Research press release


Nature Ecology & Evolution

Early mammals abandoned nightlife after dinosaur extinction



この仮説には以前から多少の支持があったが、直接的な証拠は得られていなかった。今回、Roi Maorたちは、現生するほぼ全ての哺乳類科に関して昼夜の行動パターンのデータを収集し、それぞれの種の進化的分岐時期に基づいてその行動の起源を確かめた。その結果、最初の哺乳類は夜行性であり、数千万年後までに、時として日中にも活動するようになったことが明らかになった。日中に活動する哺乳類の数は、恐竜絶滅の直後に急増した。オランウータンやマーモセットなどの霊長類は、全哺乳類の中で最も鋭敏な部類の視覚を有しているが、それは、祖先が昼間のライフスタイルに転換した最初の哺乳類の一部であったことによるものと考えられる。

The extinction of the dinosaurs facilitated a switch from a nocturnal lifestyle to an increase in daytime activity in early mammals, reports a paper published online this week in Nature Ecology & Evolution.

Today, many mammals are active by day, but mostly lack the characteristics that allow fish, reptiles and birds to be such successful daytime predators: colour vision, for instance. In fact, most mammals have vision more similar to nocturnal reptiles and birds. These observations led to the development of the 'nocturnal bottleneck' theory, which proposes that early mammals had to restrict their activity to the night time to avoid conflict with dinosaurs that were active by day. When the dinosaurs died out, it is proposed, mammals were able to move into the newly available daytime niche.

There has been some support for this hypothesis over the years, but direct evidence has been lacking. Here, Roi Maor and colleagues assembled data on day and night behaviour patterns for nearly all living mammal families and ascertained the origin of these behaviours based on the timing of the evolutionary split of each species. They find that the first mammals were nocturnal, and became occasionally active by day tens of millions of years later. The number of mammals active by day had increased rapidly shortly after the extinction of the dinosaurs. Simian primates, like orangutans and marmosets, have some of the most acute vision of all mammals, which may be due to their ancestors being among the first mammals to switch to a daytime lifestyle.

doi: 10.1038/s41559-017-0366-5

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