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Reproductive interference is the interaction between individuals of different species during mate acquisition that leads to a reduction of fitness in one or more of the individuals involved. These interactions occur when individuals make mistakes or are unable to recognise their own species, labelled as ‘incomplete species recognition'. Reproductive interference has been found within a variety of taxa, including insects, mammals, birds, amphibians, marine organisms, and plants.

There are seven causes of reproductive interference, namely signal jamming, heterospecific rivalry, misdirected courtship, heterospecific mating attempts, erroneous female choice, heterospecific mating, and hybridisation. All types have fitness costs, generally from a reduction in reproductive success, a waste of gametes, and the expenditure of energy and nutrients. These costs are variable and dependent on numerous factors, such as the type of reproductive interference, the sex of the parent, and the species involved.

Reproductive interference occurs between species that occupy the same habitat, and can play a role in influencing the coexistence of these species. It differs from competition as reproductive interference does not occur due to a shared resource. Reproductive interference can have ecological consequences, such as through the segregation of species both spatially and temporally. It can also have evolutionary consequences, for example; it can impose a selective pressure on the effected species to evolve traits that better distinguish themselves from similar heterospecifics.

Causes of reproductive interference
Reproductive interference can occur at different stages of mating, from locating a potential mate, to the fertilisation of an individual of a different species. There are seven causes of reproductive interference that each have their own consequences on the fitness of one or both of the involved individuals.

Signal jamming
Signal jamming, refers to the interference of one signal by another. Jamming can occur from signals emitted from environmental sources (e.g. noise pollution), or from other species. In the context of reproductive interference, signal jamming only refers to the disruption of the transmission or retrieval of signals by another species. The process of mate attraction and acquisition involves signals to aid in locating and recognising potential mates. Signals can also be an indicator of mate quality. Signal jamming can occur to different communication types, such as auditory signal jamming that can stem from a noisy environment created by heterospecific signals. Jamming from similar heterospecific signals, labelled as auditory masking, causes difficulties in differentiating conspecifics from heterospecifics. Likewise in chemical signals, pheromones that are meant to attract conspecifics and drive off others may overlap with heterospecific pheromones, leading to confusion. The confusion and reduced ability in recognising and locating conspecifics can result in a reduction of encounters with potential mates and a decrease in mating frequencies.

Examples
Vibrational signalling in the American grapevine leafhopper (Scaphoideus titanus) - Like many insects, the American grapevine leafhopper has receptors that detect vibrational signals from conspecifics. The vibrations can be used to identify and locate conspecific females for mating. To successfully communicate, a duet is performed between a male and female American grapevine leafhopper. However, vibrational signals are prone to disruption by heterospecific signals, and other sources of background noise that are within their species-specific sensitivity range. The presence of other signals can mask signals produced by the females or confuse the male if the other signals closely follow the female's signals. Heterospecific signals that add to background noise can lead to a reduction of male and female encounters by reducing the success of identifying and locating female American grapevine leafhoppers. Auditory signalling in the gray treefrog (Hyla versicolor) - During mating season, male gray treefrogs aggregate and produce advertisement calls to attract a female. Overlapping calls from male gray treefrogs with male Cope's gray treefrogs (Dryophytes chrysoscelis) affected the time it took females to choose a mate. Higher overlap (i.e. higher interference) led to higher amounts of time females took to choose. In addition, at high levels of interference, a number of females made errors and chose the heterospecific call. Consequently, auditory masking may reduce the success in recognising conspecifics from listening to advertisement calls, which reduces their success in locating conspecific males.

Chemical signalling in ticks - Female ticks produce a species-specific signal in the form of a pheromone that attracts conspecific males on a host. Female ticks also produce a signal that is not species-specific, which attracts male ticks that are in a close proximity to her. Pheromone emitted from closely related species may mix and lead to interference. A study on three species of reptile tick; Aponomma hydrosauri, Amblyomma albolimbatum, and Amblyomma limbatum, found that males had trouble locating a conspecific female when attached to a host that had two species of females ticks. The presence of a heterospecific female also reduced the time a male spent with conspecific females, resulting in decreased reproductive success. Furthermore, when Amblyomma albolimbatum males attached to Aponomma hydrosauri females to mate, despite being unsuccessful, they remain attached which physically inhibits following males from mating.

Heterospecific rivalry
Heterospecific rivalry occurs only between males, when a male of a different species is mistaken for a conspecific rival for mates. In particular, heterospecific rivalry is hard to differentiate from other interspecific interactions, such as competition over food and other resources. Costs associated to the males include the wastage of time and energy that is allocated to a heterospecific male, increased predation risk, and also a risk of injury if they leave their mating territory. When the males pursue a heterospecific male, females may become exposed to following intruders, whether it be a conspecific or heterospecific male.

Examples
Eastern amberwing dragonfly (Perithemis tenera) - The males defend their mating territories against conspecifics. When approached by species of horsefly and butterfly that share similar characteristics, such as body size and colour, they were chased off due to being mistaken for a conspecific. The horsefly and butterfly do not compete with the Eastern amberwing dragonfly over resources and are neither their predator or prey. The males however, did not chase off other species of dragonfly. Chequered skipper (Carterocephalus palaemon) - When males and females of the chequered skipper are not mating, they occupy different patches which are separate from regions that are high in nectar. Although they occupy different patches, male chequered skippers are territorial and attempt to defend their females against other male rivals by perching in areas close to them. To defend females, males will leave their favoured perch to intercept the flight of the individual that threatens them. Regions that are high in nectar attract other species, such as bees, hoverflies, and other species of butterfly. In regions of high or low in nectar, 69% of the interceptions the chequered skipper male makes were 'false-alarms'. Interceptions are energetically costly to males as they can fly up to 30m away from their initial perch location. In addition, after a male takes flight to intercept a potential rival, they subsequently perform more frequent flights. When these males fly away from their perch site, they may lose opportunities to mate with a female.

Misdirected courtship
Misdirected courtship occurs when males display courtship towards individuals of a different species of either sex. The misdirection is caused by a mistake during species recognition, or by an attraction towards heterospecifics that possess desirable traits. Such desirable traits are those traits that normally are an indicator of conspecific mate quality, such as body size. Misdirected courtship may be less frequent in females who have a larger reproductive investment, as they are likelier to be more 'choosy' and able to better identify conspecifics. Costs associated with misdirecting courtship for males is the wasted energy investment in the attempt to court heterospecifics, and a decrease in mating frequency within species.

Examples
Blue waxbill - A study compared the mating preferences in three species; the blue breast (Uraeginthus angolensis), red cheek (Uraeginthus bengalus), and blue cap (Uraeginthus cyanocephalus). In males, the preference for conspecific females was affected by the size of a heterospecific female, potentially due to body size being an indicator of fecundity. When the heterospecific female was larger, the male's preference for conspecific females lowered. Similarly in females, in all three species females prefer males with ornamentation, which all excluding the male blue breast have. Atlantic salmon (Salmo salar) - Atlantic salmon that were once native to Lake Ontario were reintroduced to study their spawning interactions with other species of fish, including the chinook salmon, coho salmon, brown trout. Chinook salmon interacted with Atlantic salmon the most, where male chinooks attempted to court female Atlantic salmon. Male chinooks also chased away, and in some interactions behaved aggressively towards other Atlantic salmon that approached female Atlantic salmon. A male brown trout was also observed to court a female Atlantic salmon once. Misdirected courtship towards the Atlantic salmon can cause problems in waters that the Atlantic salmon currently occupy, and towards conservation efforts to reintroduce the Atlantic salmon to Lake Ontario. Implications of misdirected courtship on the Atlantic salmon may include: the delay or prevention of spawning, and the hybridisation of the Atlantic salmon with other species.

Heterospecific mating attempts
Heterospecific mating attempts occur when males attempt to mate with females of a different species, regardless of whether courtship occurs. During each mating attempt, sperm transfer may or may not occur. Unlike misdirected courtship where only the male is impacted, both sexes can have costs when a heterospecific attempts to mate. Costs associated with heterospecific mating attempts include wasted energy, time, and potentially gametes if sperm transfer occurs. There is also a risk of injury and increased risk of predation for both sexes.

Examples
Cepero's grasshopper (Tetrix ceperoi) - When housed within the same enclosure as high numbers of slender groundhopper, the reproductive success of the Cepero's grasshopper decreased. The reduction of reproductive success stems from the increase in mating attempts towards the slender groundhopper, potentially due to their larger body size. However, these mating attempts are generally unsuccessful as the mate recognition of female slender groundhoppers are reliable.

Italian agile frog (Rana latastei) - The distribution of Italian agile frog and the agile frog (Rana dalmatina) overlap naturally in ponds and drainage ditches. The abundance of the agile frog is higher than Italian agile frog in areas where they overlap. A study found that when the abundance of Italian agile frogs are low compared to the agile frog, the mating within Italian agile frogs are interfered with. Male agile frogs will attempt to displace male Italian agile frogs during amplexus, which is a type of mating position. Amplexus between both conspecifics and heterospecifics were seen when the agile frog was present. The mating attempts by the Italian agile frog can reduce the reproductive success of the agile frog. The number of viable eggs produced by the Italian agile frog also decreased, potentially due to sperm competition between the male Italian agile frog and agile frog.

Erroneous female choice
Erroneous female choice refers to mistakes made by females when identifying males of the same species, and mistakes made when rejecting males of a different species. Female choice may occur at different stages of mating, including male courtship, copulation, or after copulation. Female choice can depend on the availability of appropriate males. More mistakes in female choice may occur if females become less choosy when there are less available conspecific males.

Examples
Bark beetles - Amongst the three bark beetles: Ips confusus, Ips paraconfusus, PinyoIps hoppingi, the females are attracted to the pheromones of males of both conspecifics and heterospecifics. The product of the pairings between the three species of bark beetles does not survive past the larval stage.

Striped ground cricket (Allonemobius fasciatus) and the Southern ground cricket (Allonemobius socius) - The striped ground and southern ground crickets use calling songs to locate potential mates. The songs are different in frequency and period. Naturally the two species of cricket co-exist in some areas and are isolated in others. Females of either species did not show preferences towards the calls from conspecific males, and appeared to be unable to differentiate between the calls. The lack of ability to differentiate the calls is proposed to be due to the weak selective pressure on females of both species. Erroneous female choice can cause costs, including energy wastage and increases in predation risk when searching for a conspecific. However, these costs may be small since both species of cricket are abundant.

Heterospecific mating
Heterospecific mating is when two individuals from different species copulate. After the male transfers his sperm into the heterospecific female, different processes can occur that may change the outcome of the copulation. The interaction results in the production of a hybrid in some pairings. Costs associated to heterospecific mating include the wastage of time, energy and gametes.

Examples
Spider mites - two different species of spider mites within the genus Panonychus, Panonychus citri and Panonychus mori, mate with each other when co-existing. Heterospecific mating in this case, can produce fertilised eggs; however, all die by the larval stage. Notably, after a female mates with a heterospecific male, they will only be able to produce male offspring when mating with a male of the same species. In addition to the other costs, including the wastage of gametes, time, and energy, heterospecific mating can affect the species' distribution and cause local extinctions. Black-legged meadow katydid (Orchelimum nigripes) and the handsome meadow katydid (Orchelimum pulchellum) - The two species of katydid co-exist along the Potomac River and are closely related. Heterospecific mating between the black-legged meadow katydid and the handsome meadow katydid leads to large costs for females. Females either produces no eggs, or very few eggs after mating with a heterospecific male. In addition, eggs thatmayvelop into male hybrids may be sterile.

Hybridisation
Main article: Hybridisation

Hybridisation, in the context of reproductive interference, is defined as the mating between individuals of different species that can lead to a hybrid, an inviable egg, or an inviable offspring. The frequency of hybridisation increases if it is hard to recognise potential mates, especially when heterospecifics share similarities, such as body size, colouration, and acoustic signals. Costs associated with hybridisation are dependent on the level of parental investment and on the product of the pairing (hybrid). If a hybrid is produced, they have the potential to become invasive, as some can be more successful than their parent species in surviving within new and changing habitats. Compared to each individual parent species, they hold a different combination of characteristics that can be more adaptable and 'fit' within particular environments. If an inviable product is produced, both parents suffer from the cost of unsuccessfully passing on their genes.

Examples
California Tiger Salamanders (Ambystoma californiense) x Barred Tiger Salamanders (Ambystoma mavortium) - California tiger salamanders are native to California, and were geographically isolated from Barred tiger salamanders. Barred tiger salamanders were then introduced by humans, and the hybridisation between these two species led to the formation of a population of hybrids. A study on the survivability of the hybrid at the early-larval stage was conducted, to test whether individual fitness is dependent on ancestry. The survivability of individuals with a mixed-ancestry is higher than individuals with a highly native or highly introduced background. Populations that started with a highly native ancestry became mixed with more introduced genes, and vice versa. The hybrids have established in their parent habitat and spread into human modified environments. Hybrids pose both ecological and conservation consequences as they threaten the population viability of the native California tiger salamanders, which is currently listed as an endangered species. The hybrids may also affect the viability of other native organisms within the invaded regions, as they consume large quantities of aquatic invertebrates and tadpoles. Red deer (Cervus elaphus) x sika deer (Cervus nippon) - The sika deer were introduced to Britain, and spread through further deliberate introductions and escape. Red deer are native to Britain and mate with the sika deer. Heterospecific mating between the pair can prodase viable hybrids, which has led to the formation of a population of hybrids. The sika deer and the hybrid, are competitors of the red deer in regions of dense woodland. As the complete eradication of sika and the hybrids is impractical, management efforts are directed at minimising spread by not planting vegetation that would facilitate their spread into regions where the red deer still persist.