Primate Social Monogamy

Based on our classwork material, we currently believe that one of the main causes of monogamous relationships in mammals is the male necessity of protecting a female mate from other males, thus ensuring that the offspring is his – in doing so, males ensure that they pass on their genes, thus increasing their reproductive fitness.  Another reason is the need for biparental care.  That is, if the mother cannot take care of the offspring herself because its needs surpass her capacity to hold or feed it, monogamous relationships form to ensure the survival of the offspring and thus the survival of both of the parent’s genes. Maternal care usually manifests itself in two ways: feeding and teaching life skills.  In providing this care, loss of time that could be used for reproduction as well as vulnerability and high energy expenditures are consequences of providing care.  When this requires too much energy, monogamous relationships in mammals are most advantageous. Males, by supplying food and other care for offspring increases their reproductive fitness by forming a socially monogamous relationship with a single female. An example of this is the male Emperor penguin, which incubates the egg for a period of time while the mother fishes.

These articles indicate different causes for monogamous relationships, specifically in primates.  Primates, like many other mammals, give parental care to their offspring.  The article “Male infanticide leads to social monogamy in primates” written by Christopher Opie, Quentin D. Atkinson, Robin I. M. Dunbar, and Susanne Shultz, claims that primates have developed social monogamy to defend primate infants from infanticidal males.  To prove this point, Opie, Atkinson, Dunbar, and Shultz, claim that when lactation is longer than gestation, as in primates, there is correlated evolution between monogamy and paternal care, female ranging patterns, and male infanticide. They found that male infanticide, evolutionarily speaking, precedes social monogamy and that stability is inherent to social monogamy with low infanticide rates. Socially monogamous species have relatively low infanticide rates because monogamy facilitates a shorter lactation period as compared to the gestation period, indicating an evolutionary advantage to the survival of offspring with biparental care. The evolution of female ranging patterns and paternal care came after the development of social monogamy, so these can be ruled out as causes, claim Opie, Atkinson, Dunbar, and Shultz.  Their research shows that reproductive rates in primates are increased by biparental care because of shortened lactation and the female’s return to state of fertility.  Therefore, male infanticide, according to these authors, remains the prime reason for monogamous relationships in primates.

D. Lukas and T. H. Clutton-Brock provide an alternative to the male infanticide theory in their article “The Evolution of Social Monogamy in Mammals”. In it, Lukas and Clutton- Brock claim that in the common ancestor of all mammalian species, the males had ranges that overlapped several solitary females. From this, different conditions caused some mammals to develop social monogamy.  This article addresses and attempts to disprove theories that paternal care, male infanticide and male defensibility of females are possible causes of monogamy in primates. It argues that there is an underlying cause to all of these results of social monogamy: evolution.  In primates, the current social monogamy seems to be derived from unstable group living, characterized by intense feeding competition between females, the intolerance of breeding females toward other female, and low population density. Under Clutton-Brock and Lukas’s interpretation, it was more advantageous, given these conditions, for males to guard and mate individual females. However, this article leaves several alternatives open as possibilities for the development of social monogamy; depending on the ancestry of the species, different conditions could have arose to precede social monogamy.

I think “The Evolution of Social Monogamy” by Lukas and Clutton-Brock provides a more convincing scientific argument, mainly because of the extensive statistical analysis done on nearly every possible cause of social monogamy.  By including the calculations, sample sizes, Z scores, and specific probabilities, they effectively provide evidence for their claim interspersed throughout the article. They were very clear about phylogenetic categorization of species and specific groupings that allowed for statistical proofs. It should be acknowledged, however, that both Lukas and Clutton-Brock and Opie, Atkinson, Dunbar and Shultz used Bayesian framework to verify their data and that Shultz et al. also provided some statistical analysis pertaining to male infanticide. Both teams agree through empirical data, that paternal care was a development, not a cause of social monogamy and thus it could not have preceded monogamy in primates. If the number of references is an indication of credibility, Lukas and Clutton-Brock have a list of forty-four references, while Shultz et al. cites thirty-one. What really seems to make Lukas and Clutton-Brock’s argument the strongest is the meticulous research in all possible causes of social monogamy; Shultz et al. does extensive analysis, but most of it is focused on male infanticide. As far as accuracy, Lukas and Clutton-Brock directly acknowledged that possible interpretations can range based on the classification of social systems, which helps explain why Shultz et al came to a different conclusion.

Having had only limited knowledge about social monogamy in primates, I found it interesting that there were so more possible causes for this type of relationship. I never knew the extent of the theories behind the development of social monogamy. I also did not realize that based on evolutionary and phylogenetic evidence, possible causes of monogamy may be proved or discarded through statistical calculations, or that this type of research would be so empirically supported.  Monogamy may have evolved for reasons much less obvious than the ones I had known- for the protection of a mate and care of an offspring. I was surprised to read that the presence or lack of a long term mate could effect lactation and gestation.  I was most surprised by the male infanticide theory, that other males would know to kill offspring so that they might have an opportunity to mate with the female. Primates seem to be unbelievably complex in their relationships and behaviors.


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