drink until itâs full.â
I nodded, recalling that these vampires had been given the species name
rotundus
because of their round-bellied appearance. Unfortunately, the naturalist Geoffroy never realized when he named them that their rotund abdomens were due to a gastrointestinal tract bloated with blood. Had he dissected a specimen he would have certainly noticed (as did Darwinâs friend and supporter Thomas H. Huxley) that the common vampireâs esophagus didnât empty directly into the stomach, a feature that typifies all mammals. Instead, the lower end of the esophagus ended in an inverted T, one serif leading to the stomach, the other leading to the intestine. Furthermore, the stomach wasnât J-shaped (as is seen in most mammals). It was tubular, a blind-ended U that was nearly two-thirds as long as the ropelike intestine that it closely resembled.
Not surprisingly, researchers who sought to determine the route of ingested blood in vampire bats found that the going was weird indeed. In an experiment using barium-laced cow blood, an X-ray machine, and five (presumably grumpy) common vampire bats, G. Clay Mitchell and James Tigner determined that freshly ingested blood moved from the vampireâs mouth to the esophagus and then into the intestines
before
passing into the stomach.
We know now that these variations in digestive anatomy and physiology (like other anatomical and behavioral adaptations) are related to the vampire batâs unique lifestyle. In all other mammals, a primary function of the stomach is bulk storage of foodâwith some breakdown of that food (digestion). There is little transfer of nutrients or other material from the lumen of the stomach to the circulatory system (which then distributes it to the body). *32 This last, and generally overlooked digestive system function (absorption), is usually carried out by the small and large intestines and the network of blood vessels that supply and drain them. In vampire bats a key role of the stomach appears to be the rapid absorption of water (which makes up approximately 80 percent of the ingested blood volume). This excess water is carried to the kidneys (via the circulatory system once again) where much of it is converted into urine (i.e., water plus dissolved nitrogenous waste products). As mentioned earlier, since blood contains a negligible amount of fat, a substance in which energy is typically stored for later use, vampire bats are required to drink about half their body weight in blood each night. â 33 This sudden weight gain is an extremely dangerous proposition for an animal that might be required to take flight at a momentâs notice. Because of this, an important adaptation for blood-feeding bats is their ability to shed weight as quickly as possible, and their digestive and excretory systems reflect this emphasis. Researchers who have observed common vampire bat feeding sessions know that the bats begin urinating well before theyâve finished feeding. Along with modifications to the typical mammalian stomach and intestines, evolution has cranked up the vampire batsâ excretory system, enhancing its ability to deal with its ownersâ rather unique dietary requirements.
Vampire bats get most of their nutrition from protein (in this case, hemoglobin and blood plasma proteins like albumin, fibrinogen, and globulin). These proteins are composed of smaller subunits called amino acids. In mammals, a problem arises when these nitrogen-bearing amino acids are broken down during digestion, releasing the toxic compound ammonia. Most mammalian excretory systems deal with ammonia by having the liver quickly convert it into a less toxic substanceâurea. Urea is not only safer to have circulating around the body than ammonia but can also be more easily excreted by the body as urine (which is basically urea diluted with water extracted by the kidneys from circulating blood plasma).
As a vampire bat begins
I nodded, recalling that these vampires had been given the species name
rotundus
because of their round-bellied appearance. Unfortunately, the naturalist Geoffroy never realized when he named them that their rotund abdomens were due to a gastrointestinal tract bloated with blood. Had he dissected a specimen he would have certainly noticed (as did Darwinâs friend and supporter Thomas H. Huxley) that the common vampireâs esophagus didnât empty directly into the stomach, a feature that typifies all mammals. Instead, the lower end of the esophagus ended in an inverted T, one serif leading to the stomach, the other leading to the intestine. Furthermore, the stomach wasnât J-shaped (as is seen in most mammals). It was tubular, a blind-ended U that was nearly two-thirds as long as the ropelike intestine that it closely resembled.
Not surprisingly, researchers who sought to determine the route of ingested blood in vampire bats found that the going was weird indeed. In an experiment using barium-laced cow blood, an X-ray machine, and five (presumably grumpy) common vampire bats, G. Clay Mitchell and James Tigner determined that freshly ingested blood moved from the vampireâs mouth to the esophagus and then into the intestines
before
passing into the stomach.
We know now that these variations in digestive anatomy and physiology (like other anatomical and behavioral adaptations) are related to the vampire batâs unique lifestyle. In all other mammals, a primary function of the stomach is bulk storage of foodâwith some breakdown of that food (digestion). There is little transfer of nutrients or other material from the lumen of the stomach to the circulatory system (which then distributes it to the body). *32 This last, and generally overlooked digestive system function (absorption), is usually carried out by the small and large intestines and the network of blood vessels that supply and drain them. In vampire bats a key role of the stomach appears to be the rapid absorption of water (which makes up approximately 80 percent of the ingested blood volume). This excess water is carried to the kidneys (via the circulatory system once again) where much of it is converted into urine (i.e., water plus dissolved nitrogenous waste products). As mentioned earlier, since blood contains a negligible amount of fat, a substance in which energy is typically stored for later use, vampire bats are required to drink about half their body weight in blood each night. â 33 This sudden weight gain is an extremely dangerous proposition for an animal that might be required to take flight at a momentâs notice. Because of this, an important adaptation for blood-feeding bats is their ability to shed weight as quickly as possible, and their digestive and excretory systems reflect this emphasis. Researchers who have observed common vampire bat feeding sessions know that the bats begin urinating well before theyâve finished feeding. Along with modifications to the typical mammalian stomach and intestines, evolution has cranked up the vampire batsâ excretory system, enhancing its ability to deal with its ownersâ rather unique dietary requirements.
Vampire bats get most of their nutrition from protein (in this case, hemoglobin and blood plasma proteins like albumin, fibrinogen, and globulin). These proteins are composed of smaller subunits called amino acids. In mammals, a problem arises when these nitrogen-bearing amino acids are broken down during digestion, releasing the toxic compound ammonia. Most mammalian excretory systems deal with ammonia by having the liver quickly convert it into a less toxic substanceâurea. Urea is not only safer to have circulating around the body than ammonia but can also be more easily excreted by the body as urine (which is basically urea diluted with water extracted by the kidneys from circulating blood plasma).
As a vampire bat begins
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