synapsid-taxonomy
you, a generalist unspecialized mouse or mouselike mammal:
- food goes from esophagus to stomach to intestine
- can eat and digest almost anything
- adaptable and can survive in many different environments
me, a specialized sanguivorous vampire bat:
- food goes from esophagus to intestine to stomach to intestine
- can only eat blood
- will die if I don’t eat for one night (unless someone vomits in my mouth)
- will die if there’s not enough humidity in the air
- will die if I exercise too much
- will become dehydrated if I drink too much
- constantly pissing so I’m not too heavy to fly
I’ve gotten a couple of requests for more info on this and also I fuckin’ love these horrible creatures so let me explain the digestive system of the vampire bat. I guarantee by the end you will be wondering how these creatures even exist.
Vampire bats are the only known obligate blood feeding vertebrates. Other animals like vampire finches supplement their blood diets with other stuff, depending on what’s available. This is because blood is a terrible food to live solely on.
Blood is, first and foremost, 92% just plain water. This means to gain appreciable nutritional value from it, you have to drink A LOT. Common vampire bats drink around 20 grams of blood each night, which doesn’t sound like much unless you realize that common vampire bats weigh, on average, around 30 grams. (Several of them could fit snugly in a teacup.) That’s like if a person who weighed 150 pounds/68 kg drank 100 lbs/45 kg of fluid every night in half an hour.
This presents an issue, because vampire bats can’t just swell up into an orb and roll off when they’re done feeding- they need to be light enough to fly. So blood needs to be processed very fast by their digestive system so they can shed the water weight. This is why vampire bats start peeing within about two minutes of feeding, and continue peeing through their approximately 30 min feeding session. It shoots through their body that fast.
Peeing this much at once has consequences on the body, though. To put it briefly, while vampire bat pee is mostly clear water at the beginning of the feeding, it is dark with urea by the end. (Urea is a waste product from food that builds up in the body and is released by urine.) Because they need to keep peeing to process the blood fast and dump toxic urea from the body, their urine becomes more and more concentrated as their bodies run out of water to dilute their urine with. So even though bats may consume 2/3 or more of their body weight of fluid each night, the vast majority of which is water, they may become dehydrated.
Their high risk of dehydration is why they can’t handle dry environments, and why you don’t find them outside of tropical environments. (Really it’s a miracle these creatures can survive at all.)
Blood isn’t just a troublesome food because you need to drink a lot of it to live. It’s troublesome because even with the water taken out, the nutritional value of what remains SUCKS, no pun intended. It’s literally basically just some proteins and iron. And while that may be why vampire bats are so jacked (seriously, they’re very muscular in places most bats aren’t), it is extremely difficult to thrive on. One big reason is that blood contains almost no fat, which is crucial to most animals because it provides spare batteries- essentially, stored energy we can use if food is scarce.
A vampire bat does not have this backup. They will literally die within about 36 hours of not feeding. Even mice can live 3-4 days without food, and they normally live for two years as opposed to a vampire bat’s 12-20 years. (Depending on their environment.) Each and every night in a vampire bat’s life is on a knife’s edge, teetering towards starvation.
These bats do help each other, however, by regurgitating small amounts of blood for their hungry colony mates who haven’t found food for the night. Without this behavior I’m not sure the species would be anywhere near the populations it has now; they might not be able to thrive at all considering how desperately mothers with pups need the food. It takes most small bats about two weeks to wean their pups. It can take vampire bats up to nine months to wean their pups (though more generally it’s around four months) because their milk suffers from the same lack of nutrition as their food. They also have unusually long pregnancies (5-7 months; other similar-sized bats average around 6 weeks) for the same reason.
The energy budgeting for the vampire bats is so severe that they actually have a sharp limit for how far they can fly before they become exhausted. Vampire bats are not known to migrate or even relocate because frankly, they might end up dropping dead out of the sky.
So, to recap, each day is a struggle between life and death, the bats teeter between drinking too much and becoming too heavy to fly and/or dehydrated, or drinking too little and dying on the way back. This is a highly successful species we’re talking about here. How they’re so successful with these constraints is a mystery to me, although it might have something to do with their high intelligence.
I haven’t covered one thing, which is the structure of the vampire bat’s digestive system. So. For the vast majority of vertebrates, food goes in the mouth, down the esophagus, into the stomach and through the intestines. Let’s call the esophagus/stomach/intestine routine ABC. Vampire bats… take a slightly different route. Using these letters, their digestive order would be ACBC.
Take a look at the following diagram. An average insect-eating bat’s organs are shown on the left, while those of a vampire bat are on the right.
You may notice that things are a bit… off. Unlike practically any other vertebrate on the planet, a vampire bat’s esophagus splits into two branches. One branch leads directly to the intestines, the other to the stomach. The stomach and intestines are not connected in any other way.
The question is: why? Why this? Why do you have to be like this, vampire bats?
Naturally, the answer is in the diet again. The bat uses its intestines to pull out the negligible nutrition from the blood quickly, then sends the resulting wastewater to the stomach, which balloons tremendously even as it rapidly sends the water to the kidneys to be processed into urine. (Then later they have sludgy black poops.) Even with their fast urination system, bats only manage to shed about a quarter of their water weight by the time they lift off into a sloshy flight, weighing easily twice as much as when they left. See the “before and after” shots below.
(Both photos taken by Jon Flanders. Hey kids, contrary to these images, never touch a bat with bare hands, much less a bat that can deliver extremely deep wounds as well as the bacteria and viruses of whatever animal they just fed on. Don’t Do That™)
Anyway. I need to stop talking about vampire bat digestion because this got uhhhhhh long. It’s a fascinating yet mystifying subject. If you want to learn more I recommend Dark Banquet: Blood and the Lives of Blood-Feeding Creatures by Bill Schutt. I learned a lot of stuff that I wrote here from that particular book, and it makes for a pretty good read (even though I disagree with his hypothesis about how vampirism evolved in bats). If you’re interested in vampire bat behavior, which is equally interesting, I recommend looking into the research of Gerry Carter.
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