For decades, the scientific consensus held that the earliest placental mammals—small, shrew-like creatures that emerged approximately 160 million years ago—were solitary animals. Because these ancestors shared physical traits with modern shrews, researchers assumed they mirrored the solitary behavior typically attributed to those species. However, a comprehensive new study suggests this long-standing assumption was incorrect.

A research team conducted a decade-long analysis of over 15,000 scientific publications to categorize the social structures of living mammals. By applying statistical modeling to this data, which included 806 extant species, researchers reconstructed the social habits of early mammalian ancestors. The findings indicate that these early creatures were not strictly solitary; instead, they frequently engaged in stable, female-male pair-living.

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The study reveals that social flexibility is an ancient trait rather than a recent evolutionary development. While early mammals were not locked into a single social system, evidence suggests that pair-living was a common strategy for the ancestors of various placental groups. By 70 million years ago, this behavior had become the dominant social structure for lineages including primates and even-toed ungulates.

Following the mass extinction event 66 million years ago, the disappearance of large predators allowed surviving mammals to increase in size and evolve more complex social structures. This transition eventually led to the group-living behaviors observed in most modern mammals today. The researchers conclude that the focus of future study should shift from questioning how pair-living evolved to understanding how mammals developed the tolerance necessary for group living.

Source: The Conversation