When the Dinos Went Away, Mammals Came Out (in Daylight) to Play

While it’s challenging to imply one caused the other, a new study shows that mammals came into the light of day soon after the dinos disappeared

Early Mammal
Mark Witton

Paleontologists believe that the first mammals to evolve on Earth were small nocturnal creatures that used a keen sense of smell and hearing to operate in the dark, which was a good place to be in the age of the dinosaurs. These days, many mammal species spend most of their time operating during the daytime, and many other species are crepuscular, which means they do most of their hunting, mating and interacting in the early morning and twilight hours.

But when did mammals make the switch from the night life to day life? Now, reports Gretchen Vogel at Science, a new study pinpoints the time in the distant past when mammals came out of the dark. And it turns out, it’s immediately after the demise of the dinosaurs.

As Vogel reports, paleontologists have had difficulty determining the behavior of ancient animals just by looking at their fossils. Typically, they assume an animal is nocturnal if it has features like large eye sockets and certain configurations of the nasal cavity. But that work is largely speculative and can’t answer the question of when mammals first waddled into the daylight.

In search of answers, a group of researchers from University of College-London and Tel Aviv University worked backward, analyzing the lifestyles and behavior of 2,415 mammal species that exist today, writes George Dvorsky for Gizmodo. Using an algorithm, they were able to reconstruct the likely behavior of their ancestral mammals back to the beginning, when mammals evolved from a reptilian ancestor 220 to 160 million years ago, Agence France-Presse reports.

The researchers used two different variations of the mammalian family tree, according to a press release, but the results were the same. Mammals came into the light between 52 and 33 million years ago. The dinosaurs died out 65 million years ago. The research appears in the journal Nature Ecology & Evolution.

“We were very surprised to find such close correlation between the disappearance of dinosaurs and the beginning of daytime activity in mammals, but we found the same result unanimously using several alternative analyses,” Ph.D students and lead author Roi Maor of Tel Aviv University says in the press release.

As quickly as 200,000 years after the disappearance of the dinosaurs creatures began emerging. For example, the ancestor of camels, hippos and deer may have begun operating at least partially during the daytime or twilight hours, Maor tells Vogel. The ancestors of primates were some of the first mammals to begin living primarily during the day, AFP reports, likely venturing into the light as early as 52 million years ago.

The eyes of primates reflect this early emergence. While most mammal species even today have lots of rods, specialized photo-receptors in the eye that can detect low-level light, they lack a fovea, an area of the retina where focus is strongest and detects color in high light. Humans and primates, however, do have the feature, probably because we’ve had a few extra million years in the sunshine to develop the adaptation.  

The researchers can’t say for certain that there’s causation between the demise of the dinosaurs and the rise of daytime mammals, but they suggest that the end of the thunder lizards reduced the number of predators and opened up many niches in the environment, ending what is sometimes called the nocturnal bottleneck. 

“It’s very difficult to relate behavior changes in mammals that lived so long ago to ecological conditions at the time, so we can’t say that the dinosaurs dying out caused mammals to start being active in the daytime,” co-author and geneticist at University College, London Kate Jones says in the release. “However, we see a clear correlation in our findings.”

While other researchers believe the study was well done, they caution that there are still a lot of gaps in our knowledge of ancient mammalian behavior that an algorithm can’t settle. “[U]ntil we find a way to look at fossils and directly figure out how these extinct animals were behaving, it will still be a prediction,” Stephen Brusatte from the University of Edinburgh, who not involved in the study, tells Dvorsky. “It may be that the end-Cretaceous extinction caused a big shift from nocturnal to diurnal mammals, but it wouldn’t surprise me if some mammals living alongside the dinosaurs were also active during the day and we just haven’t found a good way to determine that yet. That will be the next big step in testing these results.”

The team hopes to do just that, by adding more species the mammalian family tree to help refine their data.