Abstract
Behavioral ecologists, evolutionary biologists, and anthropologists have been long fascinated by the existence of in the animal kingdom. Multiple studies have explored the factors underlying its evolution and maintenance, sometimes with contradicting and contentious conclusions. These studies have been plagued by a persistent use of fuzzy terminology that often leads to researchers comparing “apples with oranges” (e.g., comparing a grouping pattern or social organization with a sexual or genetic mating system). In this review, we provide an overview of research on in mammals generally and primates in particular, and we discuss a number of problems that complicate comparative attempts to understand this issue. We first highlight why the muddled terminology has hindered our understanding of both a rare social organization and a rare mating system. Then, following a short overview of the main hypotheses explaining the evolution of pair-living and sexual monogamy, we critically discuss various claims about the principal drivers of that have been made in several recent comparative studies. We stress the importance of using only high quality and comparable data. We then propose that a productive way to frame and dissect the different components of pair-living and sexual or genetic monogamy is by considering the behavioral and evolutionary implications of those components from the perspectives of all participants in a species’ social system. In particular, we highlight the importance of integrating the perspective of floater individuals and considering their impacts on local operational sex ratios, competition, and variance in reproductive success across a population. We stress that pair-living need not imply a reduced importance of intrasexual mate competition, a situation that may have implications for the sexual selection potential that have not yet been fully explored. Finally, we note that there is no reason to assume that different taxa and lineages, even within the same radiation, should follow the same pathway to, or share a unifying evolutionary explanation. The study of the evolution of pair-living, sexual monogamy, and genetic monogamy remains a challenging and exciting area of research.
Highlights
Behavioral ecologists, evolutionary biologists, and anthropologists have been fascinated by the existence of “monogamy” in the animal kingdom
The hypothesis assumes that females losing their offspring become receptive sooner than if they continue nursing current offspring. It assumes that a female’s partner is the likely sire of her offspring and that the presence of a male increases both his and the female’s reproductive success by reducing the success rate of potentially infanticidal intruders. While this hypothesis proposes a selective pressure acting on the social organization, the degree of pair-bonding and the sexual mating system (Tecot et al, 2016), it more strongly relates to pair-living than to sexual monogamy in otherwise group-living species, since one of the assumed counter-strategies of females against infanticide is to mate with multiple males (e.g., Chakrabarti and Jhala, 2019)
We have tentatively suggested in the section on proposed terminology that, where possible, researchers should seek to abandon categorizations of potentially continuous variables. It needs to be explored in more detail how this could be best approached, because unthinking use of measurable values might create new problems, and it has been suggested that using continuous data can sometimes lead to a false perception of continuity (Rubenstein et al, 2016). Both in our work and when writing the sections above, our premise has been that evolutionary biologists attempting to understand the evolution of any aspect of an animal social system are primarily interested in two main topics
Summary
Behavioral ecologists, evolutionary biologists, and anthropologists have been fascinated by the existence of “monogamy” in the animal kingdom. The amount of time and energy invested in offspring is conspicuously sex-biased, with females often investing more in each reproductive opportunity than males This is especially true among mammals, where internal fertilization, gestation, and lactation, almost de facto, demand greater investment by females. Because of the multiple concerns we raise with these studies, we close the section with a set of guiding principles that, we argue, should be adhered to in comparative studies of animal social systems (section 4.3) The last of those principles calls for considering the behavioral and evolutionary implications of particular components of a taxon’s social system from the perspectives of all involved participants, not just certain classes of individuals. We conclude with a few suggestions for future research directions (section 6) formulated in light of the critiques of recent comparative studies we raise and the questions that arise from explicitly considering the perspectives of all classes of individuals
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