|According to a new study, many marsupials, such as the squirrel glider (Petaurus norfolcensis) shown here, can live in a variety of social groups, such as a mating pair or a larger group of males and females. GETTY/SYLVAIN CORDIER/STOCKBYTE|
Marsupials' social lives may be more varied than previously thought.
Despite their reputation as loners, pouched animals have a wide range of social relationships that have gone unnoticed, according to a new study published on October 26 in Proceedings of the Royal Society B. The findings could change the way scientists think about early mammal lifestyles.
"These findings help to move us away from the linear thinking that used to exist in some parts of evolutionary theory, that species develop from supposedly simple forms into more complex forms," says Dieter Lukas, an evolutionary ecologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, who was not involved in the study.
The evolution of the marsupial social organization, J. Qiu et al. doi: 10.1098/rspb.2022.1589
Mammals have a wide range of social organization systems, from loose, ephemeral interactions like jaguar aggregations in South American wetlands to antlike subterranean societies like naked mole rats (SN: 101321; SN: 102020).
However, marsupials — a subgroup of mammals that gives birth to relatively immature young reared in pouches — have traditionally been thought to be largely solitary. Some kangaroo species have been observed to form temporary or permanent groups of dozens of individuals. Long-term bonds between males and females were thought to be rare in marsupials, and there were no known examples of group members cooperating to raise young. Previous research on mammalian social evolution patterns assumed that roughly 90% of marsupial species studied were solitary.
"If you look at other [studies] on specific species, you will notice that [researchers] tend to assume that the marsupials are solitary," says Jingyu Qiu, a behavioral ecologist at CNRS in Strasbourg, France.
Sorting social lives
Qiu and her colleagues created a database of field studies that shed light on marsupial social organization, accounting for how populations differ within species and delving into the evolutionary history of marsupial social lives. The researchers compiled data from 120 studies on 149 populations of 65 marsupial species, classifying them as solitary, living in pairs — such as one male and one female — or falling into one of four types of group living, including one male and multiple females (or vice versa), multiple males and females, or single sex groups.
While 19 species, or 31% of those studied, appear to live solely on their own, nearly half of the species always live in pairs or groups. The researchers also discovered a lot of variation within species; 27 of the 65 species, or more than 40%, fit into multiple social organization classifications.
When the researchers compared this social variation to Australia's climatic conditions, they discovered that social variability was more common in drier environments with less predictable rainfall. It's possible that the ability to switch between solitary and group living acts as a buffer against resource volatility.
According to Lukas, the researchers' emphasis on social flexibility "highlights that there is nothing simple even about a supposedly solitary species."
Implications for the earliest mammals
Qiu and her colleagues also conducted computer analyses comparing marsupial evolutionary relationships to how they form social relationships. This allowed the researchers to forecast the social organization of the first marsupials, which diverged from placental mammals around 160 million years ago. Because modern marsupials have been assumed to be solitary, the marsupial ancestors — and the earliest mammals in general — have also been assumed to be solitary.
The team discovered that solitary was the most likely social category of the ancestral marsupials, with a 35% chance. However, Qiu points out that the other 65 percent is made up of various combinations of pair and group living options. As a result, she believes "it is more likely that the ancestor was also non-solitary." According to her, the findings also provide insight into the range of possible lifestyles experienced by the earliest mammals.
However, Robert Voss, a mammalogist at the American Museum of Natural History in New York City, questions the findings regarding a potentially social ancestral marsupial. The researchers' thresholds for what constitutes and does not constitute social behavior, he claims, are largely to blame for the uncertainty surrounding the solitary alternative. Voss, for example, disagrees with the team's description of opossum social organization.
According to Voss, "anecdotal observations of [members of the same species] occasionally denning together is not compelling evidence for social behavior." "None of the studies cited indicate that opossums are anything other than solitary."
Qiu plans to collect data on a larger subset of mammals other than marsupials in the future to gain a better understanding of how social traits have evolved among mammals.