Sexual selection in scaled reptiles

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Garter snake Garter snake tooth.jpg
Garter snake

Sexual selection in scaled reptiles studies how sexual selection manifests in snakes and lizards, which constitute the order Squamata of reptiles. Each of the over three thousand snakes use different tactics in acquiring mates. [1] Ritual combat between males for the females they want to mate with includes topping, a behavior exhibited by most viperids in which one male will twist around the vertically elevated fore body of its opponent and forcing it downward. It is common for neck biting to occur while the snakes are entwined. [2]

Contents

Male-male competition

Snakes

Japanese striped snake Elaphe quadrivirgata.JPG
Japanese striped snake
Green Anaconda Eunectes murinus2.jpg
Green Anaconda
Garter snake Coast Garter Snake.jpg
Garter snake

In the species Japanese striped snake (Elaphe quadrivirgata), competition involves males maintaining body contact with their opponent and exerting pressure by pushing, topping, or entwining in order to subdue him. [2]

Male snakes employ a variety of strategies to help them entice the female into mating. The red-sided garter snake (Thamnophis sirtalis) population in Alberta, Canada hibernates for the majority of the year, emerging in early May to copulate and feed. The communal dens have been observed to reach populations of thousands, with females often dispersing from the den rapidly to try to avoid being attacked by a flurry of males. [3] Males must then rely on pheromone-based tracking to locate the females. When multiple males come into contact with a female intra-sexual competition often occurs. Snake species that have largely male-biased sex ratios often have high levels of male-male competition. [4] Males may be under strong selection for the development of a variety of characteristics that aid them in acquiring a mate. [5] The most successful males are the ones that display the most vigor in their efforts. [1] Males who are able to align their body and cloaca, the joint outlet of the reproductive and digestive system, with that of the female for the longest period of time are often the most successful. [6] While attempting cloacal alignment, a male entices a female by pressing his chin along the length of her body and continually attempting to intertwine his own tail with hers. At the same time, the male tries to prevent other males from breeding with the female, using his body to block their access. [1]

When copulation is achieved, a male implements a coercive mating strategy known as caudocephalic waving. The male garter snake strikes his own body against the female in quick repeated intervals. These strikes drive anoxic air from the non-respiratory lung of the female into her respiratory lungs, causing an increase in her stress level. The increase in stress level causes cloacal gaping, allowing for the courting male to more easily insert his reproductive organ into the female. [1]

Snake mating systems are generally believed to be polygynous, where males copulate with multiple females. Many researchers have assumed that multiple male courtships are successful without providing paternity evidence, [7] and mating systems may be more polyandrous, supported by studies of the green anaconda ( Eunectes murinus ). A population containing hundreds of marked anacondas was observed over a period of several years with forty-five mating aggregations studied. When a female anaconda is ready to find a mate, she lies in an area of shallow water or mud until a male approached her. Some mating aggregations recorded up to thirteen individual males attempting to court the female at once. These interactions sometimes lasted for periods of up to a month, showing that the males put a large amount of their energy into mating attempts. Observation after copulation recorded no evidence of a male anaconda mating with multiple females or even searching for another female after mating. [7] High-energy cost of courting is also apparent in the red-sided garter snake, where the most vigorous males would succeed in copulation. [1] The intensity and amount of energy that male snakes put forth into the courting of females provides evidence for a polyandrous mating system among snake species.

Lizards

White's skink White's Skink, Freycinet peninsula, cropped.jpg
White's skink
Common collared lizard Common Collared Lizard.jpg
Common collared lizard
An agamid MC Siedleragame.jpg
An agamid

Males spend most of their energy in fights over a female to establish dominance and impress females by demonstrating a high quality of fitness. In White's skink (Egernia whitii), in southeastern Australia, larger males with strength and size correlating positively, intimidate rivals with their size, ward off other males from a female and claim territory space. [8] The losers of fights have an increased Stress rate and fewer breeding opportunities [9] while fighting among males, which only happens during the breeding season illustrates to a female that he would be able to provide protection for her and her eggs. [9] Some weapons used are spikes on the body and tail, teeth, and claws. [10] An iguana's bite force can result in injury to other males; this leads the weaker male to flee from the fight and abort his chance of trying to mate. Bite-force performance predicts dominance in males and who sires more offspring. [10] Male dominance correlates with a large territory size and access to females. [10] In the common collared lizard Crotaphytus collaris, males display their locomotive skills in order to attract a female by getting to territory and resources first. [11] Faster males have energy to spend when it comes to obtaining food and territory [11] and are protective of their female mate and have a higher reproductive success and mate with more females on a first-come, first-served basis. [11] In Australian agamid lizards, coloration influences competitive success; the more intimidating a male is perceived based on his color, the more likely a weaker male would not want to compete with him for a chance to mate with a female. [12]

Choice in lizards

Males in some lizard species can choose the female they want to mate with. [13] Males prefer more-ornamented females displaying better fitness and fecundity. [13] In striped plateau lizards (Sceloporus virgatus), females during the breeding season develop an orange color on their throat area signaling that they are ready to mate, and it represents a higher quality female (fewer ectoparasites, and larger egg mass). [13]

Females in many lizard species have the choice to mate with or reject males. [14] Females spend energy in investigating a male's traits in order to determine if he is healthy and has good genes. [14] In the species Side-blotched lizard ( Uta stansburiana ), males chosen by females had 76 percent less ectoparasites. [15] Females who are not sickly can spare the energy in investigating their potential mate's qualities. Females prefer males that can afford to spend the most energy in displaying their traits because it is difficult to fake good genes. [9] In the species Anolis pulchellus , females chose males who defended territory 89% of the time. [16]

Male painted dragon lizards, Ctenophorus pictus , are brightly conspicuous in their breeding coloration. However their color declines with aging. [17] Experiments in which antioxidants were administered to these males led to the conclusion that breeding coloration reflects innate anti-oxidation capability that protects against oxidative damage to DNA. [17] This finding indicates that color acts as a “health certificate” allowing females, during mating, to visualize the underlying oxidative stress induced damage in potential male mating partners.

Pheromonal cues

In snakes

Crotalus viridis nuntius Crotalus viridis nuntius.jpg
Crotalus viridis nuntius
Agkistrodon contortrix Copperhead05.jpg
Agkistrodon contortrix

Snakes have two distinctive chemical sensory systems, which have unique roles in the reception and deciphering of various chemical signals: the main olfactory system and the Vomeronasal system, which is used to decipher distinct chemosensory information. [18] In snakes this has a much greater complexity than its olfactory system and is essential for prey tracking as well as mate distinction and courtship. [19]

Snake tongues are bifurcated, or forked, to enhance their ability to pick up pheromones by means of tropotaxis, [19] and so are able to detect different stimuli intensities simultaneously using the two tips, flick them to pick up chemical cues from surrounding objects, which are delivered to the vomeronasal ducts upon reentry of the tongue into the oral cavity, activating the regions of the brain responsible for translation, allowing them to be processed and used for both mate and prey location. [19] In nearly all snake species, the males are the mate-searching sex, [20] and sometimes must travel a great distance to find a female.

Examples. A study done on prairie rattlesnakes, Crotalus viridis , indicates that males who maintain a constant search for females over fixed areas tend to be more successful in copulating. This population is very male-biased, so males had to focus more on locating a mate and were less concerned with male-male competition. [20] In red-sided garter snakes, it has found that the amount of rival males, the length of time the chemical substrates last, and the distance over which the trail is evident are significant factors influencing a male's success rate in finding a female. [21] Thamnophis sirtalis parietalis often prefer the pheromone trails of females who occupied the same communal den as males. [22] [23]

In less male-biased scenarios, a male's tongue length plays an essential role in sexual selection. Dimensions of the copperhead, Agkistrodon contortrix , show a significant tongue size difference, with males having a greater tongue bifurcation length, supporting the hypothesis that sexual selection is acting on tongue size in male snakes in order to enhance their ability to locate a potential female mate. [19]

In lizards

European green lizards Lacerta bilineata masculus et femella + Podarcis muralis (Sarthe).JPG
European green lizards
Dalmatian wall lizard Podarcis melisellensis-Vis island.jpg
Dalmatian wall lizard

Females can determine a male's fitness based on the pheromones he leaves in an environment. [24] These chemical cues inform females of a male's quality, [25] with males trying to impress future mates by displaying elaborate colors, which can signal to a female that he is healthy, disease free, or has good genes, [26] as seen in the Dalmatian wall lizard Podarcis melisellensis, whose identity is based on their morph color: yellow, orange, the dominant colour as such lizards have a larger size and bite force so they can ward off competing males in order to mate with a female of choice and claim territory, and blue. [27] The females prefer the orange males since they are bigger and healthier and can give a female's offspring higher quality benefits. [27] Even though females prefer to mate with orange morphs, females will still mate with yellow morphs, who give females direct benefits like protection and a small territory. Blue males are only able to mate by intruding on another male's territory and mating with other their females. What these lizards portray is the rock-paper-scissors game. [27] In the species Uta stansburiana, throat coloration was also used as a way for males to elaborate their good health and body conditions to females. [28] [29] This is another example of the rock-paper-scissors game. [29] The male European green lizards develop a blue throat patch, which shows high reflectance in the ultraviolet (UV) range, which females prefer since throat brightness correlates with a larger body size, a larger head size and less ectoparasites, [30] though it is costly in energy. Females can then rely on throat color in male European green lizards to be an honest signal. [31]

Sexual conflict in snakes

Sexual dimorphisms are phenotypic differences between males and females of the same species. The objective of many sexual dimorphic studies done on snakes focuses more on broad comparisons between species from different regions and less on individual species themselves. [32] Size dimorphisms are common in snakes; females tend to be larger in populations where the production of large liters is feasible. Males tend to be larger in mating systems in which male-male competition is a large factor. [33] Hydrophiid snakes, otherwise known as sea snakes, have only recently evolved from terrestrial elapids. [34] Sea snake scales differ from that of terrestrial snakes because they are rugose and wrinkled. Male sea snakes scale rugosity is more developed than that of the female snakes. The male turtle headed sea snake, Emydocephalus annulatus , provides an interesting case study because of their unique scales. [35] Females of this species display smooth scales year round. Males, on the other hand, show a shift from smooth scales to ridged scales upon the arrival of the winter breeding season. There are multiple reasons why this trait could have been selected. Scale rugosity in males allows for a better tactile resistance during mating than non rugose scales. Also, it has been shown that rugose scales provide a more efficient oxygen uptake system by means of diffusion. During courting, males often lose mating opportunities because they deplete their oxygen stores and must return to the surface to breathe. Rugose skin provides a solution to this problem; its structure helps to increase the rate of oxygen diffusion, allowing for males to dive for longer periods. [35]

Post-copulatory

In snakes

Snake sperm morphology and function is highly influenced by their ability to find, interact with, and fertilize eggs. Snake species display extended copulations and higher gonad mass/body mass proportions in males than other reptilian taxonomic groups. [36] Furthermore, their mating systems have a wide range of variability, depending on the temporal availability and predictability of the females. These factors influence sperm competition levels in both intense male-male combative species and those species that participate in prolonged mate searching. [36]

Oviparous species show relatively larger testes and sperm midpiece length than viviparous species because oviparous species often reproduce annually as opposed to the bi annual cycle of the viviparous species. Because the oviparous species reproduce less frequently, these traits may have been selected to generate stronger propulsions, develop more mitochondria and increase the amount of sperm per ejaculate to help aid in the sperms success rate. [36]

Often in snake species, females will copulate with multiple males in one mating aggregation. [6] [37] To increase his chances in paternity, a male will sometimes try to inhibit the female from re-mating. A common tactic in many species is to obstruct the reproductive pathway of the female in order to physically prevent additional copulations. The mating plug of T.s. parietalis is a jellylike blob produced by males that is implanted within the females reproductive tract. Retention of the plug can last from two days to two weeks, with maximal effectiveness declining after the two-day mark. [37] The plug functions to prevent the leakage of the male's sperm from the female's cloaca, reduce the attractiveness and receptivity of the female to further copulations, and to physically block the reproductive tract to prevent immediate re-mating. The plug is not always 100% effective, but re-mating is a rare occurrence while the plug is in place. [37]

Cryptic female lizards

Females store sperm after copulation from multiple males before ovulation, but how sperm is stored is still unknown. [38] There is a time gap between sperm copulation and fertilization. This time gap allows the female to mate with multiple males. The most competitive sperm will fertilize the female's eggs while the remaining sperm will be discarded. [39] Females also gain nutrients from sperm storage so the more a female mates with different males the more nutritional access she will obtain. [40] After copulation, some female lizard species chose the sex of their offspring based on the male's sperm. Females produce sons with sperm from larger sires, and daughters from sperm with smaller sires. [41] A reason would be to ensure that her sons will have good genes that can display stronger more elaborate traits in order to maximize his reproductive success. [41] An increase in the production of sons demonstrates a preference for larger males. [41] Daughters are produced from smaller sires since females do not need to use a large amount of energy to attract a male; more males are ready to mate than females. [41] However, if females store sperm from a variety of sires, females can produce both sons and daughters with high fitness. [41]

Inbreeding avoidance

When the female sand lizard Lacerta agilis mates with two or more males, sperm competition within the female's reproductive tract may occur. Active selection of sperm by females appears to occur in a manner that enhances female fitness. [42] On the basis of this selective process, the sperm of males that are more distantly related to the female are preferentially used for fertilization, rather than the sperm of close relatives. [42] This preference may enhance the fitness of progeny by reducing inbreeding depression.

Environmental stimuli in snakes

Seminatrix pygaea Seminatrix pygaea.jpg
Seminatrix pygaea

The semi-aquatic black swamp snake, Seminatrix pygaea , which lives in an environment where periods of drought are very common, has shown that environmental factors have a negative effect on female snakes whose large size was selected to increase fecundity as these droughts create a unique scenario to test whether survivability or reproductive pressures influence female body size more. Females grow to be larger than males because there is no male-male competition. At the same time, selection acts on female size in order to increase fecundity. [43]

After the drought period larger snakes, both male and female, were less likely to survive, with a larger decline in the female population, mainly because female snakes had a greater body size compared to males. This correlates with the majority of Seminatrix pygaea giving birth at the start of the dry season, giving them some time to recover from the energy depletion they suffered from birthing clutches of offspring, [43] showing that selection favors smaller snakes in times of drought and larger snakes during years of high food abundance, and providing evidence that environmental factors affect sexual selection.

Related Research Articles

<span class="mw-page-title-main">Sexual selection</span> Mode of natural selection involving the choosing of and competition for mates

Sexual selection is a mode of natural selection in which members of one biological sex choose mates of the other sex to mate with, and compete with members of the same sex for access to members of the opposite sex. These two forms of selection mean that some individuals have greater reproductive success than others within a population, for example because they are more attractive or prefer more attractive partners to produce offspring. Successful males benefit from frequent mating and monopolizing access to one or more fertile females. Females can maximise the return on the energy they invest in reproduction by selecting and mating with the best males.

<span class="mw-page-title-main">Sexual dimorphism</span> Condition where males and females exhibit different characteristics

Sexual dimorphism is the condition where sexes of the same species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most dioecious species, which consist of most animals and some plants. Differences may include secondary sex characteristics, size, weight, color, markings, or behavioral or cognitive traits. Male-male reproductive competition has evolved a diverse array of sexually dimorphic traits. Aggressive utility traits such as "battle" teeth and blunt heads reinforced as battering rams are used as weapons in aggressive interactions between rivals. Passive displays such as ornamental feathering or song-calling have also evolved mainly through sexual selection. These differences may be subtle or exaggerated and may be subjected to sexual selection and natural selection. The opposite of dimorphism is monomorphism, when both biological sexes are phenotypically indistinguishable from each other.

<span class="mw-page-title-main">Sperm competition</span> Reproductive process

Sperm competition is the competitive process between spermatozoa of two or more different males to fertilize the same egg during sexual reproduction. Competition can occur when females have multiple potential mating partners. Greater choice and variety of mates increases a female's chance to produce more viable offspring. However, multiple mates for a female means each individual male has decreased chances of producing offspring. Sperm competition is an evolutionary pressure on males, and has led to the development of adaptations to increase male's chance of reproductive success. Sperm competition results in a sexual conflict between males and females. Males have evolved several defensive tactics including: mate-guarding, mating plugs, and releasing toxic seminal substances to reduce female re-mating tendencies to cope with sperm competition. Offensive tactics of sperm competition involve direct interference by one male on the reproductive success of another male, for instance by physically removing another male's sperm prior to mating with a female. For an example, see Gryllus bimaculatus.

<span class="mw-page-title-main">Sand lizard</span> Species of lizard

The sand lizard is a lacertid lizard distributed across most of Europe from France and across the continent to Lake Baikal in Russia. It does not occur in European Turkey. Its distribution is often patchy. In the northern extremes of the sand lizard's distribution, such as along the southern English coast of Great Britain, it can only survive by inhabiting seaside heathlands, where the ground temperature is sufficiently elevated from the Sun's rays; the warmth is also critical to the lizard being able to successfully incubate their eggs, which are laid in sand.

<span class="mw-page-title-main">Anisogamy</span> Sexual reproduction involving a large, female gamete and a small, male gamete

Anisogamy is a form of sexual reproduction that involves the union or fusion of two gametes that differ in size and/or form. The smaller gamete is male, a sperm cell, whereas the larger gamete is female, typically an egg cell. Anisogamy is predominant among multicellular organisms. In both plants and animals gamete size difference is the fundamental difference between females and males.

<span class="mw-page-title-main">Hemipenis</span> Male sex organ in squamate reptiles

A hemipenis is one of a pair of intromittent organs of male squamates. Hemipenes are usually held inverted within the body, and are everted for reproduction via erectile tissue, much like that in the human penis. They come in a variety of shapes, depending on species, with ornamentation such as spikes.

Monogamous pairing in animals refers to the natural history of mating systems in which species pair bond to raise offspring. This is associated, usually implicitly, with sexual monogamy.

<span class="mw-page-title-main">Sexual conflict</span> Term in evolutionary biology

Sexual conflict or sexual antagonism occurs when the two sexes have conflicting optimal fitness strategies concerning reproduction, particularly over the mode and frequency of mating, potentially leading to an evolutionary arms race between males and females. 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.

<span class="mw-page-title-main">Sexual cannibalism</span> Practice of animals eating their own mating partners

Sexual cannibalism is when an animal, usually the female, cannibalizes its mate prior to, during, or after copulation. It is a trait observed in many arachnid orders and several insect and crustacean clades. Several hypotheses to explain this seemingly paradoxical behavior have been proposed. The adaptive foraging hypothesis, aggressive spillover hypothesis and mistaken identity hypothesis are among the proposed hypotheses to explain how sexual cannibalism evolved. This behavior is believed to have evolved as a manifestation of sexual conflict, occurring when the reproductive interests of males and females differ. In many species that exhibit sexual cannibalism, the female consumes the male upon detection. Females of cannibalistic species are generally hostile and unwilling to mate; thus many males of these species have developed adaptive behaviors to counteract female aggression.

<span class="mw-page-title-main">Sexy son hypothesis</span> Postulate in biology

The sexy son hypothesis in evolutionary biology and sexual selection, proposed by Patrick J. Weatherhead and Raleigh J. Robertson of Queen's University in Kingston, Ontario in 1979, states that a female's ideal mate choice among potential mates is one whose genes will produce males with the best chance of reproductive success. This implies that other benefits the father can offer the mother or offspring are less relevant than they may appear, including his capacity as a parental caregiver, territory and any nuptial gifts. Fisher's principle means that the sex ratio is always near 1:1 between males and females, yet what matters most are her "sexy sons'" future breeding successes, more likely if they have a promiscuous father, in creating large numbers of offspring carrying copies of her genes. This sexual selection hypothesis has been researched in species such as the European pied flycatcher.

<span class="mw-page-title-main">Mating plug</span> Gelatinous secretion used in the mating of some species

A mating plug, also known as a copulation plug, sperm plug, vaginal plug, or sphragis, is a gelatinous secretion used in the mating of some species. It is deposited by a male into a female genital tract, such as the vagina, and later hardens into a plug or glues the tract together. While females can expel the plugs afterwards, the male's sperm still gets a time advantage in getting to the egg, which is often the deciding factor in fertilization.

Bateman's principle, in evolutionary biology, is that in most species, variability in reproductive success is greater in males than in females. It was first proposed by Angus John Bateman (1919–1996), an English geneticist. Bateman suggested that, since males are capable of producing millions of sperm cells with little effort, while females invest much higher levels of energy in order to nurture a relatively small number of eggs, the female plays a significantly larger role in their offspring's reproductive success. Bateman's paradigm thus views females as the limiting factor of parental investment, over which males will compete in order to copulate successfully.

<span class="mw-page-title-main">Female sperm storage</span>

Female sperm storage is a biological process and often a type of sexual selection in which sperm cells transferred to a female during mating are temporarily retained within a specific part of the reproductive tract before the oocyte, or egg, is fertilized. This process takes place in some species of animals, but not in humans. The site of storage is variable among different animal taxa and ranges from structures that appear to function solely for sperm retention, such as insect spermatheca and bird sperm storage tubules, to more general regions of the reproductive tract enriched with receptors to which sperm associate before fertilization, such as the caudal portion of the cow oviduct containing sperm-associating annexins. Female sperm storage is an integral stage in the reproductive process for many animals with internal fertilization. It has several documented biological functions including:

A nuptial gift is a nutritional gift given by one partner in some animals' sexual reproduction practices.

<span class="mw-page-title-main">Sexual selection in birds</span>

Sexual selection in birds concerns how birds have evolved a variety of mating behaviors, with the peacock tail being perhaps the most famous example of sexual selection and the Fisherian runaway. Commonly occurring sexual dimorphisms such as size and color differences are energetically costly attributes that signal competitive breeding situations. Many types of avian sexual selection have been identified; intersexual selection, also known as female choice; and intrasexual competition, where individuals of the more abundant sex compete with each other for the privilege to mate. Sexually selected traits often evolve to become more pronounced in competitive breeding situations until the trait begins to limit the individual's fitness. Conflicts between an individual fitness and signaling adaptations ensure that sexually selected ornaments such as plumage coloration and courtship behavior are "honest" traits. Signals must be costly to ensure that only good-quality individuals can present these exaggerated sexual ornaments and behaviors.

<span class="mw-page-title-main">Sexual selection in mammals</span> Mode of natural selection

Sexual selection in mammals is a process the study of which started with Charles Darwin's observations concerning sexual selection, including sexual selection in humans, and in other mammals, consisting of male–male competition and mate choice that mold the development of future phenotypes in a population for a given species.

<span class="mw-page-title-main">Sexual selection in amphibians</span> Choice of and competition for mates

Sexual selection in amphibians involves sexual selection processes in amphibians, including frogs, salamanders and newts. Prolonged breeders, the majority of frog species, have breeding seasons at regular intervals where male-male competition occurs with males arriving at the waters edge first in large number and producing a wide range of vocalizations, with variations in depth of calls the speed of calls and other complex behaviours to attract mates. The fittest males will have the deepest croaks and the best territories, with females making their mate choices at least partly based on the males depth of croaking. This has led to sexual dimorphism, with females being larger than males in 90% of species, males in 10% and males fighting for groups of females.

Cryptic female choice is a form of mate choice which occurs both in pre and post copulatory circumstances when females in certain species use physical or chemical mechanisms to control a male's success of fertilizing their ova or ovum; i.e. by selecting whether sperm are successful in fertilizing their eggs or not. It occurs in internally-fertilizing species and involves differential use of sperm by females when sperm are available in the reproductive tract.

<span class="mw-page-title-main">Polyandry in animals</span> Class of mating system in non-human species

In behavioral ecology, polyandry is a class of mating system where one female mates with several males in a breeding season. Polyandry is often compared to the polygyny system based on the cost and benefits incurred by members of each sex. Polygyny is where one male mates with several females in a breeding season . A common example of polyandrous mating can be found in the field cricket of the invertebrate order Orthoptera. Polyandrous behavior is also prominent in many other insect species, including the red flour beetle and the species of spider Stegodyphus lineatus. Polyandry also occurs in some primates such as marmosets, mammal groups, the marsupial genus' Antechinus and bandicoots, around 1% of all bird species, such as jacanas and dunnocks, insects such as honeybees, and fish such as pipefish.

<span class="mw-page-title-main">Red-sided garter snake</span> Subspecies of snake

The red-sided garter snake is a subspecies of the common garter snake, in the subfamily Natricinae of the family Colubridae. This slender subspecies of natricine snake is indigenous to North America and is one of the recognized subspecies of Thamnophis sirtalis. This subspecies is widely spread across northern United States and southern Canada.

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