What is it about?
For decades, we have pictured the early mammals living on the shadows of the dinosaurs as solitary secretive animals. According to this view, more complex forms of social organisation evolved only after the extinction of the dinosaurs. Our research challenges this long-held assumption. While most early mammals were indeed solitary, about one quarter appear to have lived in female–male pairs. We reached this conclusion after compiling a high-quality database from thousands of scientific publications, which was then analysed using modern comparative statistical methods to reconstruct the social organisation of ancestral mammals. Our findings are consistent with fossil evidence indicating that early mammals and even ancestors of mammals were sonetimes social and pair-living.
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Why is it important?
Our findings challenge the traditional image of early mammals as universally solitary animals. They suggest that the roots of mammalian sociality—and ultimately some aspects of human social behaviour—extend much deeper into evolutionary history than previously thought. Our research transforms the understanding of how our own ancestors lived more than 70 million years ago in the age of the dinosaurs: Rather than being exclusively solitary, they regularly formed stable female–male pairs. The results also indicate that early mammals were not locked into one single social system. Like many modern mammals, they showed intra-specific variation in social organisation. For decades, researchers have focused on explaining how pair-living evolved in mammals. Our findings suggest that pair-living may instead be an ancient mammalian trait. This shifts attention to two different questions: 1. how did larger social groups evolve, and 2. how was it possible for multiple females and multiple males to live together while managing competition and conflict?
Perspectives
This was a huge long-term project for us. It started with data collection in 2015, which was only finished in 2022 after going through roughly 14 000 publications. At least during the Corona years it allowed to work in a dispersed worldwide group with students from many countries and from three continents. The results were surprising for myself and changed the way I see mammalian and human evolution. I would never have thought that pair-living is so ancestral. But when thinking about it, it makes sense: Early mammals were in their ecology and life history very similar to birds, and birds are most often pair living,
Carsten Schradin
Institut Pluridisciplinaire Hubert Curien
Read the Original
This page is a summary of: Pair living emerged early in placental mammals, Proceedings of the National Academy of Sciences, July 2026, Proceedings of the National Academy of Sciences,
DOI: 10.1073/pnas.2528458123.
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