are more than 100 different Acps in D. melanogaster. Acp genes have been found in a variety of species and genera. Acps have been described as displaying a conservation function because they reserve protein biochemical classes within the seminal fluid.
Drosophila hibisci use mating plugs rather than traumatic insemination. The mating plugs of Drosophila hibisci are gelatinous, hard composites that adhere to the uterus of the female in the event of copulation. A study tested two hypotheses concerning mating plugs: a) that they were nutritional gifts for females to digest to provide maintenance of the eggs during maturation, or b) that they could serve as a chastity device to prevent sperm of rivals. The study found that mating plugs had no effect on female nutrition and serve as an enforcement device against rival males. Although this species of fruit flies ( Drosophila hibisci) found success in mating plugs, they are ineffective for other Drosophila species. A study found that males who insert their mating plugs within females were unable to prevent females from remating just four hours after mating. Therefore, the assumption can be made that male Drosophila melanogaster develop other male adaptations to compensate for mating plug insufficiency, including intra-genitalic traumatic insemination to directly deposit their sperm.
Males which had multiple copulations with the same female caused greater damage to her genitals. However, those same males transferred a small quantity of ejaculate compared to the virgin males. It was also observed that males that participated in copulation with females sometimes deposit no sperm through the wounds they created on the females.
Females which mated with more than one male suffered higher mortality. Females had a decrease in longevity as a result of receiving a large single ejaculate from males. However, females which received a total of two ejaculates were less likely to die compared to those that received just one ejaculate. The assumption could be made that females that mated 48 hours after the first copulation were lacking nutrition as they do not drink or eat. The ejaculate that was provided after the second copulation was nutritionally beneficial and lengthened female longevity, allowing them to produce more offspring.
Females which mated with virgin males were less likely to suffer genital damage compared to those which mated with sexually experienced males. It was suggested that factors contributing to male virgins being less harmful were ejaculate size and the amount of sperm contained.
During forced copulation, male water striders (genus Gerris) attack females. As a result, a struggle occurs because the female is resistant. When the male water strider is successfully attached to the female, the female carries the male during and after copulation. This can be energetically costly to the female because she has to support the heavy weight of the male at the same time as she is gliding on the water surface. The speed of the female is usually reduced by 20% when the male is attached. The purpose of long copulation is for the male to achieve paternity assurance in order to restrict the female from other males. Long periods of copulation can strongly affect females because females will depart from the water surface after mating and discontinue foraging. The duration of copulation can be extremely long. For water strider Aquarius najas it was a total of 3 months. For water strider Gerris lateralis the time ranged from 4 to 7 minutes.
In water strider Gerris odontogaster, males have an abdominal clasping mechanism that grasps females in highly complex struggles before mating. Males that have clasps that are longer than those of other males were able to endure more somersaults by resistant females and achieved mating success. Males' genital structures had a particular shape to aid in female resistance.
Water striders G. gracilicornis have a behavioral mechanism and grasping structures allowing grasping. Male water striders use what is called an "intimidating courtship". This mechanism involves males using a signal vibration to attract predators in order to manipulate females to mate. Females face more risks of being captured by predators since they idle on the water's surface for long periods of time. If a male were attached to the female, it would be less likely for the male to be harmed by the predators because he would be resting on top of the female. Therefore, males will tap their legs in order to create ripples in the water to attract predators. The female become fearful, causing her to be less resistant towards the male. As a result, copulation occurs faster, during which the male stops signaling.
Male water striders Gerris odontogaster have grasping structures that can prolong copulation depending on the size of their abdominal processes. Males who had longer abdominal processes were able to restrain females longer than males who had shorter abdominal processes.
In diving beetles Dytiscidae, males approach females in the water with a grasping mechanism before copulation. When this occurs, females repeatedly resist. Males evolved an anatomical advantage towards grasping. Males have a particular structure located on their tarsae that enhances grasping of female anatomical structures, pronotum and elytra, which are located on her dorsal surface.
Sepsis cynipsea is another example of sexual conflict via grasping. Males cannot force copulation; however, while females lay eggs fertilized from a previous mating, a new male mounts the female and guards her from other males. Although the females are larger than the males, the males are still able to grasp onto a female. Females are also known to attempt to shake off the male from her back. If she does not shake him off successfully, they mate.
A possible explanation for sexual cannibalism occurring across taxa is "paternal investment". This means that females kill and consume males, sometimes after sperm exchange, in order to enhance the quality and number of her offspring. Male consumption by females serves as a blood meal since they volunteer their soma. The idea of "paternal investment" supports the concept of female choice because female spiders consume males in order to receive an increase in quality of offspring. Males may tap into female sensory biases that may influence female mate selection. Male gift-giving spiders are known to provide gifts to females in order to avoid being eaten. This is a tactic that males may use in order to manipulate females to not kill them. Females may have a strong, uncontrollable appetite, which males may use to their advantage by manipulating females through edible gifts. (2025). 9783319478296, Springer International Publishing. ISBN 9783319478296
The cricket species, Gryllus bimaculatus, is a polygamous species. Multiple matings increases the hatching success of clutch of eggs which is hypothesized to be a result of increased chances of finding compatible sperm. Therefore, it is in the female's best interest to mate with multiple males to increase the offspring genetic fitness; however, males would prefer to sire more of the females' offspring and will try to prevent the female from having multiple matings by mate guarding to exclude rival males.
Similarly, the polyandrous species of spider Pisaura mirabilis has been demonstrated to have cryptic female choice. The presence of a nuptial gift by a male increases the proportion of sperm retained by the female (With copulation duration controlled for).
In the species Nicrophorus defodiens, the burying beetle, there is biparental care; however, males of the species will resume releasing pheromones after mating with the primary female in order to attract more females to increase his reproductive output. However, it is in the female's best interest if she can monopolize the male's parental care and food providence for her offspring. Therefore, the female will bite and attempt to push the male off his signaling perch and interfere with the male's secondary mating attempts in order to impose monogamy on the male.
In Remiz pendulinus, the penduline tit, the male will build an elaborate nest and may or may not be joined by a female at any stage of construction. After eggs are laid, it is strictly uniparental incubation and offspring care; however, either parent may take the role of caregiver. Females will give care 50-70% of initiated breedings while males will give care 5-20% of the time, and approximately 30%-35% of the time, the eggs, which consist of four to five viable eggs, will be left to die, which is a considerable cost to both parents. However, being deserted also represents a large cost for the deserted parent. Therefore, timing of desertion becomes very important. Optimal timing for the males depends on the status of the clutch, and as a result the male frequently enters and remains near the nest during the egg-laying period. For females it is important not to desert too early so that the male does not also desert the eggs, but also not too late else the male deserts before the female does. Females adapt by being very aggressive towards males that try to approach the nest as well as hiding one or more eggs so that males do not have full information on the clutch status.
Breeding success of Eurasian penduline tits suggests conflicting interests between males and females in a wild population: by deserting the clutch each parent increases her (or his) reproductive success although desertion reduces the reproductive success of their mate. This tug-of-war between males and females over care provisioning has been suggested to drive flexible parenting strategies in this species. In the closely related Cape penduline tit Anthoscopus minutus, however, both parents incubate the eggs and rear the young. A contributing factor to parenting decision is extra-pair paternity since in Cape penduline tit less than 8% of young were extra-pair whereas in Eurasian penduline tit over 24% young resulted from extra-pair paternity.
In other species such as the Guianan cock-of-the-rock, as well as other lekking species, sexual conflict may not even manifest itself in parental care. The females of these species have the tendency to select males to mate with, become fertilized, and the females raise the offspring on their own in their nests. (2025). 9780761472704, Marshall Cavendish. ISBN 9780761472704 (1994). 9780198572183, University of Texas Press. ISBN 9780198572183
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