Here’s a riddle: How many legs does a frog have?
That all depends on which parasites infect it.
I’m Anna and this is Gross Science.
Throughout North America there’s a parasitic flatworm, that makes its home in the digestive
systems of water birds.
A bird’s esophagus is a great place for these guys—that’s where they find mates,
交配 然后产出成千上万的卵 这些卵穿过消化道
have sex, and pop out thousands of eggs.The eggs move through the digestive tract, and
等到鸟儿在水中排泄时 就孵化成寄生虫的幼虫 并去寻找新的家园
when the bird poops in freshwater, the eggs hatch and the baby parasites look for a new home.
But finding a new feathered friend isn’t so easy.
The parasite will need to travel through two other animals and go through four different
life stages before it find another bird and settle down.
The first stop is a freshwater snail.
When it finds one, the parasite will invade the snail’s tissues, turn into the next
larval stage, and eat away at the snail’s reproductive organs
castrating it in the process.
There it also multiplies asexually, and enters yet another life stage, turning the snail
into a mobile parasite factory.
最后 这群寄生虫从蜗牛体内游走 去寻找下一个宿主
Eventually, these hordes of parasites swim out of the snail in search of their next host
– a tadpole.
Once a tadpole is found, the larvae start to penetrate its tissue focusing on the hind
limb buds—that’s where the developing frog’s back legs will eventually grow.
幼虫会长出坚硬的保护层 称为“囊肿” 而这就是
The larvae grow a hard, protective coating called a “cyst”, and this is where things
get really gross.
这些寄生虫的囊肿中断了正常的肢体形成 一旦变形 青蛙腿的数量
These parasitic cysts interrupt proper limb formation, causing the frogs to have an unusual
number of legs once they metamorphose— anywhere from zero to ten!
And when these froggy monstrosities get eaten by birds, the parasites finally become adults
and the life cycle starts all over again.
Now, the thing I find most interesting about this is that the frog’s weird limb development
isn’t just a side effect of the parasitic infection.
Scientists think that causing frogs to have multiple, or even missing legs, is actually
advantageous for the parasite.
Manipulating a host’s morphology – in other words, how it looks – is just one strategy
that parasites use to survive.
Frogs that have an unusual number of limbs move more slowly than their four legged counterparts,
and that makes them easier for birds to catch.
Which of course makes it more likely that the parasite will end up exactly where it
其心意所属之地的可能性 进入鸟儿的食道 与同伴一起生活
wants to be, inside a bird’s esophagus, surrounded by mates.
And really, isn’t that what we all want in life?