本帖最後由 eaglelu 於 2013-8-4 07:52 PM 編輯
Millions of years ago, some bats gave up their old habits of hunting for insects and tried something new: drinking blood. These creatures evolved into today’s vampire bats, and it’s mind-boggling to explore all the ways that they evolved to make the most of their sanguine meal.
A lot of the adaptations are easy enough to see with the naked eye. Vampire bats have Dracula-style teeth, for example, which they use to puncture the tough hide of cows. When they open up a crater-shaped wound, they dip in their long tongue, which contains two straw-shaped ducts that take up the blood.
Finding these prey has led to another remarkable adaptation that you can see–at least if you’re a scientist who studies how vampire bats move. Like other bats, they can fly, but on top of that, they can also walk and, yes, even gallop. Of the 1200 or so species of bats, vampire bats are among the very few that can move quickly on the ground.
But vampire bats have many other adaptations for drinking blood that are invisible. They use their combined senses–long-range vision, a sharp sense of smell, acute hearing, and echolocation–to find their victims. In their noses, they even have heat-sensitive pits that detect the heat of warm-blooded animals. Once they land on an animal, they apply those pits to the skin to locate capillaries full of hot blood close to the surface.
When vampire bats dip their tongue into a wound, they don’t just draw out blood. They also put their saliva into their victim. And in this liquid are still more invisible adaptations for a blood-feeding life. Vampire bats, you see, are venomous.
This may sound odd. That’s because we usually think of venom as a chemical an animal sticks in your body to cause you pain or death. But biologists define venom more broadly than that: it’s a secretion produced in a specialized gland in an animal, which is delivered to another animal by inflicting a wound, where it can disrupt its victim’s physiology.
Snake venom, the sort we’re all most familiar with, can disrupt physiology to the point of death. And it does so in several ways–jamming neurons, for example, or causing tissue to rot. But other animals that don’t set out to kill their victims also produce venom. Vampire bats, for example, don’t want eat a whole cow. They just want to take a sip.
Unfortunately, drinking blood has some drawbacks. Vertebrates come equipped with lots of molecules and cells that plug up wounds. As soon as they sense even a tiny tear in a blood vessel, they start making clots to staunch the flow.
Vampire bats use venom to keep the blood flowing. In a new paper with a title worth quoting in full–“Dracula’s Children: Molecular Evolution of Vampire Bat Venom”–an international team of scientists explore the molecules that vampire bats use to subvert blood’s defenses.
What’s most striking about vampire bat venom is how it goes after its victim from so many directions. Blood clots develop through a series of reactions that involve a chain of enzymes. Vampire bats produce different proteins to go after different enzymes in that chain. Platelets, which are cell fragments, also clump around wounds to help heal wounds. Vampire bats make separate compounds that attacks platelets.
To make their venom cocktail, vampire bats have repurposed old molecules for new jobs. When any vertebrate formed a blood clot to stop a wound, it needs to break that clot down once the wound is healed. An enzyme called plasminogen activator creates a supply of molecules called plasminogen, which chops up the clots. Vampire bats produce plasminogen activators in their blood for this job. But they also produce an extra supply in their mouth glands. When the plasminogen activators get into a wound, they use the victim’s own plasminogen to keep the blood flowing.
Once bats borrowed plasminogen activators to use in their venom, the molecules became better adapted to that new job. Normal plasminogen activators get cleared from the blood stream by other enzymes. That’s important for our survival, because otherwise they would hang around and make it hard to form new clots. Vampire bat plasminogen activators have a slightly different shape that shields them from their victim’s enzymes.
Together, these molecules are so effective that a cow will keep bleeding long after a vampire has flown away. While scientists have been studying vampire bat venom for decades, they’re still finding new molecules in the cocktail. The authors of “Dracula’s Children” applied a new method to the search. They caught two vampire bats and cataloged all the genes that were highly active in their mouth glands. The scientists then identified the genes and studied the properties of the proteins they encoded. They discovered dozens of new proteins. Some of them kill microbes, keeping the bat’s food supply clean. Some expand blood vessels, increasing the flow into the wound.
When a cow gets attacked by a vampire bat, it’s not entirely helpless. Ranchers have noticed that when bats feed over and over again on their herds, the cows bleed for a shorter period of time. Scientists have found that this happens because the immune systems of the animals learn to recognize some of the venom molecules and attack them. In the new study, the researchers found venom molecules that can ward off the immune system. But the venom itself is evolving to escape the immune system’s recognition, taking on new shapes that may allow them to go unnoticed.
Reading “Dracula’s Children” gives me a potent sense of deja vu. I recently wrote a feature about ticks for Outside, and in the research for the piece I learned all about how ticks produce saliva loaded with proteins that, among other things, open blood vessels, use our own molecules to break up clots, and do many of the things that vampire bat venom does.
Vampire bats are what you get if you turn a mammal into a tick. And I mean that as the highest compliment.
Source:NG
中文補充：
小心吸血蝙蝠(作者：張樹義)：
動身去雨林之前，我翻看了幾本早期赴亞馬遜探險者的回憶錄，其中讀過這樣一段文字：
“早晨四点鐘，我從吊床上醒来，發現自己身上到處是血跡。上帝啊，到底發生了什麼？”
原来，他是遭到了吸血蝙蝠的“暗算”。作者進一步記述：“這類蝙蝠體型巨大，在人或其他動物熟睡時吸血，
有时直到后者死亡。它們具有奇特的吸血本領，可以本能地辨識出熟睡的人或其他動物，一邊扇動翅膀一邊輕輕咬破
襲擊目標的皮膚。
因為傷口極小，所以受害者感覺不到疼痛。于是，吸血蝙蝠便從這個小傷口不停地吸吮，直到幾乎飛不動為止。
”早期到亞馬遜探險的很多旅行家都有過類似的記載，人們因此錯誤地以為新大陸所有的蝙蝠都食血。
其實，全球真正以血為食的蝙蝠只有3種，主要分布于新大陸的熱帶地區，僅有普通吸血蝙蝠進入北美洲的得克薩斯南部。
可以說，吸食其他脊椎動物的血可能是蝙蝠最特别的食物習性；這種食物習性在哺乳動物，甚至可能在所有脊椎動物中都是唯一的。
吸血蝙蝠有很多與吸血食性相適應的特點：鼻葉有熱感器，以便探測皮膚的毛细血管豐富處；特化的犬齒能切断毛髮；
長而尖銳的門牙可毫無痛感地在皮膚上咬開一個口；抗凝素可以防止血液凝固；槽狀舌有助于血液迅速流向口腔内；
特化的胃和腎能迅速除去血漿；在吸血完成之前，它們通常已經開始排泄。
一般來說，一隻吸血蝙蝠不太可能吸乾受害者的血，但幾隻蝙蝠度對一隻幼畜或一個小孩的襲擊則有可能導致受害者死亡，
這也主要是由于傷口不凝，導致持續大出血造成的。
吸血蝙蝠吸血時總是採取偷襲的方法，它們還會針對不同的對象選擇不同的吸血部位。例如，遇到牛和馬，專咬背部和體側；
遇到豬，專咬腹部；遇到鳥或家禽，則咬腿部。
有人曾觀察到吸血蝙蝠與大公雞之間戲劇性的一幕：吸血蝙蝠用翼勾攀住雄雞的腿，后腿站在地上；雞走時它也走，
邊走邊吸雞的血。
吸血蝙蝠吸血時總是不厭其多，把肚子喝得鼓鼓的。有人估計，一隻吸血蝙蝠一生能吸血100升，可謂道道地地的“吸血鬼”。
更令人深惡的是，吸血蝙蝠在貪得無厭的吸血過程中，往往把各種疾病带給被害者。例如，它在吸取馬的血液時，能傳播椎蟲病；
在咬傷人和其他家畜時，最易傳染狂犬病。因此，吸血蝙蝠是居住在熱带叢林中的人和牲畜的大敵。
不過，普通吸血蝙蝠之所以成為害獸也是因為新大陸大量引入馬、牛和其他家養動物，這些廣泛的食物源使普通吸血蝙蝠的族群在過去500年間顯著的成長。
同時，那些地區的人口也在不断增長。因為地楚熱帶，當地人習慣于敞開門睡覺；而且，由于貧困，很多人露宿在無遮無掩的地方；吸血蝙蝠因此又增加了
额外的食物来源。
所以，很多關於吸血蝙蝠襲擊人的描述並非虛構。但吸血蝙蝠的體型實際上並非很大，成年個體只有30多克重，翼展30厘米。
在亞馬遜叢林生活的日子裡，我們每個人都睡在棉布做成的大蚊帳裡，這既是為了防蚊蟲叮咬，也更是為了避免受吸血蝙蝠的襲擊。
所以，幾年的時間裡，生態站裡還沒有人遭吸血蝙蝠的暗算。
來自Dracula's children: Molecular evolution of vampire bat venom. DOI: 相關文獻：
研究人員在一種吸血蝙蝠的毒液中發現了一種分子，這種分子能够逃過受害者的免疫系统。
澳大利亞昆士蘭大學的生物科學副教授Bryan Fry說：“我們的研究團隊在蝙蝠的毒液中，發現了一種全新的抗凝劑，
和一種新分子可導致靠近皮膚的小動脈的擴張。”
“就像毒蛇快速進化它們的毒液以防獵物對之產生抗體一樣，吸血蝙蝠的毒液也快速進化，防止它們的獵物的免疫系统產生可抵抗毒液分子的抗體。”
Bryan Fry的這項研究發表在Journal of Proteomics雜誌上。研究表示，吸血蝙蝠分泌的同種活性成分具有多種形式，
在活性成分的分子表面分布有無數的细微變化。
“這就意味著，即使產生了針對一種分子的抗體，還有大量其他類型的分子能躲過獵物的防御系統，保證獵物血液的流通。
這意味著蝙蝠可以每晚吸食同一個獵物的血液。”
“這項研究中發現的大量新分子在用于研發新的中風與高血壓新藥中具有巨大的潜力。”Bryan Fry表示。 |
發表於 2013-8-4 11:19 AM
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發表於 2013-8-4 02:36 PM
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