A team of scientists established that mammals remained nocturnal until after the dinosaurs died out according to a paper published in the journal Nature Ecology & Evolution. They also found that many diurnal species still present adaptations similar to those of night-prowling animals.
When Did the Transition from Nocturnal to Diurnal Occur?
Researchers from the Tel Aviv University’s Steinhardt Museum of Natural History and the University College London gathered activity records of 2415 species. These came from the PANTheria database and published sources like research articles and field guides.
The study team paid particular attention to underrepresented families and orders. While they found activity pattern data for only 44.6 percent of species, the researchers did find such data for over 91.2 percent of the mammal families.
The researchers then placed the species into five categories. Number one was nocturnal and the second diurnal. The third was crepuscular or active only at sunrise and/or sunset and the fourth cathermal or active during both night and day. The fifth and last one was ultradian or active in cycles that lasted only a few hours. About 60 percent of the species or about 1400 belonged to the first group.
The scientists used computer algorithms in their analysis. They did so partly because of the dearth of fossils and also partly because many members of the Mammalia family still have skulls and eye sockets shaped like those of a night-prowling animal. Primates are an exception and have no such traits.
That is partly because they were among the first to switch to a daytime schedule. This might have possibly happened as long ago as 55 million years. Many primates are also arboreal and thus need clear vision to navigate the treetops. Their eyesight and color perception can, as such, be more easily compared to that of diurnal birds or reptiles.
The researchers used their algorithms to estimate the activity patterns of the ancestors of closely related animals. As they continued to work backward, they got a growing number of nighttime species.
There are two competing hypotheses regarding the evolutionary history of Mammalia, and the researchers thus presented two sets of results to accommodate each hypothesis. That conflict makes it difficult, for now, to determine exactly when mammals evolved (220 million years ago or 160 million years ago?). It also makes it hard to establish when the given groups switched to daytime schedules.
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