An explanation for the evolution of male monogamy in mammals has been highly debated among researchers. In a new study, evidence suggests male monogamy emerged as a way for animals to guard their mates as females were more spread out across a region.
According to Dieter Lukas and Tim Clutton-Brock, from the University of Cambridge, the theory of the evolution of male monogamy in mammals has two main schools of thought. Monogamy offers few biological advantages as choosing just one female mate, with a gestation period that could last weeks or months, would limit the ability of a male to produce as many offspring as possible.
The first theory for male monogamy involves the ability of two parents to provide better care for their offspring, thus improving its chance of survival. The second theory states male monogamy allows for a male to protect his mate as there are fewer females within a region, and the spreading out of a population reduces the number of potential mates.
As the researchers explain, male mammals follow the lead of their mates as females become solitary and spread out in order to secure resources for their, and their offspring’s, survival. To determine the benefit of pairing in mammals, Lukas and Clutton-Brock studied 2,500 mammal species and created genetic tree maps to study relationships. These maps, called phylogenies, help researchers trace the different relationships between species and determine when breeding patterns changed in their ancestors. The study was published in the journal Science.
It took the researchers two years to acquire the information for the study and Lukas said, “We searched for information for every known mammal (there are 5,400 species), consulting multiple sources, and we discussed our classification with researchers who are conducting field studies.”
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Dividing the different species into three groups, solitary, socially monogamous or group-living, the researchers attempted to make species that were solitary or group-living become socially monogamous. In the phylogenies, the researchers evaluated which method worked the best.
The researchers were able to determine 61 methods that would explain why a solitary or group-living species would become socially monogamous. Out of all the methods, 60 involved having a solitary female ancestor, indicating these ancestors had established a “home range” that would reduce competition for resources with other females, note the researchers. Because females were so spread out in a region, males could not cover such a large terrain to protect multiple mates and had no choice but to become monogamous.
According to the researchers, the second theory surrounding mammal monogamy, having two parents to ensure a healthier offspring, was a secondary benefit that was a later adaptation. Future research could focus on determining the time and circumstances for the switch in breeding behavior. “One next step would be to test whether similar ecological pressures were involved in the evolution of social monogamy in other taxonomic groups,” says Lukas. For humans, the evolution of monogamy is unclear, but researchers could use phylogenies to determine at what point in history humans chose to become monogamous. Lukas said, “It could potentially have evolved during a stage in which ecological factors led to females becoming separated and solitary, but it is also possible that monogamy is a very recent, cultural arrangement of marriage within groups."