For one thing, pair-bonds differ markedly with respect to duration. Most monogamous birds pair only for a single breeding season.
In these species, the whole courtship and mating process starts afresh every year. Other species a minority pair for several seasons and even for life. Most people know that swans mate for life. But there are other examples too, including cranes, vultures, eagles, geese and albatrosses.
And of course, scientists have pondered why. These species are all long-lived. We believe that lifelong pair-bonds are beneficial because they allow pairs to optimise their effectiveness as parents, resulting in more surviving young.
This hypothesis has been studied and validated in kittiwakes coastal gulls. Maybe humans are most like long-lived birds. Yet, according to the charity Relate, 42 per cent of UK marriages end in divorce.
Data from the ONS suggest that divorce usually occurs within the first few years of marriage the mode is five years. Could it be, as some have suggested, that our pair-bonds are intended to endure just long enough to rear one infant through the most intensive and demanding years? More recent work discussed below has demonstrated that co-evolutionary feedback can occur and lead to patterns that are more complex than those initially predicted e.
Indeed, as noted by Wittenberger and Tilson , no single hypothesis alone is sufficient to explain monogamy; instead a series of hypotheses and an understanding of when each should apply is required to understand monogamy Wittenberger and Tison, Nonetheless, the relatively simple verbal arguments of Emlen and Oring highlighted the key role that sexual selection, resources, and parental care can play in influencing monogamy.
Wittenberger and Tilson expanded the conditions under which monogamy is expected. They focused more explicitly on the role that female fitness benefits have in maintaining monogamy and hypothesized that monogamous mating systems require: 1 female benefits of monogamous pair bonds that cannot be obtained in the absence of monogamy; 2 the ability of females to assess the mated status of males; and 3 a lack of male desertion.
Importantly, these hypotheses are inter-related and not mutually exclusive. In the years following the work of Emlen and Oring and Wittenberger and Tilson , numerous authors refined the conditions under which monogamy is expected e. Many of these early studies that focused explicitly on monogamy utilized verbal arguments that were largely based on our knowledge of mating systems at the time but see, e. Below, I review the insights that such work has provided. A frequent prediction of early work is that paternal care is associated with monogamy Emlen and Oring, ; Wittenberger and Tilson, ; Gowaty, ; Figure 1.
Indeed, some theoretical work has found that paternal care influences the occurrence of monogamy. Iwasa and Harada found that monogamy can occur when parental ability is the same between parents if females choose their mates and males invest in paternal care. If females and males vary in quality i. More recent work has suggested that understanding the origin and persistence of any mating system, including monogamy, requires that we develop a framework that accounts for interactions among mating dynamics, parental investment, and costs of care, and mating Figure 1.
The likelihood of paternal care, which as discussed above is in some cases predicted to influence monogamy, is affected by a range of factors, including the costs and benefits of caring vs.
Kokko and Jennions developed a model focused on sex roles that accounted for feedback associated with the costs and benefits of caring vs. Their work revealed that when providing parental care is associated with higher mortality than competing for mates, individuals of the deserting sex i. While monogamy was not explicitly focused on in the modeling work of Kokko and Jennions , if bi-parental care is associated with monogamy see above , we might expect monogamy to be more likely in systems in which parental care is associated with high adult mortality.
While bi-parental care and monogamy commonly co-occur reviewed in Brotherton and Komers, , some researchers have questioned whether bi-parental care is necessary for, or rather a consequence of, monogamy. In humans, Schacht and Bell found that mate guarding rather than paternal care leads to monogamy, as monogamy allows males to maintain high paternity.
Likewise, Lukas and Clutton-Brock suggested that in non-human mammals, male care is a consequence rather than a cause of monogamy. They instead suggested that monogamy is caused by low female density and the inability of males to defend multiple females. However, Dobson et al. Instead, Dobson et al. In primates, Opie et al. In some cases, paternal care is decoupled from monogamy.
In extreme cases, males die after mating and are thus unable to provide paternal care. Fromhage et al. Thus, while bi-parental care and monogamy frequently co-occur, it does not appear that bi-parental care is always a pre-requisite for the origin or maintenance of monogamy.
Indeed, in some cases mate guarding and paternity assurance can directly favor monogamy. In summary, bi-parental care commonly co-occurs with monogamy, but the role that bi-parental care plays in driving the origin of monogamy is unclear, particularly in mammals. While bi-parental care is not always essential for monogamy, parental care and offspring need can in some cases interact with ecological factors to drive the origin of monogamy.
For instance, Brown et al. In poison frogs, they found that feeding of offspring co-evolved with the use of small pools and that feeding behavior was associated with the origin of bi-parental care, as bi-parental care is essential to offspring survival in small but not large pools.
Molecular and field analyzes revealed that social and genetic monogamy occur in Ranitomeya imitator , a species that utilizes small pools, but not Ranitomeya variabilis , a species that uses large pools, suggesting that an ecological factor pool size led to the co-evolution of bi-parental care and monogamy in R. These results highlight the important interactions that can occur between ecological factors, parental care, offspring need, and monogamy.
Decades of empirical and theoretical research suggest that there is no single factor that drives monogamy across animals. This is perhaps not surprising given the immense variation in life history, evolutionary history, and ecological factors in animals. Numerous studies, however, have demonstrated that the following likely influence monogamy: 1 spatial and temporal distribution of females, 2 parental care costs and benefits, 3 offspring need, 4 infanticide, 5 costs and benefits of multiple mating, 7 mate competition, 8 paternity assurance, 9 the potential for mate guarding, and 10 resource use Figure 1.
This is a broad list, and the relative importance of each factor likely varies across systems. Additionally, the factors above are likely to interact. For example, ecology can affect mate distribution and offspring need; infanticide can affect offspring need; costs of parental care will influence mate availability, which will in turn affect benefits of mate searching.
Additional research on such interactions warrants further attention discussed below. I suggest that there are two primary areas of research that are needed to more fully understand monogamy from an ultimate perspective. First, we need to recognize that the factors that promote the origin vs. Within animals, it will be important to better understand if the factors that promote the origin of monogamy are the same or different than those that promote the maintenance of monogamy. Second, it will be critical to better understand how life-history, ecological, and mating factors interact to influence monogamy.
As mentioned earlier, monogamous mating dynamics can create selective pressures that influence evolutionary trajectories; for example, West found that the evolution of large brain size is associated with social but not genetic monogamy in birds. Further, previous work has found that mate guarding, male attractiveness, and paternity can interact in complicated ways to influence social monogamy Kokko and Morrell, Recent studies have begun to focus on the interplay between such factors, but there is still more work to do in identifying the interactions rather than the individual factors that lead to the origin and maintenance of monogamy.
Focusing on such interactions will likely also be necessary to better understand how monogamy varies within a population and through time. Indeed, understanding variation in the propensity to be monogamous within and across individuals of a population has received relatively little attention from an ultimate perspective and warrants future attention.
The author confirms being the sole contributor of this work and approved it for publication. The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
I am grateful to Dylan Hackett for his discussion and research on the ideas discussed herein. Hunt holds a B. She is also a professional dog trainer. By using the site, you agree to the uses of cookies and other technology as outlined in our Policy, and to our Terms of Use.
Why Monogamy? Social Monogamy Social monogamy is much more common than sexual monogamy. Sexual Monogamy Sexual monogamy is rare. How to Tell the Sex of a Canadian Goose. The Mating Behaviors of the Golden Eagle. What Color Is a Wasp?
The parasitic female may be monogamous, but she is "stealing" parental investment from another pair. Therefore the situation is not one in which mated pairs rear only their own offspring, as traditional use of the term monogamy has implied.
Second, a few recent studies employing new techniques of genetic analysis have allowed investigators to determine whether one or both members of a pair are the parents of all of the nestlings or fledglings they are rearing.
Because so few species have been investigated using this technique, the results of future analyses may lead to a further reevaluation of the evolutionary significance of monogamy. At the moment it is perhaps best simply to consider monogamy as a social pattern in which one male and one female associate during the breeding season, and not to make too many assumptions about fidelity or parentage. Ehrlich, David S.
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