Introduction
Mating systems (MS) in animals are extremely numerous and very complex, mainly because a range of factors by which they are predisposed, as well as the ones that determine their further evolution, are yet to be explored. Hinging on not only the biological characteristics and development of the species but also the environment in which they live, the availability of the required resources, the presence of various risk factors, etc., MS may differ significantly for various species.
It should also be borne in mind that the reproductive success of a particular species defines its further survival in the target environment. Therefore, the concept of sexual adaptation is not restricted merely to mating with as many females of the species in question as possible but, instead, extends to the ability to develop the adaptive pathways that will allow for the further increase in the fecundity of the specimens of the said species and, therefore, contribute to the rise in the number of offspring.
These patterns imply greater levels of female investment in the process of raising the posterity (Shuker and Simmons 7). Therefore, for the most part, superior estimated breeding values determine the choices made by the representatives of various species before the mating process.
It should be borne in mind, though, that there are certain social dimensions of mating that may determine the further evolution of the MS.
Types of Mating Systems (MS) in Different Types of Organisms
Hermaphroditic Species: MS Description, Examples, and Advantages
Hermaphroditic animals are typically defined as those that possess the reproductive organs of both male and female sexes (Escobar et al. 1234). It should be borne in mind, though, that the identified definition is rather loose and does not fully encompass the variety of ways in which hermaphroditic elements manifest themselves in various animal species. Particularly, different types of hermaphrodites can be identified based on whether male and female reproductive organs are present in the target species simultaneously or whether the identified organism can alter its sex at a certain point in its development. As a rule, simultaneous hermaphroditism is mentioned when talking about the organisms that combine the characteristics of male and female reproductive systems at the same time at a specific point of their development (Escobar et al. 1235).
Selfing
When considering the instances of simultaneous hermaphroditism in animals and the MS options that can be witnessed in the specified setting, one typically mentions selfing as one of the common MS. The concept of selfing, or self-fertilization, is practically self-explanatory; by definition, it implies that the gametes which create a zygote should come from the same organism (Escobar et al. 1235). It should be noted that selfing is not necessarily the only option for hermaphroditic species; however, in certain circumstances, hermaphrodites resort to selfing as one of the means of keeping their population levels at the required high rate.
A recent study shows that the factors determining the rates of selfing in hermaphrodites are, in fact, rather basic. Particularly, the levels of inbreeding depression and the propensity toward selfing among hermaphrodites are typically in inverse proportion, as the study by Escobar et al. (1234) shows quite clearly. The specified phenomenon can be observed among Basommatophoran snails.
Biparental Cross-Fertilization
The study also points to the fact that biparental cross-fertilization may also occur among hermaphrodites as a mode of mating (Escobar et al. 1235). In contrast to selfing, which requires that both gametes should come from a single individual, biparental cross-fertilization implies that two hermaphrodites should assume the roles of a male and a female organism correspondingly, thus, engaging in the mating process and producing a zygote (Escobar et al. 1235). In the process, the simultaneous reciprocity of the population is not required, although it may occur.
When considering the advantages of the identified types of MS, one must mention the fact that both are very helpful in case of inbreeding depression among the target species. In case the inbreeding depression levels increase, the possibility of extinction becomes very high, thus, causing the number of species in question to shrink significantly. Simultaneous hermaphroditism, in turn, serves as the mean of preventing the specified scenarios, therefore, creating the environment in which the species may remain numerous.
Simultaneous Polyandry
Simultaneous polyandry (SP) implies that each fertile female of a particular species engages in relationships with two or more males, typically within the boundaries of a certain area that the female in question marks as its domain. The scenario in question also involves mixed parentage quite often, though not necessarily. While, in some cases, the female stays with the male members of the species for a certain period of time, helping them to defend their territory. They identified behaviors are common for Cephalopods (Squires et al. 1).
Cooperative Simultaneous Polyandry
Cooperative simultaneous polyandry (CSP) is a type of the MS that can also be observed among certain hermaphroditic species and implies that a group of male species with one female member engage in polyandric relationships that extend to rearing the posterity. The phenomenon of CSP is, therefore, defined by the long-term relationships which can be observed among the target species, as well as the social hierarchy that they develop when forming groups (Shuker and Simmons 11).
Sequential Hermaphroditic Species: MS Description, Examples, and Advantages
In contrast to simultaneous hermaphroditic species, sequential ones suggest that the representatives of certain species should change their biological sex at a certain moment (Shuker and Simmons 24). When considering the specimens thereof, one may view fishes as the prime example of sequential hermaphrodites. Sequential hermaphrodites are traditionally classified based on the velocity of sexual maturity (i.e., the sex that matures at a faster pace is viewed as the dominant one and, therefore, defines the further behavior of its owner).
Protogynous species of fish show faster maturity rates in their female sex and then change it to the male one as they grow, whereas, in protoandrous fishes, the opposite phenomenon (i.e., the male to female transfer) can be observed. It should be noted, though, that alternative development pathways are also a possibility; for instance, the Goddiae species have shown the propensity toward bidirectional sequential hermaphroditic behaviors (i.e., the ability to change their sex two or more times). The identified ability allows the species to adapt to the ever-changing environment, react toward the adverse factors, and maintain the levels of fertility high.
Sequential Polyandry
Sequential polyandry is typically defined as the MS in which the posterity is produced with different males in sequence, hence the name of the MS. The effects of sequential polyandry include a significant change in the post-mating sexual selection strategies among females. Particularly, significant changes in the importance of male fitness levels in the post-mating process can be viewed as the direct effect of sequential polyandry in hermaphroditic species (Shuster et al. 3).
Hermaphroditic snails can be viewed as the prime example of the phenomenon described above. Physa acuta shows the propensity toward producing the posterity with the help of both male and female reproductive functions. The consistent reproductive success thereof shows that the identified MS can be deemed as quite efficient in maintaining the population levels of the target species high.
Polygyny
Though the phenomenon of Polygyny is not quite common among hermaphrodites, it can also be observed in some species. Reef fish is typically viewed as one of the most common examples of Polygyny in the animal kingdom. Particularly, the Labroides dimidiatus genus can be mentioned among the best-known polygynic hermaphrodites. Similarly to the phenomenon of simultaneous polygamy, the phenomenon of simultaneous polygyny becomes increasingly more likely in the scenarios where the chances to encounter the representatives of the male sex are very scarce (Kuwamura et al. 2).
Location-Defence Polygyny
The Location- Defence Polygyny (LDP) as a specimen of MS implies that, with a high density of spatial resource distribution rates, the female members of a particular species should control the area and, therefore, attract males. The latter, in turn, engage in activities such as the tasks of building the nesting sites, as well as the process of impregnation of the female species.
The significance of the identified MS cannot possibly be overrated since it allows creating the defense clusters for females, as the name of the MS suggests. As a result, the female members of the species are safeguarded from the adverse environmental factors, whereas male-to-male competition intensifies. Consequently, the prerequisites for producing more plentiful posterity can be created, and the threat of extinction is avoided successfully. LDP can be observed among marine amphipods, e.g., Jassa marmorata (Dennenmoser and Thiel 306).
Sperm-Cast Mating System
Another SM that is used by hermaphrodite species, the sperm-cast mating system (SCMS), can be described as the release of aquatic spermatozoa for the further fertilization of the eggs that have been laid by the said specimen of a particular species. The identified approach to MS may imply either copulation, i.e., may involve the representatives of both sexes, or pseudocopulation, which implies the mimicry of the sexual act but does not typically involve one (Ewers-Saucedo et al. 4).
As a rule, sessile marine invertebrates are mentioned as the direct example of the organisms that have SCMS as the basis for their reproduction process. Similarly, flatworms display the same patterns of SCSM in the course of their reproduction.
When considering the benefits of the identified approach, one must admit that, while it causes rather low maternal investment, it, nevertheless, triggers a significant rise in the sperm competition rates, contributing to the increase in the number of species extensively (Ewers-Saucedo et al. 3). Furthermore, the fact that the identified MS is likely to lead to reciprocity among the hermaphrodites involved in the mating process needs to be mentioned. Creating the prerequisites for a reference for a certain sexual role in the relationships between the species, SCMS contributes to the increase in reproductive success rates. Indeed, with the opportunity for selective mating that the SCMS provides, the members of a certain species will be able to select the preferred mating partners, thus, building the foundation for a more efficient reproductive process (Ewers-Saucedo et al. 7).
Gonochoristic Species: MS Description, Examples, and Advantages
Last but definitely not least, the phenomenon of gonochorism in animals needs to be considered. Gonochoristic species are typically defined as those that produce either male or female gametes but never produce both (Sunobe et al. 11). Therefore, monogamous and monomorphic MS are usually representative of the identified species.
As a rule, the representatives of gonochoristic species are a part of polygamy and polygyny MS. However, group spawning can also be observed quite often among ceratin gonochoristic species, particularly among n Epinephelinae (Serranidae) and Labridae (Sunobe et al. 15).
It should be borne in mind, though, that gonochoristic species are also known to display the tendency toward alternative mating strategies. For instance, the identified phenomenon can be observed among sexually dimorphic organisms belonging to the gonochoristic species. Monomorphic species may also engage in alternative mating strategies; however, their integration into the alternative MS is viewed as rather uncommon and, therefore, is considered untypical.
Unisexual fishes can be considered a typical representative of gonochoristic species (Ross 194). It should also be borne in mind, though, that gonochoristic fish are prone to sex reversal, which occurs under the influence of a change in temperature and other environmental factors (Baroiller and D’Cotta 246).
The increase in male-to-male competition among the representatives of the target species can be deemed as the primary effect of gonochorism in animals. Furthermore, in the species that are predominantly gonochoristic, there is usually a propensity among females to choose the males that are larger in size or have any other advantages that allow for higher opportunities of survival than the rest of the male representatives of the target species have (Baroiller and D’Cotta 243).
While the identified effects of gonochorism in animals may potentially cause side effects, they can generally be considered positive for the representatives of the corresponding species. Moreover, the choice of group spawning allows for a significant increase in the levels of fecundity among females in gonochoristic species (Ross 194).
Ecological Model of Social Organization and Mating Systems
For years, researchers have been trying to identify and explain the principles that make the foundation of MS by which the animal kingdom is represented (Koenig et al. 2). Because of the connections that MS have to the social structure of relationships between the members of a particular group, numerous theories have been developed, yet the Ecological Model of Social Organization and Mating Systems (EMSOMS) is the one that stands out most. The framework is derived from the Socio-Ecological Model (SEM) by Crook and Gartlan (Koenig et al. 3), which suggests that specific environmental factors determine the differences in social systems, thus, making the latter comparatively predictable.
EMSOMS, however, is more intricate in its way of classifying the relationships between the environmental factors and the MS developed by certain species. Particularly, EMSOMS suggests that there is a tangible connection between the spatio-temporal distribution of females, the spatial distribution of resources (mainly, food, water, and nest sites), as well as risks (e.g., the threat of diseases, the presence of predators, and the possibility of infanticide) (Koenig et al. 2).
The transfer to a new interpretation of how MS emerge and develop can be attributed to the shift from studying the characteristics of MS in animals, in general, to the study of the MS among primates. Because of the link between the ecological factors and the MS in primates, the focus on the social organization of the groups and, therefore, the social principles as the foundation for the MS development became a possibility (Koenig et al. 3).
Consequently, the EMSOMS framework was created to explain the connection between the social risks, the risks associated with the scarcity of resources, and various types of MS. Narrowing down the essential principles of EMSOMS will lead to the following generalization that is currently viewed as true for MS in the animal kingdom: the availability of resources (i.e., food, water, nest sites, etc.), the presence of predators (e.g., wild animals, as well as parasites and diseases), and the existence of social risks (particularly, infanticide) defines the grouping patterns among animals, affecting the size, composition, and cohesion thereof. As a result, both the social structure and the MS are defined by the type of the social organization in the species.
The identified theory allows understanding the specifics of MS in different types of organisms, including hermaphroditic species, the animals that fall under the category of sequential hermaphroditic specimens, and the representatives of gonochoristic species.
Conclusion
The concept of MS is fairly complicated due to the intricate nature thereof, including its development and the factors that shape it. While the very idea of MS being shaped by the necessity to produce as many offspring specimens as possible, the unique principles by which the interactions between various species are guided make the existing MS rather intricate. In the species that belong to hermaphrodites, sequential hermaphrodites, and gonochoristic animals, the process of mating typically involved polygamy and polyandry, whereas monogamic relationships remain a rarity. However, the said feature can be deemed as the only similarity between the identified classes of animals.
A closer look at hermaphrodites, sequential hermaphrodites, and gonochoristic animals will show that they tend to selfing, SCMS, and polygamy or polygyny, correspondingly. However, it would be wrong to claim that the adherence of each of the said species to the corresponding MS is completely rigid; quite on the contrary, there seems to be a propensity toward a flexible choice of mating systems, particularly, in hermaphroditic species.
The use of selfing can be deemed as the means of maintaining the population levels high and, therefore, is not a regular occurrence among a range of species. Similarly, gonochoristic animals do not necessarily engage in polygamy and polygyny; instead, group swapping is often observed among the identified animals as one of the key MS for sustaining the fecundity levels high.
Therefore, while seemingly rigid, MS prove to be prone to a certain degree of flexibility, especially when the target species are affected by certain factors posing a threat to their fecundity and possibly affecting the number of their posterity. The further exploration of the said types of organisms has also shown that there is a tangible connection between the presence of a particular sex or sexes and the choice of the mating system.
Particularly, both hermaphroditic and sequential hermaphroditic species have proven to be mostly polygamic and polyandric, whereas the gonochoristic ones showed the tendency to engage in group spawning. A general overview of the choices made by the said types of organisms, in turn, has shown that MS are developed under and shaped by the factors that determine the fertility rates of females and the male-to-male competition.
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