Mammals ventured out for daytime activities after dinosaurs were wiped out

The extinction of the dinosaurs around 66 million years ago gradually caused mammals to change from a nocturnal to diurnal schedule, according to new research published by Tel Aviv University and University College London scientists.

The change did not happen quickly but, rather, involved an intermediate stage of mixed day-andnight activity over millions of years, according to the study, which has just been published in the journal Nature Ecology and Evolution.

Scientists have long thought that the common ancestor to all mammals was active only at night, but the new discovery reveals when mammals started being awake and active in daylight for the first time and provides insight into which species changed their behavior first.

According to the lead author and TAU doctoral student Roi Maor, mammals started being active in the daytime after non-avian dinosaurs were wiped out.

The 33-page study analyzed data of 2,415 different species of mammals that are alive today. The scientists used computer algorithms to reconstruct the likely activity patterns of their ancient ancestors who lived millions of years ago.

Two different mammalian family trees portraying alternative timelines for the evolution of mammals were used in the analysis. The results from both show that mammals switched to daytime activity shortly after the dinosaurs had disappeared. The intermediate stage of mixed day and night activity over millions of years coincided with the events that eliminated the dinosaurs.

“We were very surprised to find such close correlation between the disappearance of dinosaurs and the beginning of diurnal activity in mammals, but we unanimously found the same result using several alternative analyses,” explained Maor, who is also affiliated with UCL.

The team found that the ancestors of simian primates – gorillas, gibbons and tamarins – were among the first to completely forgo nocturnal activity. But the two evolutionary timelines varied.

This discovery fits well with the fact that simian primates are the only mammals that have evolved adaptations to seeing well in daylight. The visual acuity and color perception of simians is comparable to those of diurnal reptiles and birds – groups that never left the daytime niche.

“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. However, we see a clear correlation in our findings,” added co-author, UCL genetics, evolution and environment Prof. Kate Jones.

“We analyzed a lot of data on the behavior and ancestry of living animals because the fossil record from that era is very limited and since behavior as a trait is very hard to infer from fossils,” explained another co-author, Prof. Tamar Dayan, chairman of TAU’s Steinhardt Museum of Natural History. “You have to observe a living mammal to see if it is active at night or in the day. Fossil evidence from mammals often suggest that they were nocturnal even if they were not. Many subsequent adaptations that allow us to live in daylight are in our soft tissues.”

Scientists have been asking for years how mammals have survived in an age when dinosaurs dominated the world – for 150 million years, continued Dayan.

“The mammals were small and not diverse at the time, and inferior in any competition against the dinosaurs. One hypothesis is that the early mammals adapted themselves to nighttime activity, while the dinosaurs, like reptiles in need of solar radiation to warm up and live, were day laborers. Thus the mammals could exist in the same geographical area as the dinosaurs, yet in an entirely different ecological niche – the night environment.

“This hypothesis is based, among other things, on the fact that, to this day, most mammals except for monkeys are still active at night. In fact, primates – which include humans – are the only mammals whose sense of sight is clearly adapted to daylight.”

The team say further research is needed to better populate the mammalian family tree to give more accurate information on when the behavior of species changed from nighttime to daytime activity. NEW RESEARCH indicates that mammals began being active in the daytime after non-avian dinosaurs were wiped out.