Many solitary marsupials have long had complex social lives

Marsupials may have a richer social life than previously thought.

Packed animals are generally considered solitary variety of social relationships that have not been identified, a new analysis published October 26 in Proceedings of the Royal Society b She suggests. The findings could have implications for how scientists think about the lifestyles of early mammals.

“These findings are useful for moving us away from the linear thinking that has existed in some parts of evolution theory, which is that species evolve from supposedly simple forms to more complex forms,” ​​says Dieter Lucas, an evolutionary ecologist at the Max Planck Institute for Evolution. anthropologist in Leipzig, Germany, who was not involved in the study.

Mammals run the gamut of systems of social organization, from loose, ephemeral interactions such as Jaguar aggregates in the wetlands of South America to subterranean plant-like communities Naked mole rat (SN: 10/13/21; SN: 10/20/20).

But marsupials—a subgroup of mammals that give birth to relatively underdeveloped young that are kept in pouches—have traditionally been considered largely solitary. Some species of kangaroos are known to form transient or permanent groups of dozens of individuals. Among marsupials, however, long-term bonds between males and females were thought to be rare and there were no known examples of group members cooperating to raise the young. previous job Based on the patterns of mammalian social evolution, about 90 percent of the marsupial species examined were considered solitary.

If you look at the other [studies] About some specific species, you will see [the researchers] “You tend to assume that marsupials are solitary,” says Jingyu Qiu, a behavioral ecologist at the National Center for Scientific Research in Strasbourg, France.

Sort out social life

Qiu and her colleagues developed a database of field studies that shed light on marsupial social organization, taking into account how populations differ within a species and delving into the evolutionary history of marsupial social life. The researchers combined data from 120 studies on 149 populations of 65 marsupial species, classifying each group as solitary, living in pairs – as one male and female – or falling into four types of living groups, including one male and several females (or vice versa). , plurality of males and females, or single-sex groups.

While 19 species, or 31 percent of those studied, appear to act entirely solitary, nearly half of the species always live in pairs or groups. The team also found a lot of differences within species; Twenty-seven of the 65 species — more than 40 percent — fell into multiple social regulatory classifications.

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When the researchers looked at this social disparity against climatic conditions in Australia, they found that social disparity was more common in drier environments with less predictable rainfall. The ability to switch between individual and group lives can act as a buffer against the unpredictability of resources.

The researchers’ focus on social resilience “highlights that there is nothing even simple about a supposedly solitary species,” says Lucas.

Implications for the oldest mammals

Keough and her colleagues also conducted computer analyzes comparing the marsupials’ evolutionary relationships to how social relationships were formed. This allowed the team to predict the social organization of the earliest marsupials, which split off from placental mammals about 160 million years ago. Since modern marsupials were considered solitary, it has generally been assumed that the ancestors of marsupials—and older mammals in general—were also solitary.

The team found that solitary was the most likely ancestral social group for marsupials, a probability of 35 percent. But Qiu points out that various combinations where married and group lives are possible options make up the other 65 percent. So, “the progenitor was also likely to be non-isolated,” she says. The findings also provide insight into the range of possible lifestyles that early mammals led, she says.

But Robert Foss, a mammalogist at the American Museum of Natural History in New York City, questions the analyzes’ insights into possible social ancestors of the marsupial. He says the uncertainty about the solitary alternative is largely due to the researchers’ standards for what does and does not constitute prosocial behavior—thresholds that Voss considers too lenient. For example, Voss disagrees with the team’s characterization of opossum social organization.

Anecdotal notes from [members of the same species] Sometimes possums together are not a definitive proof of social behavior,” says Foss. “None of the studies cited indicate that opossums are anything other than solitary.”

Qiu says future work will include collecting data on a larger subgroup of mammals outside of marsupials to get a clearer picture of how social traits evolve among mammals.

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