How our small mammal ancestors survived when dinosaurs ruled the Earth.

The Kayentatherium, a Mesozoic-era mammal, pictured in this illustration with a pair of Dilophosaurus, background, and the prehistoric turtle Kayentachelys, foreground. A new study suggests that mammals, which were largely nocturnal during the age of dinosaurs, became more active in daytime shortly after the dinosaurs went extinct. CreditMark Witton 

New York Times: After the Dinosaurs’ Demise, Many Mammals Seized the Day

When the dinosaurs are away, the mammals will play.

That’s the basic idea behind the “nocturnal bottleneck” hypothesis, a concept proposed in 1942 that suggested mammals could have only survived in a dinosaur-dominated world by avoiding the sharp-toothed beasts during the day and coming out at night.

Mammalogists have long thought the earliest shared ancestor of all mammals was nocturnal, and now a study published Monday provides a potential date for when the furry creatures stopped cowering in the shadows and started venturing into the daylight.

The researchers found that the first mammals to be active during both day and night appeared around 65.8 million years ago, just 200,000 years after the extinction event that led to the demise of most dinosaurs. They were most likely the ancestors of even-toed ungulates, like today’s cattle, llamas and hippopotamuses, as well as the cetaceans like whales and dolphins.

“In evolutionary time 200,000 years are hardly anything. It’s almost immediately,” said Roi Maor, a doctoral student at Tel Aviv University in Israel and University College London, and lead author of the paper that appeared in Nature Ecology & Evolution.

They also found the first mammals that were clearly diurnal, or only active during the daytime, appeared about 52.4 million years ago, some 13 million years after the dinosaurs died out. Among these mammals were early monkeys and apes, the ancestors of today’s gorillas, gibbons and humans.

Mr. Maor cautioned that his study only showed a correlation, not causation, between when nonavian dinosaurs went extinct and when mammals became daytime creatures. But the finding adds support for the 75-year-old hypothesis describing how our ancestors inherited the day after dinosaurs disappeared.

Using a computer program, Mr. Maor and his colleagues plugged in behavioral data from 2,415 mammal species that noted whether the species was nocturnal, diurnal, or cathemeral (irregularly active at day or night). The analysis also evaluated ancestry information that showed how closely related the species were to one another.

“Think of it as an atlas. We show all the species alive today and each one of their ancestors is mapped onto that road map,” he said. “Our algorithm told us whether or not their ancestors were diurnal or nocturnal.”

It showed the expected behavioral patterns of ancestors going back at least 166 million years ago during the Mesozoic, and highlighted a shift from nocturnal to daylight activity among some mammalian ancestors after about 66 million years ago, when calamity struck the planet.

Their data set represented 91 percent of all mammal families. About 60 percent were nocturnal, like the vampire bat, the fennec foxand the four-toed hedgehog, and 26 percent were diurnal like the eastern gray squirrel, giraffes and humans. Most of the rest were characterized as cathemeral like the star-nosed mole, the European rabbit and the muskrat.

Lars Schmitz, an evolutionary biologist from The Claremont Colleges, in California who was not involved in the study, said he was excited to see such a large comparative study, and that the study’s finding of when mammals emerged in the daylight supports the nocturnal bottleneck hypothesis.

“This seems to support what most people have been saying, that mammals were predominantly in the nocturnal niche and could not expand into the daytime until the dinosaurs were gone,” he said.

But he noted that a limitation of the work is that it only includes living species and not extinct ones. Without knowing their behavior, the researchers may be missing some important clues about when mammals first carpe diem-ed.