A New Day for Mammals

Artist’s impression of Kayentatherium (mammal) with Dilophosaurus (dinosaurs) in the background and Kayentachelys (turtle) in the foreground. Recent analyses suggest that mammals such as kayentatherium switched from being nocturnal to diurnal after the dinosaurs became extinct.

Mark Witton

For millions of years, dinosaurs likely dominated the day, while prehistoric mammals kept to the protective shadows of the night. According to the longstanding “nocturnal bottleneck” hypothesis, mammals only expanded into daytime living after nonavian dinosaurs disappeared, roughly 66 million years ago. Support for this hypothesis is hard to come by, as reading clues from fossils, such as the orientation of eyes in a skull, can be unreliable. Recently, researchers have used the behavior of modern living mammals and our knowledge of their evolutionary tree to infer changes in the behavior of long-extinct mammals. 

Ecologist Roi Maor and collaborators at Tel Aviv University, Israel, and University College London, UK gathered data from over 2,400 animal species, representing all living orders of mammals and nearly 45 percent of all known mammal species. Each species’ behavior was classified as being diurnal (mostly active during the day), nocturnal (mostly active at night), or cathemeral (active both in the day and at night). The team next mapped those behaviors onto two possible versions of the mammalian family tree that traces the evolutionary history of mammals. Finally, they used a computer model to extrapolate back in time the likely behaviors of the prehistoric mammals that evolved into modern-day mammals. 

This approach allowed the researchers to estimate the rates by which mammals switched their schedules, such as when a cathemeral or nocturnal species became diurnal. Under both versions of mammalian evolutionary history, the model showed that ancient mammals started out nocturnal and that strictly daytime-dwelling mammals did not appear until after the nonavian dinosaurs died out. “We were very surprised to find such close correlation between the disappearance of dinosaurs and the beginning of daytime activity in mammals,” said Maor. The team’s models support the nocturnal bottleneck hypothesis and helps explain why many living diurnal mammals possess special adaptations to seeing at night—they may not need them today, but such adaptations helped their evolutionary ancestors survive in the time of the dinosaurs. (Nature Ecology and Evolution