The extinction of the dinosaurs facilitated a switch from a nocturnal lifestyle to an increase in daytime activity in early mammals, reports a paper published online this week in Nature Ecology & Evolution.

Today, many mammals are active by day, but mostly lack the characteristics that allow fish, reptiles and birds to be such successful daytime predators: colour vision, for instance. In fact, most mammals have vision more similar to nocturnal reptiles and birds. These observations led to the development of the 'nocturnal bottleneck' theory, which proposes that early mammals had to restrict their activity to the night time to avoid conflict with dinosaurs that were active by day. When the dinosaurs died out, it is proposed, mammals were able to move into the newly available daytime niche.

There has been some support for this hypothesis over the years, but direct evidence has been lacking. Here, Roi Maor and colleagues assembled data on day and night behaviour patterns for nearly all living mammal families and ascertained the origin of these behaviours based on the timing of the evolutionary split of each species. They find that the first mammals were nocturnal, and became occasionally active by day tens of millions of years later. The number of mammals active by day had increased rapidly shortly after the extinction of the dinosaurs. Simian primates, like orangutans and marmosets, have some of the most acute vision of all mammals, which may be due to their ancestors being among the first mammals to switch to a daytime lifestyle.