They Want to Suck Your Blood!
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Vampires are a Halloween staple. We’ve discussed their scientific origins here on the Bruce Science blog, but what about the real-life vampires, the creatures that bite and sometimes go bump in the night?
Hematophagy, the practice of feeding on blood, is a surprisingly common evolutionary adaptation. It evolved six different times in insects and arachnids, and over 15,000 species evolved from those evolutionary pioneers. There are many vertebrate hemovores as well. The most famous of these is the vampire bat, but there are many lesser-known varieties of bird and fish species that partake in blood too. You may also be familiar with blood-sucking worms, such as leeches, or parasitic intestinal nematodes.
For many of these animals, blood only comprises a part of their diet. The oxpecker bird is an African species which picks parasitic insects and ticks off of large mammals, but it also will peck at scabs and drink the blood that pools out. Even mosquitoes have a far more varied diet than one might expect. They usually use their puncturing mouthparts to dine on nectar and other plant fluids. It is only a female mosquito about to lay eggs that searches for blood. These animals which only occasionally drink blood are called facultative hemovores. Others, the obligatory hemovores, eat blood and blood alone.
Blood is the only food that bed bugs, ticks, and candiru fish will ever eat. It is a very nutritious, full of proteins and lipids. However, the high fluid content of blood, and the inherent challenges of acquiring blood in the first place, can make the hemovore lifestyle a difficult one. Obligatory hemovores have numerous specialized adaptations for this reason. Some hemovore features are unique to certain species, but others are surprisingly common among animals that drink blood.
Finding a Host
Before an organism can partake in a blood meal, first it must find a host.
Many hematophagous parasites are born upon or nearby their hosts, such as lice laying eggs upon the head of their hapless humans, or fleas seeding eggs throughout your carpet. Most lice will remain on a single host throughout their lifetime.
Wingless arthropod hemovores tend to rely on their hosts to carry them anywhere they need to go. Interestingly, the ancestors of many of these species had wings, but those wings were lost after generations of increased specialization. If you look at an enlarged photo of a bed bug, you might notice tiny vestigial wings. Another example of this evolutionary strategy is bat flies. While their ancestors looked much like a normal fly, a bat fly looks more like a wingless spider. They latch onto bats to feast on their blood, and females never leave their host, except to lay eggs.
For more mobile hemovores, detecting their host is critically important. Hemovores use a variety of different strategies to do this, but it is often a combination of factors such as temperature, smell, and visual recognition that allows them to recognize a proper host. Many can sense carbon dioxide emission, and smelling skin or sweat is a common method for detection as well.
Leeches have poor vision, but have sensitive hairs that can detect movement and vibrations in water. When they are younger, they rely more on changes in light and shadow to find prey, but they shift to being attracted to vibrations as adults. Think you can escape a leech by being perfectly still? Think again. They have numerous other senses that they can use to find even the most motionless prey.
Bed bugs are not known to transmit any disease, but infestations are notoriously hard to get rid of.
Holding On
Once a hungry hemovore has found its host, it has to stay attached for long enough to feed. When a vampire bat finds a chicken to prey on, it becomes especially nefarious. It might settle onto the back of a chicken, or nestle into its brood patch. This imitates the sensation a chicken might feel when she is brooding chicks, or being mounted by a rooster. The chicken is less likely to throw off the hungry bat under those conditions.
Often, hematophagous flying insects will land on or climb to places that the host would have a hard time reaching to groom them off, such as the nostrils or the back. This is one of the benefits of animals evolving social grooming. If you can’t reach a certain spot where a parasite has latched on, maybe another member of your species could help scratch that itch.
Parasites that stay on their hosts for longer than a quick bite to eat will have adaptations to reflect this. One common feature is a flattened body, which makes it easier to move between feathers and fur, and stay pressed close to the skin to avoid removal.
Many blood-sucking parasites are challenging even for humans to pry off. If you find a tick latched onto you, it is very important that you remove it with care. Their heads can be so firmly attached to human skin that pulling them while holding their body will decapitate the tick, leaving the head still embedded and capable of transmitting disease.
Vampire bats showing off their teeth. Image by Oasalehm.
The Meal
Once a hemovore has found and made contact with a host, the next hurdle it has to overcome is gaining access to that tasty blood meal. Puncturing through the skin is only part of this equation, though still one that necessitates adaptation on the part of an aspiring hemovore. Mouths tend to be heavily modified in blood-drinking organisms for just this purpose. Many insect and arachnids have needle-like proboscises, sharp enough to pierce, and hollow, capable of sucking up blood like a straw.
Other hemovores drink from pooling blood. The vampire bat uses razor-sharp teeth to cut through its prey’s skin, and then laps blood up that wells out of the open wound.
The process of drinking blood is where some of the most fascinating traits of hemovores come into play: Chemical warfare. Many hematophagous species secrete chemicals into their prey that facilitate their feeding. Some inject natural anticoagulants or substances that dilate blood vessels, causing the blood to flow more easily.
Studying these substances has led to several breakthroughs in medicine, including more effective anticoagulant drugs. In other cases, the animals themselves are used by the medical community.
Leeches have been in use for therapeutic purposes for thousands of years. In the modern day, leeches are sometimes attached to the site of injuries, or where limbs have been reattached. Their saliva contains powerful natural anticoagulants that are able to promote greater blood flow to injured areas, speeding healing and reducing infection risk.
Vampire bats may play a role in human health in the future too. Draculin, a protein found in their saliva, is currently being studied as a treatment for heart attacks and strokes.
Digestion
Digesting blood presents the final challenge. It takes a high volume of blood to gain enough nutrients for a meal, which can cause parasites to become engorged with water. Ticks and leeches that swell massively in size have a harder time moving, and become more vulnerable to predation. Ticks can swell up to 600 times their original size after feeding! Swollen with blood, ticks and leeches will drop off their hosts, and find somewhere to hide until they need to search for food again. Some hemovores can last for months on a single meal. Larger creatures, like the vampire bat, need to feed more often.
Vampire bats need to find a blood meal every night, and digest that meal swiftly and efficiently. As they drink blood, it enters the intestine before the stomach, facilitating swift water removal. This process is so fast that vampire bats often urinate while they drink. Still, they do gain a significant amount of weight when drinking, and sometimes need to fly away in a hurry. They can use their forelimbs to catapult them into the sky, launching themselves and their full stomachs off to safety.
Vampire bats have one more adaptation that helps them survive off a blood diet: Altruism. A vampire bat can only survive 70 hours without food, meaning that even a single missed meal can leave them on the brink of death. Luckily for the bats, they live in large colony groups. If a vampire bat is sick and unable to fly, or otherwise successful in its hunt, it is almost always fed by its colony members. Why do vampire bats feed each other? It is a complicated topic, fit for its own article on Storage Room No. 2!
A mosquito.
Final Thoughts
The prevalence of hematophagy in the natural world speaks to how beneficial a life strategy it is. While we might admire the hematophage for its evolutionary ingenuity, the sad reality is that they cause vast amounts of suffering. Insect hematophages are some of the most prolific spreaders of disease in the world. Mosquitoes alone can spread malaria, dengue, Zika virus fever, yellow fever, and West Nile fever. Combined, mosquitoes transmit diseases that kill around a million people per year. Ticks, fleas, lice, tsetse flies, and numerous other hemovore insects spread yet more illnesses across the globe.
If you were asked which you were more afraid of, a vampire bat or a mosquito, which would you say? Vampire bats only rarely bite humans, and it is even rarer that their bites cause serious harm. Mosquitoes, on the other hand, are the deadliest animals known to humanity by far.
Millions of years of evolution have diversified and specialized these many animals to feed on the blood of other animals, sometimes even our own. However, some of these animals, like the vampire bat, perhaps we can admire and learn from. Others, like the mosquito, we’d be much better off without.
- Kate Dzikiewicz, Science Curatorial Associate and Seaside Center Manager
