"Long before it's in the papers"
January 27, 2015


Did monogamy arise to prevent infanticide?

July 29, 2013
Courtesy of PNAS
and World Science staff

Mo­nog­a­my, the pair­ing of a male and female to mate and raise off­spring, may have evolved among hu­man an­ces­tors to re­duce the threat of in­fanti­cide, a study sug­gests. 

Mo­nog­a­my is rare among mam­mals, and the fac­tors that fa­vored its ev­o­lu­tion re­main un­clear. But in­fanti­cide, the kill­ing of in­fants, is wide­spread among pri­ma­tes, the ev­o­lu­tion­ary line­age that in­cludes mon­keys, apes and hu­mans.

In the stu­dy, Chris­to­pher Opie of Uni­vers­ity Col­lege Lon­don and col­leagues used sta­tis­ti­cal meth­ods to as­sess three com­pet­ing sce­nar­i­os pre­vi­ously pro­posed to ex­plain why mo­nog­a­my evolved. 

The sce­nar­i­os are: a male forms a pair to pre­vent the female from mat­ing with ri­val ma­les, there­by im­prov­ing his chances of re­pro­duc­ing; a pa­ter­nal con­tri­bu­tion to the care of off­spring im­proves the re­pro­duc­tive suc­cess of the pair; or a paired male pro­tects his off­spring from oth­er ma­les, who may kill an un­re­lat­ed in­fant to im­prove their chances of re­pro­duc­ing with the moth­er.

Us­ing trait da­ta from 230 spe­cies of pri­ma­tes, the sci­en­tists tested for cor­rela­t­ions be­tween the ev­o­lu­tion of mo­nog­a­my and a “mark­er trait” for each hy­poth­e­sis. The traits are female rang­ing pat­terns, pa­ter­nal care, and in­fanti­cide by ma­les, re­spec­tive­ly. 

The re­search­ers found that each trait “co-e­volved” with mo­nog­a­my, but that only male in­fanti­cide ap­peared be­fore the emer­gence of pair liv­ing.

“The most com­pel­ling ex­plana­t­ion for the ap­pear­ance of mo­nog­a­my is male in­fanti­cide. It is only the pres­ence of in­fanti­cide that re­liably in­creases the prob­a­bil­ity of a shift to so­cial mo­nog­a­my,” they wrote. The find­ings are pub­lished in this week’s early on­line is­sue of the jour­nal Pro­ceed­ings of the Na­tio­n­al Aca­de­my of Sci­en­ces.

Once es­tab­lished, the authors added, pair liv­ing may have fa­cil­i­tat­ed the emer­gence of “mate-guarding” and pa­ter­nal care.

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Monogamy, the pairing of a male and female to mate and raise offspring, may have evolved among human ancestors to reduce the threat of infanticide, a study suggests. Monogamy is rare among mammals, and the factors that favored its evolution remain unclear. But infanticide, the killing of infants, is widespread among primates, the evolutionary lineage that includes monkeys, apes and humans. In the study, Christopher Opie of University College London and colleagues used statistical methods to assess three competing scenarios previously proposed to explain why monogamy evolved. The scenarios are: a male forms a pair to prevent the female from mating with rival males, thereby improving his chances of reproducing; a paternal contribution to the care of offspring improves the reproductive success of the pair; or a paired male protects his offspring from other males, who may kill an unrelated infant to improve their chances of reproducing with the mother. Using trait data from 230 species of primates, the scientists tested for correlations between the evolution of monogamy and a marker trait for each hypothesis. The traits are female ranging patterns, paternal care, and infanticide by males, respectively. The researchers found that each trait “co-evolved” with monogamy, but that only male infanticide appeared before the emergence of pair living. “The most compelling explanation for the appearance of monogamy is male infanticide. It is only the presence of infanticide that reliably increases the probability of a shift to social monogamy,” they wrote. The findings are published in this week’s early online issue of the journal pnas. According to the authors, the findings suggest that monogamy may have evolved in primates due to the threat of male infanticide, and once established, pair living may have facilitated the emergence of “mate-guarding” and paternal care.