two tyrannosaurus rexes fighting illustration two tyrannosaurus rexes fighting illustration Rather than politely asking these T-Rexes to break up the fight, mammals in their midst simply slept it off. (Photo: metha1819/Shutterstock)

If not for dinosaur extinction, we might all be nocturnal

Scientists reveal that ancient mammals may have spent their days asleep – and hiding out from dinosaurs.

When dinosaurs and early mammals coexisted, it wasn't exactly harmonious. In fact, new research suggests that ancient species remained nocturnal for many years to avoid running into the notoriously fierce reptiles, and then switched to a daytime schedule, pointing to a major transition in early animal behavior millions of years ago.

It's all part of a study conducted by researchers at Tel Aviv University in Israel and the University College London, published in the journal Nature Ecology & Evolution. It's more evidence of a long-standing concept known as the nocturnal bottleneck hypothesis.

These early mammals, which include gorillas, gibbons and tamarins, were nocturnal while dinosaurs roamed the Earth. Then, after a very long (millions of years!) period of mixed day- and nighttime activity, research shows mammals eventually gave up being nocturnal altogether. This, to be clear, was all long before humans existed.

Emperor tamarin This emperor tamarin monkey may have had night owls for ancestors. (Photo: Flavio~ / Flickr)

"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," explained lead author Roi Maor, a Ph.D student at Tel Aviv University and University College London.

The scientists used computer algorithms to reconstruct activity patterns in these mammals. They also used existing findings of visual adaptations over time to support their hypothesis. Namely, they compared the visual acuity and color perception of ancient primates to those of diurnal reptiles and birds that lived in or near the same time period, and found several similarities.

While the scientists can't actually determine why so many mammal species were night owls during the Mezozoic Era, which is the period between 252 million and 66 million years ago, they do have fascinating new evidence that many of the ancestors of modern mammals were nocturnal when dinosaurs existed, and then switched to diurnal after they went extinct.

"We analyzed a lot of data on the behavior and ancestry of living animals for two reasons – firstly, because the fossil record from that era is very limited and secondly, behavior as a trait is very hard to infer from fossils," said Professor Tamar Dayan of Tel Aviv University, a study co-author.

The research was conducted as part of a new initiative out of the Steinhardt Museum of Natural History, scheduled to open soon at Tel Aviv University.

The Steinhardt Museum of Natural History building. The Steinhardt Museum of Natural History, seen here recently still under construction, will open later this year on the University of Tel Aviv campus in Tel Aviv, Israel. (Photo: Zach Pontz)


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If not for dinosaur extinction, we might all be nocturnal
A new study by scientists at Tel Aviv University in Israel and the University College London shows that mammals weren't always awake during the day.