A long-standing theory holds that the common ancestor to all mammals was nocturnal, but new research from Tel Aviv University (TAU) and University College London (UCL) reveals when mammals started living in the daytime for the first time and provides insight into which species changed behavior first.
Lead author Roi Maor, a researcher at TAU’s School of Zoology and UCL’s Centre for Biodiversity and Environment Research, and colleagues analyzed data of 2,415 extant species of mammals using computer algorithms to reconstruct the likely activity patterns of their ancient ancestors.
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 non-avian dinosaurs had disappeared.
This change did not happen in an instant — it involved an intermediate stage of mixed day and night activity over millions of years, which coincided with the events that decimated the dinosaurs.
“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,” Maor said.
The researchers found that the ancestors of simian primates — such as gorillas, gibbons and tamarins — were among the first to give up nocturnal activity altogether.
However, the two evolutionary timelines varied, giving a window between 52 and 33 million years ago for this to have occurred.
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,” said co-author Professor Kate Jones, from UCL’s Genetics, Evolution Environment.
“However, we see a clear correlation in our findings.”
“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,” added co-author Professor Tamar Dayan, Chair 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.”
The findings appear in the journal Nature Ecology Evolution.
Roi Maor et al. Temporal niche expansion in mammals from a nocturnal ancestor after dinosaur extinction. Nature Ecology Evolution, published online November 6, 2017; doi: 10.1038/s41559-017-0366-5