Marsupials may have richer social lives than previously thought.
Generally considered loners, marsupial animals have a diversity of social relationships that are unknown, according to a new analysis published on October 26. Proceedings of the Royal Society B. suggests The findings could have implications for how scientists think about the lives of early mammals.
“These findings are useful to move us away from the linear thinking that usually exists in some parts of evolutionary theory, that species evolve from simple to more complex forms,” says Dieter Lucas, evolutionary ecologist at the Max Planck Institute for Evolution. Anthropology in Leipzig, Germany, who was not involved in the study.
Mammals are characterized by systems of social organization ranging from loose and ephemeral interventions such as aggregations of jaguars in South American wetlands, to similar subterranean societies of bare mound rats (SN: 10/13/21; SN: 10/20/20).
But marsupials — a subgroup of mammals that give birth to relatively young in pouches — have traditionally been considered largely solitary. Certain species were known to form transient or permanent groups of dozens of individuals. But among marsupials, long-term bonds between males and females were thought to be rare and there were no known examples of group members working together to raise young. Previous work on patterns of social evolution in mammals has shown that about 90 percent of marsupial species examined are solitary.
“If they look at someone else [studies] about some specific species, you will see [the researchers] they tend to be solitary marsupials, says Jingyu Qiu, a humanistic ecologist at the CNRS in Argentorato, France.
Kind of social life
Qiu and his colleagues developed a database of field studies that shed light on marsupial social organization, accounting for population variation within species and encoding the evolutionary history of marsupial social life. The researchers compiled data from 120 studies on 149 populations of 65 marsupials, categorizing solitary individuals, living in pairs, such as one male and one female – or into four types of living groups, including one male and several females (or vice versa) several males and women, or each sex.
While 19 species, or 31 percent of those studied, seem to go strictly alone, almost half of the species always live in pairs or groups. The team also found a lot of variation in species; 27 of 65 species, more than 40 percent — fall into multiple classifications of social organization.
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When the researchers looked at this social variation against climatic conditions in Australia, they found that social variation was more common in drier environments, with less predictable rainfall. It is possible to switch between solitary acts and living groups as a buffer against a random resource.
The researchers’ focus on social flexibility “emphasizes that nothing is simple even about isolated species,” Lucas says.
Effects on early mammals
Qiu and his colleagues are also running computer analyzes comparing the evolutionary relationships of marsupials with how they form social relationships. This team predicts the social organization of the first marsupials, which split from placental mammals about 160 million years ago. Because modern marsupials were thought to be solitary, older marsupials—and early mammals as a whole—were generally thought to be solitary as well.
The team found that solitary was the most likely social category of ancestral marsupials, at 35 percent probability. But Qiu points out that the various combinations in which pairs and groups of animals are possible options make up the other 65 percent. Thus, “it is likely that the ancestor was not isolated either,” he said. The findings also provide insights into the possible ways of life experienced by early mammals, he says.
But Robert Vossius, a mammalogist at the American Museum of Natural History in New York City, analyzes the findings and questions the social power of the marsupial ancestor. Doubts about solitary behavior, he says, are largely due to researchers benching what does and does not constitute social behavior—thresholds that Vossius also feels are permissive. For example, Vossius disagrees with the characterization of the opposite of society.
“Anecdotal observations about” [members of the same species] sometimes the connection does not force documents about social behavior,” says Vossius. “None of the cited studies suggest that opossums are anything other than solitary.”
Future work, Qiu says, will involve collecting data on a larger set of mammals outside of marsupials to get a clearer picture of how social traits have evolved among mammals.
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