Monogamy in animals

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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^{[citation needed]}.

L'appariement monogame chez les animaux fait référence à l'histoire naturelle des systèmes d'accouplement dans lesquels les espèces se lient pour élever des descendants. Ceci est associé, généralement implicitement, à la monogamie sexuelle

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Monogamous Mating

Monogamy is defined as a pair bond between a male and female of the same species. This pair may cohabitate in an area or territory for some duration of time, and in some cases may copulate and reproduce with only each other. Monogamy can be partitioned into two categories, social monogamy and genetic monogamy which may occur together in some combination, or completely independently of one another.^[1] As an example the cichlid species Variabilichromis moorii the monogamous pair will care for eggs and young together, but the eggs may not all be fertilized by the male giving the care.^[2] Monogamy in mammals is rather rare, only occurring in less than 3% of these animal species . A larger percentage of avian species are known to have monogamous relationships, but this may be biased by the vast amount of research conducted on these species compared to other animal species.^[1]^[3]

^{Accouplement monogame

La monogamie est définie comme une liaison de paire entre un mâle et une femelle de la même espèce. Cette paire peut cohabiter dans une zone ou un territoire pendant une certaine durée et, dans certains cas, peut copuler et se reproduire les uns avec les autres.}
^{La monogamie peut être divisée en deux catégories}
^{- la monogamie sociale}
^{- et la monogamie génétique}
^{qui peuvent se produire ensemble dans une certaine combinaison, ou complètement indépendamment l'une de l'autre. [1]}
^{Par exemple, l'espèce de cichlidés Variabilichromis moorii, la paire monogame, prendra soin des œufs et des jeunes ensemble, mais les œufs peuvent ne pas tous être fécondés par le mâle qui donne les soins. [2]}
^{La monogamie chez les mammifères est plutôt rare, ne se produisant que dans moins de 3% de ces espèces animales.}
^{Un plus grand pourcentage d'espèces aviaires sont connues pour avoir des relations monogames, mais cela peut être biaisé par la grande quantité de recherches menées sur ces espèces par rapport à d'autres espèces animales. [1] [3]}

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Variabilichromis moorii

Breeding and mating

Reproduction et accouplement

V. moorii is a substrate-breeder, with a male and female pair providing biparental care to their eggs and the young after they hatch.^[5] This species exhibits serial monogamy, where a male and female are monogamous for at least one spawning cycle.^[9] A female lays 200–500 eggs per brood, usually under an overhanging rock or within a rock crevice.^[5]^[10] Eggs take at least 3 days to hatch and newly hatched larvae are approximately 3.62 millimetres (0.143 in) (TL). They remain in the larval stage for 4 to 5 days, attached to the substrate by their head glands. They then develop into free-swimming fry and swim in a tight school close to the spawning site. Parents may defend the brood for up to 100 days, which is when fry reach about 3.3 centimetres (1.3 in) (TL).^[11]

^{V. moorii est un reproducteur de substrat, avec une paire mâle et femelle fournissant des soins biparentaux à leurs œufs et aux jeunes après l'éclosion. [5]}
^{Cette espèce présente une monogamie en série, où un mâle et une femelle sont monogames pendant au moins un cycle de frai. [9]}
^{Une femelle dépose 200 à 500 œufs par couvée, généralement sous une roche en surplomb ou dans une crevasse rocheuse [5] [10].}
^{Les œufs mettent au moins 3 jours à éclore et les larves nouvellement écloses mesurent environ 3,62 millimètres (0,143 po) (TL).}
^{Ils restent au stade larvaire pendant 4 à 5 jours, attachés au substrat par leurs glandes céphaliques. Ils se développent ensuite en alevins en nage libre et nagent dans un banc serré près du site de frai. Les parents peuvent défendre la couvée jusqu'à 100 jours, c'est-à-dire lorsque les alevins atteignent environ 3,3 centimètres (1,3 po) (TL). [11]}

Lunar synchronicity in spawning
Synchronicité lunaire au frai

Spawning cycles in V. moorii correspond to the lunar cycle, with egg-laying taking place just before the full moon. This lunar-related reproductive periodicity is rare in freshwater organisms and far more common in organisms in marine environments. This periodicity probably evolved to decrease risk of predation when the fry are in their most vulnerable stages (as eggs and larvae), since the moon is at its brightest and the parents can best defend against nocturnal fry-eating fish. Such nocturnal predators also tend to avoid or move away from light and thus hunt less during the full moon. This lunar synchronicity confers greater benefit to V. moorii compared to other freshwater fish species because of V. moorii’s location in the tropical zone where the moonlight is more intense (since it is more perpendicular to the surface) and V. moorii tend to live in shallow waters that are easily penetrated by moonlight.^[11]

^{Les cycles de frai chez V. moorii correspondent au cycle lunaire, la ponte ayant lieu juste avant la pleine lune.}
^{Cette périodicité de reproduction liée à la lune est rare dans les organismes d'eau douce et beaucoup plus courante dans les organismes en milieu marin.}
^{Cette périodicité a probablement évolué pour diminuer le risque de prédation lorsque les alevins sont dans leurs stades les plus vulnérables (comme les œufs et les larves), car la lune est à son plus brillant et les parents peuvent mieux se défendre contre les poissons mangeurs d'alevins nocturnes. Ces prédateurs nocturnes ont également tendance à éviter ou à s'éloigner de la lumière et donc à chasser moins pendant la pleine lune.}
^{Cette synchronicité lunaire confère un plus grand avantage à V. moorii par rapport aux autres espèces de poissons d'eau douce en raison de l'emplacement de V. moorii dans la zone tropicale où le clair de lune est plus intense (car il est plus perpendiculaire à la surface) et V. moorii a tendance à vivre dans eaux peu profondes qui sont facilement pénétrées par le clair de lune. [11]}

Parasitic spawning and multiple paternity
Frai parasite et paternité multiple

V. moorii shows social but not genetic monogamy. Breeding adults spawn according to the lunar cycle and thus spawn synchronously (see Lunar synchronicity in spawning). This allows males to spawn on eggs that do not belong to the female he is mated to (also known as parasitic spawning).^[12] All offspring within a single brood share the same mother but not the same father. A study found that broods are almost always sired by multiple males (at least two but sometimes more than ten), a rate which is exceptionally high compared to other multiply mating fish. The male mated to the mother of the brood will invest significant resources to protect the territory and brood even though he did not sire the majority of the fry. The female is related to the entire brood and the male is related only to a fraction of the brood, so the female would be expected to provide more care than the male. However, there is no great difference in male parental care and female parental care.^[12]

V. moorii montre une monogamie sociale mais pas génétique.
Les adultes reproducteurs fraient selon le cycle lunaire et fraient donc de façon synchrone (voir Synchronicité lunaire en période de frai).
Cela permet aux mâles de pondre sur des œufs qui n'appartiennent pas à la femelle à laquelle il est accouplé (également connu sous le nom de frai parasite). [12]
Tous les descendants d'une même couvée partagent la même mère mais pas le même père.
Une étude a révélé que les couvées sont presque toujours engendrées par plusieurs mâles (au moins deux mais parfois plus de dix), un taux qui est exceptionnellement élevé par rapport à d'autres poissons reproducteurs multipliés.
Le mâle accouplé à la mère de la couvée investira des ressources importantes pour protéger le territoire et la couvée même s'il n'a pas engendré la majorité des alevins. La femelle est liée à l'ensemble de la couvée et le mâle n'est lié qu'à une fraction de la couvée, de sorte que la femelle devrait fournir plus de soins que le mâle.
Cependant, il n'y a pas de grande différence entre les soins parentaux masculins et les soins parentaux féminins. [12]

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Types of Monogamy

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Social Monogamy

Social Monogamy refers to the cohabitation of males and females. The two individuals cooperate in search of resources such as food and shelter.^[1] With social monogamy there may not be an expected sexual fidelity between the males and the females.^[1]^[3] The existence of purely social monogamy is a polygamous social pair with extra pair coupling.^[4] Social monogamy has been shown to increase fitness in prairie voles. It has been shown that female prairie voles live longer when paired with males in a social monogamous relationship. This could be because of the shared energy expenditure by the males and females lower each individuals input.^[1] In largemouth bass females are sometimes seen to exhibit cuckold behavior by laying some of their eggs in another females nest, thus “stealing” fertilizations from other females.^[5] Sexual conflicts that have been proposed to arise from social monogamy include infidelity and parental investment. The proposed conflict is derived from the conflict- centric differential allocation hypothesis which states that there is a tradeoff between investment and attractiveness.^[3]

Monogamie sociale

La monogamie sociale fait référence à la cohabitation des hommes et des femmes.

Les deux individus coopèrent à la recherche de ressources telles que nourriture et abri. [1]

Avec la monogamie sociale, il ne peut y avoir de fidélité sexuelle attendue entre les hommes et les femmes. [1] [3]

L'existence d'une monogamie purement sociale est un couple social polygame avec un couplage supplémentaire. [4]

Il a été démontré que la monogamie sociale augmente la condition physique des campagnols des Prairies. Il a été démontré que les campagnols femelles des Prairies vivent plus longtemps lorsqu'ils sont associés à des mâles dans une relation sociale monogame.

Cela pourrait être dû au fait que les dépenses énergétiques partagées entre les hommes et les femmes réduisent l'apport de chaque individu. [1]

Dans l'achigan à grande bouche, les femelles présentent parfois un comportement cocu en pondant certains de leurs œufs dans un autre nid de femelles, «volant» ainsi les fertilisations des autres femelles [5].

Les conflits sexuels qui ont été proposés pour résulter de la monogamie sociale incluent l'infidélité et l'investissement parental.

Le conflit proposé découle de l'hypothèse d'allocation différentielle centrée sur le conflit qui stipule qu'il existe un compromis entre l'investissement et l'attractivité. [3]

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Largemouth bass

Spawning

Largemouth bass usually reach sexual maturity and begin spawning when they are about a year old.^[18] Spawning takes place in the spring season when the water temperature first holds steady above 60˚F. In the northern region of the United States, this usually occurs anywhere from late April until early July. In the southern states, where the largest and healthiest specimens typically inhabit, this process can begin in March and is usually over by June.^[19] Males create nests by moving debris from the bottom of the body of water using their tails. These nests are usually about twice the length of the males, although this can vary.^[18] Bass prefer sand, muck, or gravel bottoms, but will also use rocky and weedy bottoms where there is cover for their nest, such as roots or twigs.^[20] After finishing the nest, the males swim near the nest looking for a female to mate with. After one is found, the two bass swim around the nest together, turning their bodies so that the eggs and sperm that are being released will come in contact on the way down to the nest. Bass will usually spawn twice per spring, with some spawning three or four times, although this is not as common. The male will then guard the nest until the eggs hatch, which can take about 2 to 4 days in the southern U.S and Northern Mexico, and slightly longer in the northern part of its Native Range. Finally, depending on the water temperature, the male will stay with the nest until the infant bass are ready to swim out on their own, which can be about two more weeks after they hatch. After this, the male, female, and newborns will switch to more of a summer mode, in which they then focus more on feeding.^[18]

^{Le frai

L'achigan à grande bouche atteint généralement la maturité sexuelle et commence à se reproduire vers l'âge d'un an. [18]}
^{Le frai a lieu au printemps, lorsque la température de l'eau reste stable au-dessus de 60 ° F. Dans la région nord des États-Unis, cela se produit généralement de fin avril à début juillet. Dans les États du sud, où habitent généralement les spécimens les plus gros et les plus sains, ce processus peut commencer en mars et se termine généralement en juin [19].}
^{Les mâles créent des nids en déplaçant les débris du fond de la masse d'eau à l'aide de leur queue. Ces nids ont généralement environ deux fois la longueur des mâles, bien que cela puisse varier. [18] Les bars préfèrent le sable, la boue ou les fonds de gravier, mais utiliseront également des fonds rocheux et herbeux où il y a une couverture pour leur nid, comme des racines ou des brindilles. [20]}

^{Après avoir terminé le nid, les mâles nagent près du nid à la recherche d'une femelle pour s'accoupler. Après en avoir trouvé une, les deux bars nagent ensemble autour du nid, tournant leur corps de sorte que les ovules et le sperme qui sont libérés entrent en contact en descendant vers le nid.}
^{L'achigan se reproduit généralement deux fois par printemps, certains frayant trois ou quatre fois, bien que ce ne soit pas aussi courant.}
^{Le mâle gardera ensuite le nid jusqu'à l'éclosion des œufs, ce qui peut prendre environ 2 à 4 jours dans le sud des États-Unis et le nord du Mexique, et un peu plus longtemps dans la partie nord de son aire de répartition indigène.}
^{Enfin, selon la température de l'eau, le mâle restera avec le nid jusqu'à ce que le bar infantile soit prêt à nager seul, ce qui peut prendre environ deux semaines de plus après l'éclosion.}
^{Après cela, les mâles, les femelles et les nouveau-nés passent davantage à un mode d'été, dans lequel ils se concentrent davantage sur l'alimentation. [18]}

Achigan à grande bouche

L'Achigan à grande bouche¹ (Micropterus salmoides), aussi appelé black-bass², perche truitée², perche truite³ ou perche d'Amérique³ ou encore perche noire³ en Europe francophone, est une espèce de poissons d'eau douce de la famille des Centrarchidae originaire d'Amérique du Nord et introduite en Europe à la fin du XIX^e siècle. C'est une espèce très recherchée et élevée pour la pêche de loisir.

Reproduction
La maturité sexuelle est atteinte vers 2 ou 3 ans (deux ans en pisciculture)³.
Les femelles ne pondent qu’un tiers des œufs qu’elles possèdent, le fraie se déroule de fin avril à juillet, à des températures de 15 à 25 °C, en eau peu profonde sur un substrat de sable plus ou moins graveleux.
Il semblerait que les plus gros spécimens pondent en premier dans des eaux plus profondes. Les femelles peuvent émettre plus d'un million d'œufs durant la période de reproduction.
Les œufs mesurent entre 1,3 et 1,8 mm de diamètre³. Les larves mesurent jusqu’à 5 mm et les alevins d’une même fratrie ont tendance à suivre un banc, protégés par le mâle.
On estime qu’après avoir atteint 3 cm, les jeunes individus se dispersent progressivement.

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Genetic Monogamy

Genetic monogamy refers to a mating system in which fidelity of the bonding pair is exhibited.^[1] In some species genetic monogamy has been enforced.^[6] Female voles have shown no difference in fecundity with genetic monogamy, but it may be enforced by males in some instances.^[1] Mate guarding is a typical tactic in monogamous species.^[3]^[5]^[7] It is present in many animal species and can sometimes be expressed in leu of parental care by males. This may be for many reasons including paternity assurance.^[6]^[7]

^{Monogamie génétique

La monogamie génétique fait référence à un système d'accouplement dans lequel la fidélité de la paire de liaison est manifestée. [1]}
^{Chez certaines espèces, la monogamie génétique a été appliquée. [6]}
^{Les campagnols femelles n'ont montré aucune différence de fécondité avec la monogamie génétique, mais elle peut être imposée par les mâles dans certains cas. [1]}
^{La garde des compagnons est une tactique typique chez les espèces monogames. [3] [5] [7]}
^{Il est présent dans de nombreuses espèces animales et peut parfois être exprimé en lieu de soins parentaux par les mâles.}
^{Cela peut être dû à de nombreuses raisons, notamment l'assurance de paternité. [6] [7]}

^{L ANISOGAMIE}
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Evolution of Monogamy in Animals

Monogamy may have arisen as a consequence of anisogamy and other ecological factors. Anisogamy is a form of sexual reproduction which involves the fusion of two unequally sized gametes. In animals there are two sexes, the male, in which the gamete is small, motile, usually plentiful, and less energetically expensive, and the female, in which the gamete is larger, more energetically expensive, made at a lower rate, and largely immobile. Anisogamy is thought to have evolved from isogamy, the fusion of similar gametes, multiple times in many different species.^[8] The introduction of anisogamy has caused males and females tend to have different optimal mating strategies.^[4] This is because males may increase their fitness by mating with many females where as females are limited by their own fecundity. Females are therefore typically more likely to be selective in choosing mates.^[9] Monogamy is suggested to limit fitness differences as males and females will mate in pairs.^[5] This would seem to be non beneficial to males, but may not be in all cases. Several behaviors and ecological concerns may have led to the evolution of monogamy as a relevant mating strategy. Partner and resource availability, enforcement, mate assistance, and territory defense may be some of the most prevalent factors effecting animal behavior.

Évolution de la monogamie chez les animaux

La monogamie est peut-être née de l'anisogamie et d'autres facteurs écologiques.

L'anisogamie est une forme de reproduction sexuelle qui implique la fusion de deux gamètes de taille inégale.
Chez les animaux, il y a deux sexes, le mâle, dans lequel le gamète est petit, mobile, généralement abondant et moins énergétiquement cher, et la femelle, dans laquelle le gamète est plus grand, plus énergétiquement cher, fabriqué à un taux inférieur, et largement immobile.
On pense que l'anisogamie a évolué à partir de l'isogamie, la fusion de gamètes similaires, plusieurs fois dans de nombreuses espèces différentes. [8]
L'introduction de l'anisogamie a amené les mâles et les femelles à avoir des stratégies d'accouplement optimales différentes. [4]
En effet, les mâles peuvent augmenter leur condition physique en s'accouplant avec de nombreuses femelles alors que les femelles sont limitées par leur propre fécondité.

Les femelles sont donc généralement plus susceptibles d'être sélectives dans le choix des partenaires. [9]
La monogamie est suggérée pour limiter les différences de forme physique car les mâles et les femelles s'accoupleront par paires. [5]
Cela ne semble pas être bénéfique pour les hommes, mais peut ne pas l'être dans tous les cas.
Plusieurs comportements et préoccupations écologiques peuvent avoir conduit à l'évolution de la monogamie en tant que stratégie d'accouplement pertinente.
La disponibilité des partenaires et des ressources, l'application de la loi, l'aide aux compagnons et la défense du territoire peuvent être certains des facteurs les plus répandus affectant le comportement des animaux.
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Spatial Concerns (Partner & Resource Availability)

In some species females are solitary and spread over a vast landscape. This sparse distribution may be due to the location and amount of resources available.^[6]^[7] In these instances there is less of a chance for a given male to find another female once copulation occurs. In such a case it is more advantageous for a male to remain with a female. He may exhibit mate guarding or similar behaviors in order to do so. With this low population density the males have less competition from other males and female choice is limited. The end result is that the mate choice is more random than in a more dense population which has a number of effects including limiting dimorphism and sexual selection.^[6]^[7] With resource availability limited mating with multiple mates may be harder because the density of individuals is lowered. The habitat cannot sustain multiple mates so monogamy may be more prevalent. This is because resources may be found more easily for the pair than for the individual. The argument for resource availability has been shown in many species, but in several species once resource availability increases monogamy is still apparent.^[4]^[6]^[7] With increased resource availability males may be offsetting the restriction of their fitness through several means. In instances of social monogamy males may offset any lowered fitness through extra pair coupling. Extra pair coupling refers to male and females mating with several mates, but only raising offspring with one mate. The male may not be related to all of the offspring of his main mate, but some offspring are being raised in other broods by other males and females thereby offsetting any limitation of monogamy.^[4] Males are cuckolds, but because they have other female sexual partners they cuckold other males and increase their own fitness. Males exhibit parental care habits in order to be an acceptable mate to the female. Any males which do not exhibit parental care would not be accepted as a sexual partner for socially monogamous females in an enforcement pattern.^[4]

Enforcement

Monogamous mating may also be caused simply by enforcement through tactics such as mate guarding.^[3]^[5] In these species the males will prevent other males from copulating with their chosen female or vice versa.^[6] Males will help to fend off other aggressive males, and keep their mate for themselves. This is not seen in all species such as some primates in which the female may be more dominant than the male and may not need help to avoid unwanted mating, the pair may still benefit from some form of mate assistance however and therefore monogamy may be enforced to ensure the assistance of males. Bi-parental care is not seen in all monogamous species however so this may not be the only cause of female enforcement.^[4]

Mate Assistance & Territory Defense

In species where mate guarding is not needed there may still be a need for the pair to protect each other. An example of this would be sentinel behavior in avian species.^[3] The main advantage of sentinel behavior is that many survival tactics are improved. As stated, the male or female will act as a sentinel and signal to their mate if a predator is present. This can lead to an increase in survivorship, foraging, and incubation of eggs.^[3] Male care for offspring is rather rare in some taxa of species. This is because males may increase their fitness by searching for multiple mates.^[6]^[10] Females are limited in fitness by their fecundity so multiple mating does not affect their fitness to the same extent.^[9] Males have the opportunity to find a new mate earlier than females when there is internal fertilization or the females exhibit the majority of the care for the offspring.^[10] When males are shown to care for offspring as well as females it is referred to as bi- parental care. Bi-parental care may occur when there is a lower chance of survival of the offspring without male care. The evolution of this care has been associated with energetically expensive offspring.^[6] Bi-parental care is exhibited in many avian species.^[3] In these cases the male has a greater chance to increase his own fitness by seeing that his offspring live long enough to reproduce. If the male is not present in these populations the survivorship of the offspring is drastically lowered and there is a lowering in male fitness.^[3]^[10] Without monogamy bi- parental care is less common and there is an increased chance of infanticide.^[1] Infanticide with monogamous pairing would lead to a lowered fitness for socially monogamous males and is not seen to a wide extent.^[6]

Consequences of Monogamous Mating

Monogamy as a mating system in animals has been thought to lower level of some pre and post copulatory competition methods.^[5]^[7]^[11] Because of this reduction in competition in some instances the regulation of certain morphological characteristics may be lowered. This would result in a vast variety of morphological and physiological differences such as sexual dimorphism and sperm quality.

Sexual Dimorphism

Sexual dimorphism denotes the differences in males and females of the same species. Even in animals with seemingly no morphological sexual dimorphism visible there is still dimorphism in the gametes. Males have the smaller gametes and females have the larger gametes. As soon as the two sexes emerge the dimorphism in the gamete structures and sizes may lead to further dimorphism in the species.^[12] Sexual dimorphism is often caused through evolution in response to male male competition and female choice.^[5] In polygamous species there is a noted sexual dimorphism. The sexual dimorphism is seen typically in sexual signaling aspects of morphology. Males typically exhibit these dimorphic traits and they are typically traits which help in signaling to females or male male competition.^[7]^[13] In monogamous species sexual conflict is thought to be lessened, and typically little to no sexual dimorphism is noted as there is less ornamentation and armor. This is because there is a relaxation of sexual selection.^[7] This may have something to do with a feedback loop caused by a low population density. If sexual selection is too strenuous in a population where there is a low density the population will shrink. In the continuing generations sexual selection will become less and less relevant as mating becomes more random.^[7] A similar feedback loop is thought to occur for the sperm quality in genetically monogamous pairs.

Sperm Quality

Once anisogamy has emerged in a species due to gamete dimorphism there is an inherent level of competition. This could be seen as sperm competition in the very least. Sperm competition is defined as a post copulatory mode of sexual selection which causes the diversity of sperm across species.^[11] As soon as sperm and egg are the predominant mating types there is an increase in the need for the male gametes. This is because there will be a large number of unsuccessful sperm which will cost a certain level of expenditure on energy without a benefit from the individual sperm. Sperm in polygamous sexual encounters have evolved for size, speed, structure, and quantity.^[11] This competition causes selection for competitive traits which can be pre or post copulatory.^[12] In species where cryptic female choice is one of the main sources of competition females are able to choose sperm from among various male suitors.^[11]^[14]^[15] Typically the sperm of the highest quality are selected.^[11] In genetically monogamous species it can be expected that sperm competition is absent or otherwise severely limited. There is no selection for the highest quality sperm amongst the sperm of multiple males, and copulation is more random than it is in polygamous situations. Therefore sperm quality for monogamous species has a higher variation and lower quality sperm have been noted in several species. The lack of sperm competition is not advantageous for sperm quality. An example of this is in the Eurasian bullfinch which exhibits relaxed selection and sperm competition. The sperm of these males have a lower velocity then other closely related but polygamous passerine bird species and the amount of abnormalities in sperm structure, length, and count when compared to similar bird families is increased.^[11]

Animals

The evolution of mating systems in animals has received an enormous amount of attention from biologists. This section briefly reviews three main findings about the evolution of monogamy in animals.
The amount of social monogamy in animals varies across taxa, with over 90% of birds engaging in social monogamy while only 3% of mammals are known to do the same.^[16]^[17]
This list is not complete. Other factors may also contribute to the evolution of social monogamy. Moreover, different sets of factors may explain the evolution of social monogamy in different species. There is no one-size-fits-all explanation of why different species evolved monogamous mating systems.

Sexual dimorphism

Sexual dimorphism refers to differences in body characteristics between females and males. A frequently studied type of sexual dimorphism is body size. For example among mammals, males typically have larger bodies than females. In other orders, however, females have larger bodies than males. Sexual dimorphism in body size has been linked to mating behavior.^[18]^[19]^[20]^[21] In polygynous species, males compete for control over sexual access to females. Large males have an advantage in the competition for access to females, and they consequently pass their genes along to a greater number of offspring. This eventually leads to large differences in body size between females and males. Polygynous males are often 1.5 to 2.0 times larger in size than females. In monogamous species, on the other hand, females and males have more equal access to mates, so there is little or no sexual dimorphism in body size. From a new biological point of view, monogamy could result from mate guarding and is engaged as a result of sexual conflict.^[22]
Some researchers have attempted to infer the evolution of human mating systems from the evolution of sexual dimorphism. Several studies have reported a large amount of sexual dimorphism in Australopithecus, an evolutionary ancestor of human beings that lived between 2 and 5 million years ago.^[19]^[20]^[23]^[24] These studies raise the possibility that Australopithecus had a polygamous mating system. Sexual dimorphism then began to decrease. Studies suggest sexual dimorphism reached modern human levels around the time of Homo erectus 0.5 to 2 million years ago.^[19]^[20]^[23]^[25] This line of reasoning suggests human ancestors started out polygamous and began the transition to monogamy somewhere between 0.5 million and 2 million years ago.
Attempts to infer the evolution of monogamy based on sexual dimorphism remain controversial for three reasons:

The skeletal remains of Australopithecus are quite fragmentary. This makes it difficult to identify the sex of the fossils. Researchers sometimes identify the sex of the fossils by their size, which, of course, can exaggerate findings of sexual dimorphism.

Recent studies using new methods of measurement suggest Australopithecus had the same amount of sexual dimorphism as modern humans.^[26]^[27] This raises questions about the amount of sexual dimorphism in Australopithecus.

Humans may have been partially unique in that selection pressures for sexual dimorphism might have been related to the new niches that humans were entering at the time, and how that might have interacted with potential early cultures and tool use. If these early humans had a differentiation of gender roles, with men hunting and women gathering, selection pressures in favor of increased size may have been distributed unequally between the sexes.

Even if future studies clearly establish sexual dimorphism in Australopithecus, other studies have shown the relationship between sexual dimorphism and mating system is unreliable.^[18]^[19] Some polygamous species show little or no sexual dimorphism. Some monogamous species show a large amount of sexual dimorphism.

Studies of sexual dimorphism raise the possibility that early human ancestors were polygamous rather than monogamous. But this line of research remains highly controversial. It may be that early human ancestors showed little sexual dimorphism, and it may be that sexual dimorphism in early human ancestors had no relationship to their mating systems.

Testis size

Chimpanzee

Male and female gorilla

The relative sizes of male testes often reflect mating systems.^[28]^[29]^[30]^[31] In species with promiscuous mating systems, where many males mate with many females, the testes tend to be relatively large. This appears to be the result of sperm competition. Males with large testes produce more sperm and thereby gain an advantage impregnating females. In polygynous species, where one male controls sexual access to females, the testes tend to be small. One male defends exclusive sexual access to a group of females and thereby eliminates sperm competition.
Studies of primates, including humans, support the relationship between testis size and mating system.^[30]^[31]^[32] Chimpanzees, which have a promiscuous mating system, have large testes compared to other primates. Gorillas, which have a polygynous mating system, have smaller testes than other primates. Humans, which have a socially monogamous mating system, accompanied by moderate amounts of sexual non-monogamy (see incidence of monogamy), have moderately sized testes. The moderate amounts of sexual non-monogamy in humans may result in a low to moderate amount of sperm competition. Also, notably, in the case of an avowedly sexually monogamous society, the occurrence of sexual nonmonogamy is typically culturally stigmatized, and therefore detecting its prevalence is inherently difficult, if indeed it is at all possible. At best, such statistics can be viewed as general approximations with a wide margin of error.

Monogamy as a best response

In species where the young are particularly vulnerable and may benefit from protection by both parents, monogamy may be an optimal strategy. Monogamy tends to also occur when populations are small and dispersed. This is not conductive to polygamous behavior as the male would spend far more time searching for another mate. The monogamous behavior allows the male to have a mate consistently, without having to waste energy searching for other females. Furthermore, there is an apparent connection between the time a male invests in their offspring and their monogamous behavior. A male which is required to care for the offspring to ensure their survival is much more likely to exhibit monogamous behavior over one that does not. The selection factors in favor of different mating strategies for a species of animal, however, may potentially operate on a large number of factors throughout that animal's life cycle. For instance, with many species of bear, the female will often drive a male off soon after mating, and will later guard her cubs from him. It is thought that this may be due to the fact that too many bears close to one another may deplete the food available to the relatively small but growing cubs. Monogamy may be social but rarely genetic.^{[citation needed]} For example, in the cichlid species Variabilichromis moorii, a monogamous pair will care for their eggs and young but the eggs are not all fertilized by the same male.^[33] Thierry Lodé^[34] argued that monogamy should result from conflict of interest between the sexes called sexual conflict.

Monogamous species

While it is difficult to find monogamous relationships in nature, there are a few species which have adopted monogamy with great success. For instance, the prairie vole will mate exclusively with the first female he ever mates with. The vole is extremely loyal and will go as far as to even attack other females that may approach him. This type of behavior has been linked to the hormone vasopressin. This hormone is released when a male mates and cares for young. Due to this hormone's rewarding effects, the male experiences a positive feeling when they maintain a monogamous relationship. To further test this theory, the receptors that control vasopressin were placed into another species of vole that is promiscuous. After this addition, the originally unfaithful voles became monogamous with their selected partner. These very same receptors can be found in human brain, and have been found to vary at the individual level—which could explain why some human males tend to be more loyal than others. ^[35]^[36]^[37]
Black vultures stay together as it is more beneficial for their young to be taken care of by both parents. They take turns incubating the eggs, and then supplying their fledglings with food. Black vultures will also attack other vultures that are participating in extra pair copulation, this is an attempt to increase monogamy and decrease promiscuous behavior.^[35] Similarly, emperor penguins also stay together to care for their young. This is due to the harshness of the arctic weather, predators and the scarcity of food. One parent will protect the chick, while the other finds food. However, these penguins only remain monogamous until the chick is able to go off on their own. After the chick no longer needs their care, approximately 85% of parents will part ways and typically find a new partner every breeding season.
Hornbills are a socially monogamous bird species that usually only have one mate throughout their lives, much like the prairie vole. The females will close herself up in a nest cavity, sealed with a nest plug, for two months. At this time, she will lay eggs and will be cared for by her mate. The males are willing to work to support himself, his mate, and his offspring in order for survival; however, unlike the emperor penguin, the hornbills do not find new partners each season.^[38]
It is relatively uncommon to find monogamous relationships in fish, amphibians and reptiles; however, the red-backed salamander as well as the Caribbean cleaner goby practice monogamy as well. However, the male Caribbean cleaner goby fish has been found to separate from the female suddenly, leaving her abandoned. In a study conducted by Oregon State University, it was found that this fish practices not true monogamy, but serial monogamy. This essentially means that the goby will have multiple monogamous relationships throughout its life - but only be in one relationship at a time.^[39] The red-backed salamander exhibited signs of social monogamy, which is the idea that animals form pairs to mate and raise offspring, but still will partake in extra pair copulation with various males or females in order to increase their biological fitness. This is a relatively new concept in salamanders, and has not been seen frequently - it is also concerning that the act of monogamy may inhibit the salamanders reproductive rates and biological success. However, the study which was conducted in cooperation by the University of Louisiana, Lafayette, and the University of Virginia showed that the salamanders are not inhibited by this monogamy if they show alternative strategies with other mates.^[40]
Azara's night monkeys are another species that proved to be monogamous. In an 18 year study conducted by the University of Pennsylvania, these monkeys proved to be entirely monogamous, exhibiting no genetic information or visual information that could lead to the assumption that extra pair copulation was occurring. This explained the question as to why the male owl monkey invested so much time in protecting and raising their own offspring. Because monogamy is often referred to as "placing all your eggs in one basket" the male wants to ensure his young survive, and thus pass on his genes.^[41]
Other monogamous species include wolves, otters, a few hooved animals, some bats, certain species of fox, and the Eurasian beaver. This beaver is particularly interesting as it is practicing monogamy in its reintroduction to certain parts of Europe; however, its' American counterpart is not monogamous at all and often partakes in promiscuous behavior. The two species are quite similar in ecology but American beavers tend to be less aggressive than European beavers. In this instance, the scarcity of the European beavers' population could drive its monogamous behavior; moreover, it lowers the risk of parasite transmission which is correlated with biological fitness. Monogamy is proving to be very efficient for this beaver, as their population is climbing.^[42]

R2017 - La monogamie animale

dimanche 19 février 2017

Fiche anglaise - monogamy in animals