After reading the two articles and their different theories on the evolution of the monogamy in mammals I had been swayed to believe that infanticide was the leading cause in monogamy among mammals, but upon further inspection I have come to realize that there must have been and there is evidence to support the idea that multiple causes have led to monogamy among certain mammals. One cause may have been more influential than the others, but I feel that the conflicting information in the two articles makes it difficult to deduce which cause had a greater effect on the evolution of monogamy. Both papers have reduced the possible cause of mammal monogamy to include the three most likely: male care, male infanticide, and definability of females.
The idea behind male care is that the cost of raising offspring became so high that females were forced to rely on others to help raise her young. An example of this is humans, because humans are bipedal we have evolved narrower hips and thus a narrower birth canals in order to walk and run efficiently. Where as human infants have large heads in order to accommodate the size of their developing brains. The combination of narrow hips and relatively large skulls has made it necessary for humans to be born long before they are fully developed compared to other animals. The extended period of intensive care of human children requires a lot of energy from the mother and leaves her venerable. It would be nearly impossible for a human mother to carry a helpless infant in one arm while fending off an attack or hunting with the other. It would have become advantageous for the male to protect a single female and offspring in order to have highest chances of passing his genes to the next generation.
The argument for male infanticide also suggests that it would be advantageous for the male to protect his offspring from other males. When a female is caring for and feeding her offspring she cannot mate with other males. This decreases the number of reproductive opportunities available to him, and so by killing the offspring the male makes it so the female is again fertile. Lukas and Cluttin-Brock write in their paper that 27% of offspring succumb to infanticide in mammalian societies that are not monogamous where as only 9% of offspring are killed in monogamous species. An example of a species that has surprisingly high infanticide rates is gorillas, 34%. A reason for this may be because gorillas are not able to be monogamous due to their environment. These statistics clearly show that it would be advantageous for mammals to be monogamous in order to perpetuate their genes.
The last theory that was suggested was the ecological defensibility of females. This theory suggests that monogamy developed in species where females were intolerant of each other and so did not live together in a single area. The wide dispersion of females made it impossible for males to effectively defend a territory with more than one female in it. Thus leading to monogamy because it was a more effective breeding style.
I believe that all of these factors had role in the evolution of monogamy in mammals. Both articles agree that the mammalian ancestor was not monogamous so the trait for monogamy must have evolved due to natural selection. The evolution of a species to monogamy takes place over many generations. Even if evolution is not gradual but punctuated there must have been enough time for each of these factors to have an effect on at least a single species if not more than one.