进食同类的食肉性蝌蚪 @David Pfennig现实世界中并不存在狼人，不过，有一种名为锄足蟾（spadefoot toad）的动物，特别是它们的蝌蚪形态，可能是自然界中最接近狼人的动物。这是一个关于同类相食蝌蚪的离奇而真实的故事。刚出生时，锄足蟾蝌蚪是和平的底栖动物，以漂浮在水面上的小块藻类和排泄物为食。但在适当条件下，每只蝌蚪都有机会变成体型庞大、行动敏捷的捕食者，它们首先吃的就是其他蝌蚪。
在美国北卡罗来纳大学教堂山分校（University of North Carolina at Chapel Hill）生物系三楼达维德·普芬尼希（David Pfennig）的办公室里，有一个装满乙醇的小瓶子，里面有一些蝌蚪，有的体型很小，有的体型很大，体型巨大的蝌蚪是一种“食肉性变体”（carnivore morph），也就是“蝌蚪狼人”。小瓶中的这些蝌蚪来自美国亚利桑那州的沙漠。对于任何生物来说，沙漠都是相当艰难的生存环境，对于蟾蜍来说尤其如此，因为蟾蜍须要保持湿润才能呼吸，而且它们通常在水中产卵。两栖动物生存在沙漠中就像冰块被置于烤箱里一样，须要采取一些非常极端的适应方法，比如休眠。
锄足蟾会进入永久洞穴进行休眠，在干旱的环境中，它们会在地下睡上好几个月、一年甚至两年，等待最佳的出洞时机。 等到终于下雨时锄足蟾们就会从地底爬出来，开始寻找新形成的水池。这些水池遍布各地，形状各异，大小不一，有的可能只有浴缸那么大，有的可能和足球场差不多，这取决于位置和降雨量，但所有这些蟾蜍繁殖的地方都有一个共同特点：它们都是临时的。
雄性锄足蟾通常会率先到达，然后开始呱呱叫。它们的叫声非常响亮，非常聒噪，这会把雌性吸引过来，然后所有的繁殖都会在一个晚上进行。经过几个月的蛰居，锄足蟾们的动作出奇的快，每只雌性会在当晚产下800~1500枚卵，这些卵第二天就会在水池里孵化。干净雨水形成的水池是蝌蚪宝宝的完美托儿所，但这样的日子并不长久，第一晚过后，干净得可以饮用的水慢慢变成了一滩污水，藻类开始在水面上绽放，牛等大型动物也会来喝水、排便。
随着时间的推移，原本清澈的水池开始变得越来越污浊、越来越难闻。起初，这些都是新生蝌蚪的食物，但随着水分蒸发，这些污泥会越来越集中，蝌蚪们会越来越难以用鳃呼吸。如果水池干得太快，可能连水都没有了，蝌蚪越聚越多，奄奄一息。除此之外，寄生虫和捕食者也开始出现，蝌蚪体内没有毒素，所以很容易成为蛇、鸟甚至黄蜂和甲虫等昆虫的点心。普芬尼希说：“虎甲虫（tiger beetle）会在水池边排成一排，一旦有蝌蚪靠近，它们就会把腿伸进水里，试图抓住蝌蚪，然后拉到岸上吃掉。”污泥水、干涸和饥饿的虎甲虫：这些都是锄足蟾小蝌蚪尽快长大并钻进地下的理由。锄足蟾蝌蚪的发育极为迅速，有一种锄足蟾蝌蚪从卵发育到上岸大约需要七到八天：周一，小蝌蚪从卵中出来，睁开它们的小眼睛，到下周一，它们就须要甩掉尾巴，长出肺，长出腿，然后用这些腿跳出水池。这是一种极为激进、字面意义的死线，为了做到这些，蝌蚪们须要快速长大，所以它们一出生就是小饿鬼。它们几乎什么都吃，比如水池底部的东西，也就是细菌和藻类的混合物，还有其他蝌蚪的便便，它们埋头干饭，暴风吸入。大多数锄足蟾蝌蚪一开始都是底栖动物，普芬尼希称之为“杂食性变体”（omnivore morph）。有些蝌蚪一生都是这样，除非……它们碰巧尝到了肉的味道，而这就是故事真正奇怪的地方。普芬尼希：“如果一只小蝌蚪在生命早期碰巧吃进了一些丰年虾，甚至可能吃掉了另一只蝌蚪，那么它们就会发生巨大的变化，变成我们称之为‘食肉性变体’的大头蝌蚪。”这些食肉性变体与原来的底栖蝌蚪极为不同，以至于近100年来生物学家一直认为它们是完全不同的物种。它们的变化就像是《暮光之城》（Twilight）中狼人雅各布的蝌蚪版：颜色从深灰色变成金色，体型变大一倍或两倍，肠道……[查看全文]
These Creatures Are Probably the Closest Thing Nature Has to Real Werewolves
Brian Gutierrez: Werewolves aren't real. But stay with me for a moment while I tell you about the spadefoot frogs—specifically, their tadpoles, which just might be the closest things to werewolves in nature.I want to tell you about the bizarre and true story of cannibal tadpoles.I'm Brian Gutierrez, and this is Scientific American's Science, Quickly.Gutierrez: Spadefoot tadpoles are born as peaceful bottom-feeders that eat little bits of algae and poo floating in the water.But under the right conditions, each tadpole has a chance to transform into a hulking, agile predator. And the first things on the menu are other tadpoles.David Pfennig: And so, and I have some right here.Gutierrez: I'm in the office of David Pfennig on the third floor of the biology department at the University of North Carolina at Chapel Hill.Gutierrez (tape): Oh, okay. Could you describe what ...Pfennig: Yeah, so, so what you're seeing here is: I'm showing you a little vial filled with ethanol, and it's got some tadpoles in it. And some of these tadpoles you see are small; some of the tadpoles you see are large.Gutierrez (tape): Oh, okay, so this is one of the huge ones. Yeah.Pfennig: And so that huge one is what we call the carnivore morph.Gutierrez: Learning about the carnivore morph—what I think of as a tadpole werewolf—is why I came to see David.The tadpoles in this particular vial are from the deserts of Arizona. The desert is a pretty tough environment for any organism to survive, but that's especially true for frogs, which need to stay moist to breathe and they usually lay their eggs in water.An amphibian surviving in the desert is like an ice cube surviving in a dutch oven. Making it work requires some pretty extreme adaptations.One of those is hibernation.Pfennig: They'll go into their permanent burrow, and they'll dig with their hind feet. That's why they're called spadefoots, because they've got this little keratinized spade on the back of their feet. And they basically sort of do this little wiggle dance, you know, to sort of dig into the ground. So they're digging backwards, if you want to think about it that way.Gutierrez (tape): How deep do they go underground?Pfennig: Well, it depends upon the time of the year. So they have been recorded as digging almost a meter deep—so 90 centimeters deep.Gutierrez: That's almost three feet into the earth.In arid environments, spadefoot frogs sleep patiently underground for months, a year or even two years if they have to—waiting for the perfect moment to emerge. When he goes out looking for them, David waits for the trigger that will bring the frogs out of their deep sleep: rain.Pfennig: We don't really know how they know it's raining, but somehow they know it's raining.Gutierrez: The frogs come up from under the ground and start looking for newly formed pools that dot the landscape in all shapes and sizes.Pfennig: Some of them could be as small as your bathtub. Some of them could be as, as big as your bedroom. Some of these ponds can be as big as your whole house and maybe your backyard, you know, and so it just depends on the location. It depends on how much rain you get. But one thing that unites all the places where these toads breed is that they're all temporary.The males typically will arrive first, and then they'll start to call. They have very loud calls, like a lot of frogs, but these guys in particular have really loud calls.Pfennig: It's, like, really raucous. You can't even hear yourself talking, you know, this can be so loud. You have to yell for somebody else to hear you. So it's very, very noisy. The water is just full of frogs calling. And that will attract the females to the site as well. And then all the breeding will take place in one night.Gutierrez: After months of barely moving, the frogs get to work incredibly quickly. That night, each female will lay between 800 and 1,500 eggs. Those eggs will hatch as soon as the next day in these fresh rainwater pools.Pfennig: It's just, it's clean rainwater just falling on dirt, right? And so there's nothing, there's no algae or anything like that growing in them initially. Presumably, if you wanted to, you could probably drink out of them, I guess.Gutierrez: It's the perfect nursery for baby tadpoles. But it doesn't stay that way for long. After that first night, the pristine water that's clean enough to drink slowly turns into sludge. Algae starts to bloom across the surface, and large animals such as cattle come by to drink and do their business around the pool.Pfennig: And so then they'll start getting a little smelly, you know, and so then you wouldn't want to drink the water out of them, of course. And so basically, over time, it just starts, you know, it starts getting nastier and nastier, basically.Gutierrez: At first, this is all food for the new tadpoles. But as the water starts to evaporate, that nastiness gets more and more concentrated, making it harder and harder for them to breathe through their gills. If the pool dries up too quickly, there might not be any water left at all.Pfennig: You'll go in there, and there'll be thousands, tens of thousands of tadpoles all dying because the, the water has disappeared, and they're just basically desiccating in the sun.Gutierrez: David showed me a picture. It's really tragic. As the pool of sludgy water gets smaller, the tadpoles gather closer and closer together until there's no water left, just a mound of tadpoles on top of the drying mud.On top of that, parasites and predators start to arrive. The tadpoles aren't poisonous, so they're an easy snack for snakes, birds and even insects such as wasps and beetles.Pfennig: You get all these—what are called tiger beetles. And this is a type of beetle that will just, like, line up along the shoreline, and they'll just be waiting for a tadpole to come close enough to them, and they'll, like, reach into the water and try to grab it. And I've actually seen a number of times where they'll grab a tadpole, and they'll pull it up on the shore and then eat it.Gutierrez: Sludge water, drying out and hungry tiger beetles: these are all very good reasons for these little tadpoles to grow up and bury themselves in the ground as quickly as possible.Pfennig: These guys have really, really rapid development…the spadefoot tadpoles. One species can develop from egg to moving onto land at about seven to eight days.Gutierrez: That's really fast. On Monday the tadpoles emerge from their eggs and open their little tadpole eyes. By next Monday they need to lose their tail, grow lungs, grow legs and then use those legs to hop out of the pool.It's a very aggressive and very literal deadline. To meet it, the tadpoles need to bulk up fast. So they are born hungry.Pfennig: They pretty much will eat anything. So mostly what they're eating is just ... what we call detritus. So they'll just eat stuff on the bottom of the pond.And so that is an amalgamation of, like, bacteria, some algae, poop from other tadpoles. They're reprocessing. And so they're just, you see them just shoveling that, you know, like, going along and sort of eating this, the mud or the dirt on the bottom of the pond.Gutierrez: Most spadefoots start life as these bottom-feeders, what David calls "the omnivore morph". Some of them stay that way for their entire tadpole life unless—and this is the truly strange part of the story—they happen to get a taste of flesh.Pfennig: If a little tadpole happens to eat some fairy shrimp, or even maybe another tadpole, early on in life, then these really dramatic changes will take place, and they'll become this big-headed form that we call the carnivore morph.Gutierrez: These carnivore morphs are so different from the bottom-feeders that for almost 100 years, biologists thought they were an entirely different species.The transformation is like a tadpole version of that scene in An American Werewolf in London.Their color changes from dark gray to gold.They double or triple in size.
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