The reason that monogamous relationships have been deemed advantageous by natural selection is the result that extended parental care has on the K-selected offspring. With the inclusion of parental care in an offspring’s early development, the offspring becomes better equipped to deal with the harsh environment around them. K-selected offspring are born in smaller litters, which means that the parent invests more time and energy into the offspring, which is always beneficial to its survival. The offspring’s average lifespan is increased, the offspring is able to learn from the parents (including, but not limited to, unique mating rituals, foraging skills, and techniques to avoid predators), the offspring has protectors from predators, and the offspring is more developed when the time comes for it to live on its own. Having monogamously related parents leads to the parental care that is required to allow a more socially sophisticated offspring to be born.
The Lukas and Clutton-Brock article states a myriad of causes that, combined, resulted in the evolvement of socially monogamous relationships. These include: the inability of males to claim and protect territory over more than one female since females are not densely populated and they occupy a large territory, the intolerance between breeding females, and the reliance on highly nutritious food that occurs in a low frequency which increases the competition between organisms. These things, together, have lead to the evolutionary adaptation of socially monogamous relationships in mammals. The Opie et al. article leans on male infanticide as the main cause of socially monogamous relationships in mammals. Due to the fact that females can only suckle one infant at a time, and since the lactation period is longer for mammals, this has lead to the trend of males eliminating the offspring of another in order to impregnate that female, therefore fulfilling his biological destiny as stated by Darwin. The female cannot return to a oestrus period if she is lactating for an offspring, so a male of that species must kill off the offspring in order to bring her back into a fertile state. According to Opie et al., socially monogamous relationships are also caused by a low density of females of that species. This low concentration increases the difficulty of a male fending off competition and is indicative of the male need to conquer the territory. The high incidence of male infanticide led directly to the evolvement of mammalian monogamous relationships by enforcing the need to protect the young.
I most support the hypothesis that monogamous relationships in mammalian species is due to the need to lessen the incidence of male infanticide, as stated by Opie et al. This hypothesis seems more reasonable since it gives more concrete chemical evidence as to the cause. The majority, if not all, of the activities an organism carries out seems to be due to chemical influences and interactions. A high incidence of male infanticide would have promoted an evolutionary change to reverse that trend. And that evolutionary change would be monogamous relationships. This would have to had to start at a chemical level, as seen in the decrease in the lactation period in conjunction with monogamous relationships. A decrease in male infanticide would also result from another chemical drive — the natural instinct to have your young survive. Males need to be more territorial and protective in order to keep their offspring safe, a thing that is only possible, due to the low density of females, if the relations became monogamous. The male can only claim one female at a time because of the vast territory he needs to defend.
Being that I was raised around almost complete social monogamy in my human world, it was extremely shocking that only 3% of mammalian species practice it. I thought it would be much higher, since most human populations practice either monogamy or serial monogamy. I am so used to the way things are in the human world that I sometimes forget that it is a lot different, if not completely, in the animal world.