In einem Artikel im Standard werden Gründe für Monogamie besprochen, im wesentlichen sollen drei Gründe eine Rolle spielen:
Wie es dazu kam, darüber streiten Evolutionsbiologen seit langem. Im Wesentlichen werden drei Theorien diskutiert: Erstens verhindern Paarbeziehungen, dass Weibchen mit rivalisierenden Männchen fremdgehen; zweitens tragen Männchen in Paarbeziehungen zur Aufzucht bei und erhöhen so den Fortpflanzungserfolg; drittens schützen Männchen ihren eigenen Nachwuchs so besser vor Kindsmord durch andere Männchen.
Zwei Studien werden dort näher besprochen.
Die erste stellt auf den Kindsmord ab:
Although common in birds, social monogamy, or pair-living, is rare among mammals because internal gestation and lactation in mammals makes it advantageous for males to seek additional mating opportunities. A number of hypotheses have been proposed to explain the evolution of social monogamy among mammals: as a male mate-guarding strategy, because of the benefits of biparental care, or as a defense against infanticidal males. However, comparative analyses have been unable to resolve the root causes of monogamy. Primates are unusual among mammals because monogamy has evolved independently in all of the major clades. Here we combine trait data across 230 primate species with a Bayesian likelihood framework to test for correlated evolution between monogamy and a range of traits to evaluate the competing hypotheses. We find evidence of correlated evolution between social monogamy and both female ranging patterns and biparental care, but the most compelling explanation for the appearance of monogamy is male infanticide. It is only the presence of infanticide that reliably increases the probability of a shift to social monogamy, whereas monogamy allows the secondary adoption of paternal care and is associated with a shift to discrete ranges. The origin of social monogamy in primates is best explained by long lactation periods caused by altriciality, making primate infants particularly vulnerable to infanticidal males. We show that biparental care shortens relative lactation length, thereby reducing infanticide risk and increasing reproductive rates. These phylogenetic analyses support a key role for infanticide in the social evolution of primates, and potentially, humans.
Aus dem Artikel dazu:
Zu einer etwas anderen Lösung kommt die Forschergruppe um Christopher Opie: Ihren Berechnungen zufolge waren die Kindstötungen der stärkste Motor für die Entwicklung von Monogamie – zumindest bei den Primaten. Bei vielen Tierarten (bekanntestes Beispiel sind die Löwen) töten Männchen den Nachwuchs anderer Männchen, damit die Weibchen schneller wieder empfängnisbereit sind. In einer monogamen Beziehung kann der Vater seinen Nachwuchs vor solchen Angriffen schützen.
Werden die Kosten für die Aufzucht des Nachwuchses unter den Eltern geteilt, haben die Mütter mehr Ressourcen für das Stillen, wie die Forscher weiter erläutern. Dies wiederum verkürze die Stillzeit, wodurch die Weibchen schneller wieder schwanger werden können. Davon profitierten auch die treuen Männchen. Mitsorgende Väter würden aber auch – insbesondere beim Menschen – eine lange Kindheit und die lange Entwicklungszeit des Gehirns ermöglichen.
Das Infantizid eine so hohe Bedeutung hatte halte ich für eher für unwahrscheinlich. Natürlich ist in Spezien mit enger Paarbindung die Wahrscheinlichkeit geringer, dass ein anderes Männchen die Kinder tötet. Aber die Frage ist, ob das Ursache oder nur Nebeneffekt der Monogamie ist.
Die zweite Studie:
The evolution of social monogamy has intrigued biologists for over a century. Here, we show that the ancestral condition for all mammalian groups is of solitary individuals and that social monogamy is derived almost exclusively from this social system. The evolution of social monogamy does not appear to have been associated with a high risk of male infanticide, and paternal care is a consequence rather than a cause of social monogamy. Social monogamy has evolved in nonhuman mammals where breeding females are intolerant of each other and female density is low, suggesting that it represents a mating strategy that has developed where males are unable to defend access to multiple females.
Aus dem Artikel dazu:
Die Antwort, die sie auf diese Frage liefern, ist eindeutig: Zunächst war wohl das Problem der Partnerwahl zu lösen – insbesondere bei jenen Arten, wo die Weibchen eher weit verstreut und nicht ganz leicht zu finden sind. Da würde Monogamie den Fortpflanzungserfolg deutlich erhöhen. Die gemeinsame Aufzucht wäre dann nur eine Folge des monogamen Paarungsverhaltens.
Die weitere Streuung der Frauen würde in der Tat eine Monogamie vorteilhafter machen.
Hier auch noch mal ein paar Gründe aus der Wikipedia:
There are several hypotheses for the evolution of mammalian monogamy that have been extensively studied. While some of these hypotheses apply to a majority of monogamous species, other apply to a very limited number of them.
Vesopressin, a hormone that induces male Prairie Voles (Microtus ochrogasters) to mate with one female and stay by her side in order to protect her, has been found to influence the level of monogamous behavior in this species of mammals. There is a significant difference in the distribution of protein receptors in the brains of monogamous and polygamous Voles, which led to the identification of chemicals that fit into those receptors. In addition to its known role, vasopressin (which decreases urine flow) also transmits messages between the different nerve cells in the brain.
A typical male Prairie Vole displays a rather timid behavior during daily interactions; however, these individuals become aggressive after mating with a female, especially towards another male. The study’s results indicated that the Vole males that were injected with vasopressin blocker remained timid and displayed behavior similar to polygamous Voles. Polygamous Voles have a significantly smaller number of vasopressin receptors and remain shy even after mating. Thus, the scientists concluded that vasopressin is responsible for the aggressive behavior in male Prairie Voles. After further testing they also concluded that vasopressin is also responsible for forming attachment between the male and a female of his choice.
Female distribution seems to be one of the best predictors of the evolution of monogamy in some species of mammals. It is possible that monogamy evolved due to a low female availability or high female dispersion where males were unable to monopolize more than one mate over a period of time. In species such as Kirk’s dik-dik (Madoqua kirki) and Elephant shrews (Elephantulus rufescens), bi-parental care is not very common. These species do, however, exhibit monogamous mating systems presumably due to high dispersal rates. Komers and Brotherton (1997) indicated that there is a significant correlation between mating systems and grouping patterns in these species. Furthermore, monogamous mating system and female dispersion are found to be closely related. Some of the main conclusions of the occurrence of monogamy in mammals include:
Monogamy occurs when males are unable to monopolize more than one female
Monogamy should be more likely if female under-dispersion occurs
Female home range is larger for monogamous species
When females are solitary and occupy large ranges
This phenomenon is not common for all species, but species such as the Japanese serow (Capricornis crispus) are perfect examples of such behavior.
It is believed that bi-parental care had an important role in the evolution of monogamy. Because mammalian females undergo periods of gestation and lactation, they are well adapted to take care of their young for a long period of time, as opposed to their male partners who don’t necessarily contribute to this rearing process. Such differences in parental contribution could be a result of the males’ drive to seek other females in order to increase their reproductive success, which may prevent them from spending extra time helping raise their offspring. Helping a female in young rearing could potentially jeopardize a male’s fitness and result in the loss of mating opportunities. There are some monogamous species that exhibit this type of care mainly to improve their offspring’s survivorship; however it does not occur in more than 5% of all mammals.
Bi-Parental care has been extensively studied in the California mouse (Peromyscus californicus). This species of mice is known to be strictly monogamous; mates pair for a long period of time, and the level of extra-pair paternity is considerably low. It has been shown that in the event of female removal, it is the male that takes direct care of the offspring and acts as the primary hope for the survival of his young. Females who attempt to raise their young in cases where their mate is removed often do not succeed due to high maintenance costs that have to do with raising an offspring. With the presence of males, the survival of the offspring is much more probable; thus, it is in the best interest for both parents to contribute. This concept also applies to other species, including dwarf lemurs (Cheirogaleus medius), where females were also not successful at raising their offspring without paternal help. Lastly, in a study performed by Wynne-Edwards (1987), 95% of Djungarian hamsters (Phodopus campbelli) survived in the presence of both parents, but only 47% survived if the father was removed. There are several key factors that may affect the extent to which males care for their young:
Intrinsic Ability to Aid Offspring: the male’s ability to exhibit parental care.
Sociality: male paternal behavior shaped by permanent group living. There is a closer association between the male and his offspring in small groups that are often composed of individuals that are genetically related. Common examples include mongooses (Mungos mungos), wolfs (Cams lupus), and naked mole rats (Heterocephalus glaber).
High Costs to Polygyny: some males could evolve to care for their offspring in cases where females were too dispersed over the given territory and the male could not find consistent females to mate with. In those territories, individuals such as elephant shrews (Macroscelididae), and dasyproctids (Agouti, Dasyprocta, and Myoprocta), stay within their known territories rather than going outside of their limits in order to search for another mate, which would be more costly than staying around his adapted territory.
Paternity Certainty: There are cases where males care for offspring that they are not genetically related to especially in groups where cooperative breeding is practiced. However, in some species, males are able to identify their own offspring, especially in threat of infanticide. In these groups, paternity certainty could be a factor deciding about biparental care.
Infanticide threat in larger mammals 
Infanticide, or the killing of the offspring by adult individuals, has been reported in many mammalian species and it is considered as an adaptive strategy to enhance fitness; it is common in groups where male to female ratio and male tenure are relatively low. One benefit for a male perpetrator includes having multiple mating partners; females could benefit from killing other female’s offspring by gaining access to food resources or shelter.
In some species of mammals, such as Gibbons (Hylobatidae), it is thought that monogamy could have evolved as a response to the threat of infanticide, especially if performed by a foreign male.
The rates of infanticide are very low in monogamous groups of larger mammals, which can be explained by the fact that males care for their offspring and their mother by protecting them from predators and the threat of other males. This is consistent with the findings of Borries, Savini, & Koeng (2011) who indicated that the percentage of infant loss was significantly lower in monogamous species than in polyandrous ones. However, there still needs to be more empirical evidence in order to further test this hypothesis.