A Far-Reaching Analysis?
While the study offers insight into the evolution of monogamy, the results are highly dependent on how the researchers classified the various species of primates, said Eduardo Fernandez-Duque, an associate professor of anthropology at the University of Pennsylvania in Philadelphia, who was not involved in the new study.
Fernandez-Duque, who has studied monogamy and paternal care in primates for 20 years, noted some inconsistencies in the descriptions of a few of the species, such as the classification that some primates in the genus Callicebus are sexually monogamous but not socially monogamous (they don't stay together to raise the offspring, for instance).
In addition, "the researchers treat infanticide as binary, which makes me a little uncomfortable," Fernandez-Duque told LiveScience. "For example, they categorize infanticide as high or low, but there's no room for species that don't show infanticide."
Still, Fernandez-Duque says the research represents exciting progress in the field of primatology, and he hopes to look deeper into the data.
Tracing the Evolution of Monogamy
Another study, this one detailed today in the journal Science, suggests monogamy evolved to allow males to protect females.
Using a new genetic classification technique, the researchers of the new study inferred how species were related and when they split off from one another in the evolutionary tree. The scientists classified each species as solitary (living alone), socially monogamous (living in breeding pairs) or as group-living. A total of 2,500 mammalian species were involved. (The Wild Kingdom: Take Our Animal Sex Quiz)
Then the scientists simulated how solitary females might evolve social monogamy versus how group-living females might evolve the trait. Researchers used sophisticated statistical methods to determine which scenarios were more likely.
Social monogamy evolved 61 times among the animals studied, the analysis showed. All but one of these transitions involved solitary females, rather than group-living females. In addition, the common ancestor of all mammals was solitary.
The findings suggest that for species in which females lived alone in large territories to avoid competition for food and other resources, males were unable to defend multiple females, and therefore became monogamous.
"In mammals, social monogamy is the result of resource distribution," study researcher Dieter Lukas, of the University of Cambridge, said in a press briefing today. Females were limited by the distribution of food, and males were limited by the distribution of females, Lukas said.
Social monogamy was also more common among primates and carnivores than other species, the study found. The more specialized diets of these animals may have increased competition for food, leading females to isolate themselves.
The findings failed to support the idea that the risk of infanticide led to monogamy in mammals, even in primates. The researchers suggest the discrepancy between the two studies could be explained by differences in how group-living is classified. For instance, some species that Opie's team classified as group-living were classified by Lukas as socially monogamous. Alternatively, the smaller sample of animals in Opie's study could have skewed their findings, Lukas and his colleagues said.
However, both studies found that parental care was more likely a consequence, not a cause, of the evolution of monogamy.
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