Free from competition from massive reptiles, mammals ate a lot and used their new size to stay warm and fend off predators, according to a global study on how mammals grew after the extinction of the dinosaurs.
When dinosaurs roamed the earth, the average size of mammals was rather small, ranging from the size of a baby rabbit to that of a beagle.
“When dinosaurs went extinct, maximum mammal size was between one and 10kg, in that size range,” said researcher Jessica Theodor of the University of Calgary in Canada.
But it did not take long for mammals to start growing after the dinosaurs died out 65 million years ago, leaving loads of vegetation for others to eat, she explained.
“And 25 million years later we have mammals that are a thousand times bigger,” she said, noting that they grew to up to a maximum of 15.4 tonnes.
Until then, quick-metabolizing mammals had to battle larger reptilian dinosaurs for food, and since there wasn’t much to be had, the mammals never had a chance.
“Probably what was going on is that the things that they were eating were being eaten by the dinosaurs already so there was no way they could really break into that way of living.”
Once the beasts died out, mammals surged. However, those gains leveled out about 40 million years ago and have not changed much since.
The research was funded by a US National Science Foundation grant, and brought together scientists from around the world to track how big mammals got across the globe and to analyze their patterns of growth as time passed.
The study appears in the journal Science.
(AFP)
根據最近關於哺乳類如何在恐龍滅絕之後生長的研究報告顯示，自從哺乳類不用與大型爬蟲類相互競爭後，便開始大量地吃東西，並且利用新體型保暖與抵擋其他掠食者。
正當恐龍還存在時，哺乳類的平均體型還算小，大約介於幼兔到米格魯之間。
加拿大卡加利大學研究員潔西卡‧西奧多表示，「當恐龍滅絕時，哺乳類的尺寸約莫一公斤到十公斤。」
她解釋說，恐龍於六千五百萬年前死亡時，給其他物種留下許多植物吃。
她說，「兩千五百萬年後，就出現比當時重一千倍的哺乳類。」並又補充表示哺乳類最大可以長到十五點四公噸。
直到當時，新陳代謝較快的哺乳類還必須與較大的爬蟲類恐龍相互爭奪食物，但由於食物所剩不多，哺乳類也沒什麼機會爭贏。
「也許當時的情況就是，哺乳類的食物已經被恐龍吃掉，所以牠們絕對無法這樣生活。」
一旦恐龍死光，哺乳類就崛起。然而這一切都在四千萬年前保持平衡，至今沒有什麼改變。
這項研究是由美國國家科學基金會所資助的，並集合了世界各地的科學家，研究過去世界上大型哺乳類如何成長與發展。
該報告刊登於《科學》雜誌。
（法新社／翻譯：吳岱璟）
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