In 1796, Thomas Jefferson received a box of bones he couldn't identify. A long, sharp claw reminded him of a lion, but the arm bones suggested a larger animal, one about three meters long. Thinking it might be huge unknown species of North American lion, Jefferson warned explorers Lewis and Clark to keep an eye out for this mysterious predator. But Jefferson's box of bones didn't come from a lion. They came from an extinct giant sloth. Prehistoric ground sloths first appeared around 35 million years ago. Dozens of species lived across North, Central and South America, alongside other ancient creatures like mastodons and giant armadillos. Some ground sloths, like the megalonychid, were cat-sized, but many were massive. Jefferson's sloth, Megalonyx, weighed about a ton, and that was small compared to megatherium, which could reach six metric tons, as much as an elephant. They ambled through the forests and savannas using their strong arms and sharp claws to uproot plants and climb trees, grazing on grasses, leaves, and prehistoric avocados. In fact, we might not have avocados today if not for the giant sloths. Smaller animals couldn't swallow the avocado's huge seed, but the sloths could, and they spread avocado trees far and wide. Ground sloths flourished for millions of years, but around 10,000 years ago, they started disappearing along with the Western Hemisphere's other giant mammals. Researchers think that ground sloths could have been pushed out by an oncoming ice age, or competition with other species, maybe humans, who arrived in the region around the time most of the sloths went extinct. Some of the smaller sloths did survive and migrated to the treetops. Today, there are six species left living in the rainforest canopies of Central and South America. Hanging out in the trees is a good way to avoid predators, and there are plenty of leaves to eat. But this diet has its drawbacks. Animals extract energy from food and use that energy to move around, maintain their body temperature, keep their organs working, and all the other activities necessary for survival. But leaves don't contain much energy, and that which they do have is tough to extract. Most herbivores supplement a leafy diet with higher energy foods like fruit and seeds. But sloths, especially three-toed sloths, rely on leaves almost exclusively. They've evolved finely tuned strategies for coping with this restricted diet. First, they extract as much energy from their food as possible. Sloths have a multi-chambered stomach that takes up a third of their body, and depending on the species, they can spend five to seven days, or even weeks, processing a meal. The other piece of the puzzle is to use as little energy as possible. One way sloths do this is, of course, by not moving very much. They spend most of their time eating, resting, or sleeping. They descend from the canopy just once a week for a bathroom break. When sloths do move, it's not very fast. It would take a sloth about five minutes to cross an average neighborhood street. This unhurried approach to life means that sloths don't need very much muscle. In fact, they have about 30% less muscle mass than other animals their size. Sloths also use less energy to keep themselves warm because their body temperature can fluctuate by about five degrees Celsius, less than a cold-blooded reptile, but more than most mammals. These physical and behavioral adaptations minimize the sloth's energy expenditure, or metabolic rate. Three-toed sloths have the slowest metabolism of any mammal. The giant panda is second slowest, and two-toed sloths come in third. Moving slowly has allowed sloths to thrive in their treetop habitat. But it's also made the sloths themselves a great habitat for other organisms, including algae, which provides a little extra camouflage, and maybe even a snack. Sloths may not be giant anymore, but that doesn't make them any less remarkable.
1796 年,湯瑪斯·傑佛遜 收到了一隻不明生物的骸骨 又長又尖的爪子讓他想起了獅子 但牠的手臂骨頭看起來 像是更大隻的動物 一個大概三公尺長的生物 傑佛遜認為這可能是一種 新發現的巨大美洲擬獅 他特別警告了探險家路易斯和克拉克 一定要特別小心這種神秘的掠食者 但是這些骸骨並不屬於獅子 它們來自於已絕種的巨大地懶 史前地懶最早 是在 3 千 5 百萬年前出現 史前北美、中美及南美 有數十種地懶居住 與許多古老生物同時期 比如乳齒象 還有大犰狳 一些地懶,像是二趾樹懶 大小和家貓差不多 但也有許多地懶體型巨大 傑佛遜收到的骨頭屬於巨爪地懶 體重約一噸 而大地懶的體型更加巨大 可以達到 6 公噸 相當於一頭大象的體重 牠們漫步於森林及莽原之中 強壯的手臂及尖利的爪子 可以將植物連根拔起 或是用來爬樹 牠們吃草、樹葉和史前酪梨 事實上 我們今天能有酪梨可吃 還得歸功於大地懶 體型較小的動物無法吞下酪梨的種子 但是地懶可以 牠們將酪梨的種子散佈地又廣又遠 地懶繁盛地生存了數百萬年 但是在大約一萬年前 牠們開始漸漸消失 其他西半球的巨大生物也是如此 研究人員認為地懶 可能是因為冰河期而開始滅絕 或是因為和其他生物競爭而消失 地懶的競爭對手可能是人類 人類正好是在大部份 地懶消失的時期出現 一些體型較小的樹懶存活了下來 並且遷移到樹梢生活 時至今日,仍有六種樹懶 生活在中美及南美的 熱帶雨林林冠之中 在樹上生活可以避開掠食者 而且還有很多樹葉可吃 不過這種飲食習慣也有缺點 動物透過從食物攝取的 能量供自己移動、 維持體溫、 使器官正常運作、 以及其他所有生存必須的活動 但樹葉含有的能量少之又少 而樹葉之中僅有的能量 也很難被攝取利用 大部分的草食動物除了樹葉以外 還會補充高能量的食物 像是種子還有水果 但是樹懶,尤其三趾樹懶 幾乎完全只吃樹葉而已 牠們演化出了專門 應付這種飲食的策略 首先 牠們盡量從樹葉中攝取足夠的能量 樹懶的胃佔了身體的三分之一 而且有許多隔間 根據品種的不同 樹懶可以花上五至七天 甚至是數週 來消化胃裡的食物 另一個策略就是減少能量的消耗 其中一個達成目標的方法 很明顯地,就是盡量少動 樹懶大部分的時間都在 吃飯、休息或者睡覺 牠們一個禮拜只會 從樹冠上下來上一次廁所 當樹懶移動的時候,通常都不是太快 讓一隻樹懶跨越住宅區的馬路 會花上牠整整五分鐘 這種超緩慢的生活步調 使樹懶不需要太多的肌肉 事實上牠們比其他體型相近的 動物少了 30% 的肌肉量 樹懶們也盡量減少消耗 在保暖方面的能量 牠們的體溫變化範圍 可以達到攝氏五度 體溫變化較冷血動物的爬蟲類小 但是比大多數的哺乳類動物要大 這些生理上或者行為上的適應策略 將樹懶消耗的能量 代謝速率降到最低 三趾樹懶的代謝速率 在哺乳類之中是最慢的 大貓熊的代謝是第二慢的 而二趾樹懶則是第三慢 緩慢的動作讓樹懶 得以在樹冠層棲地中繁衍昌盛 但這也使得樹懶自己 變成了其他有機生物的棲地 比如長在樹懶身上的藻類 能幫助樹懶偽裝 還能當零嘴吃 儘管樹懶的體型不再巨大 人們對樹懶的興趣還是絲毫未減