Sexual Selection Theory
Batemanís principle of sexual selection states that males with more mates have more offspring, but females have the same amount of offspring regardless of number of mates (Bateman, 1948). Thus, we would expect females to be indifferent to the number of males she mates with and we would expect males to try and mate with as many females as possible. Considering this, how could staying with only one mate be adaptive for males? The following hypotheses were proposed by Emlen and Oring in 1977 (described in Alcock, p 362-3).
Mate guarding hypothesis
If females remain receptive after mating, then it would be adaptive for males to guard females against other males, as the latter's sperm might compete with the former's, especially when females are few and widely distributed. This is known as facultative monogamy. Example: in clown shrimp, females are scarce and only receptive for a short time, so when a male encounters a female he will remain with her (Wickler and Seibt, 1981). Another animal that engages in this behavior is the Kokanee salmon (Morbey, 2002).
Mate assistance hypothesis
If a male remains with a female to give parental care, then the offspring will have a better chance of surviving in certain environments. This is also called obligate monogamy. Monogamy is even more adaptive in animals where the male actually broods the young, such as the seahorse. Since the male can only carry one brood of eggs at a time, there is no incentive for him to find another mate. (Alcock, 2001)
In most mammals, there is high maternal investment in offspring. But in cases where biparental care increases survival of offspring enough to offset the cost of not mating, we would expect monogamy to be adaptive for the female, male, and the offspring.
Less than 10% of mammals give paternal (fatherly) care (Woodroffe and Goodstein, 1994, cited in Alcock, 365) and some who do are socially monogamous. Many rodents give parental care due to the vulnerablility of the offspring when they are born, but the parental care is primarily maternal. One exception is the male dwarf hamster (Phodopus campbelli) actually helps deliver his mate's babies, contributing to their survival (Jones and K.E. Wynne-Edwards, 2000). Parental care also allows one of the parents to stay with offspring while the other forages for food, shown to increase survival in the California mouse (Cantoni, D. and R. Brown. 1997).
Prairie voles are also biparental. During the early postpartum period (3 days after birth), young voles must be kept warm by at least one parent to survive (McGuire, et al., 2007). In a phenomenon called "nest relief", the father and mother take turns taking care of the young while the other forages(McGuire, et al., 2007). Wilson (1982) found that in father-absent nests, the mother did not compensate for the absence of nest relief and spent the same amount of time with the young as in father-present nests. Biparental care is adaptive in prairie voles because it allows for more combined time spent with the vulnerable offspring.
In many primates, females will not mate again while caring for a newborn; so some males will kill unrelated infants in order to mate with the mother, in a phenomenon known as infanticide (Hrdy, 1977). Monogamy is adaptive for males if they travel with their mate and offspring and are able to protect their offspring from infanticide.
Which hypothesis explains mammalian monogamy overall?
If the mate assistance hypothesis holds for mammals, we would expect mammals that give parental care to be monogamous, while those who not give parental care to be polygynous. However, comparative studies by Komers and Brotherton (1997) have shown that mate parental care and social monogamy are not tightly correlated among mammals. Their data on primates show that 8 out of 16 of monogamous primate taxa give parental care, which is not significantly different from 7 out of 20 polygamous primate taxa that give parental care. It also seemed that monogamy evolved more often in the absence of paternal care than in its presence. So while in certain species monogamy may have evolved due to the need for parental care, Komers and Brotherton think those species should be considered exceptions to the rule.
Komers and Brotherton observed that monogamy does not occur in mammals where female ranges are large, but rather monogamy seemed to evolve where females were solitary and had small, exclusive ranges. They suggest that monogamy evolved in these mammals because a single female could be monopolized successfully, which would reduce variance in mating success for males. A reduction in variance means that a male might not have a chance to father hundreds of offspring, but at least he isn't wasting energy or risking predation in the search for extra mates that he may not find. Leaving to search for other mates would also allow other males to mate with his unguarded female. So mate guarding, not mate assistance, provides a more fitting hypothesis for the evolution of monogamy in mammals. Prairie voles' social pair-bond formation behavior supports this hypothesis, contributing to reproductive success and fitness for the male and female respectively.