Komodo Dragons: Their Bite is Worse than Their Bark (A Guest Post)

By Shelly Sonsalla


Komodo Dragon.
Image by Arturo de Frias Marques on Wikimedia.

Komodo dragons are the world’s largest living lizard and can be found only on select islands in the Indonesian archipelago. These massive lizards can grow to be 10 feet in length and up to 150 pounds! Their natural prey includes wild boars, deer, and water buffalo—animals which may outweigh them by several hundred pounds. So how does a lizard, even such a large one, manage to take down prey so much larger than them? The answer lies in their bite.

Komodo dragons’ mouths are a complex interplay of force, toxins, and bacteria. A study by Brian Fry and his colleagues at the Howard Florey Institute in Australia determined the amount of force that a komodo dragon could generate with its bite. What did they find the answer to be? Not much. They found that a komodo dragon’s bite was 6.5 times less than that of an Australian saltwater crocodile. That’s comparable to a 3.5 pound fennec fox! Obviously, this means that the komodo dragon couldn’t possibly bring down such large prey by strength alone. Luckily for them, there are two more factors at play.


Size comparison between a komodo dragon and a fennec fox.
Computer Rendered by Michelle Sonsalla.

The first is venom secreted by a number of venom glands found on the lower jaw. The amount of venom that can be held in these glands totals less than half a teaspoon! This venom has a number of properties meant to kill its prey, properties which prevent the prey’s blood from coagulating and cause painful cramping in the intestines, paralysis, and loss of consciousness. These effects alone would be enough to bring down most prey, but in case they aren’t, there is a final piece of the puzzle—bacteria.

All living things have a multitude of bacteria and fungi that are naturally present on their skin and in their digestive system, but the bacteria found in the mouths of komodo dragons are specialized. According to Joel Montgomery, a researcher at the University of Texas at Arlington, there are 54 species of bacteria found in the mouths of komodo dragons which cause illness and 1 species which has been found to be lethal to mice. These bacteria enter the prey’s bloodstream through its bite and work to infect the creature slowly, causing severe infection within days or weeks.

All three factors of a komodo dragon’s bite work together to take down its prey efficiently and effectively. The bite, though weak, is enough to open the skin and allow the venom and bacteria into the prey’s bloodstream. Once in the bloodstream, the venom works to weaken the animal, which in turn allows the bacteria to gain a foothold to infect, and eventually kill, the victim. These factors allow this large, magnificent lizard, this dragon among beasts, to take down prey much larger than themselves and have helped them survive the extinction of the past’s other great lizards.


References:

Christiansen P, & Wroe S (2007). Bite forces and evolutionary adaptations to feeding ecology in carnivores. Ecology, 88 (2), 347-58 PMID: 17479753

Fry, B., Wroe, S., Teeuwisse, W., van Osch, M., Moreno, K., Ingle, J., McHenry, C., Ferrara, T., Clausen, P., Scheib, H., Winter, K., Greisman, L., Roelants, K., van der Weerd, L., Clemente, C., Giannakis, E., Hodgson, W., Luz, S., Martelli, P., Krishnasamy, K., Kochva, E., Kwok, H., Scanlon, D., Karas, J., Citron, D., Goldstein, E., Mcnaughtan, J., & Norman, J. (2009). A central role for venom in predation by Varanus komodoensis (Komodo Dragon) and the extinct giant Varanus (Megalania) priscus Proceedings of the National Academy of Sciences, 106 (22), 8969-8974 DOI: 10.1073/pnas.0810883106

Merchant, M., Henry, D., Falconi, R., Muscher, B., & Bryja, J. (2013). Antibacterial activities of serum from the Komodo Dragon (Varanus komodoensis) Microbiology Research, 4 (1) DOI: 10.4081/mr.2013.e4

Montgomery JM, Gillespie D, Sastrawan P, Fredeking TM, & Stewart GL (2002). Aerobic salivary bacteria in wild and captive Komodo dragons. Journal of wildlife diseases, 38 (3), 545-51 PMID: 12238371

Is That Lizard Possessed!? (A Guest Post)

By Tawny Liebe

Image from Chuck Heston on Flickr.

A creature straight from the depths of hell… or at least as close as you can get on this planet. The Texas horned lizard or “horny toad” is found in the deserts of the southwest United States and has an unusual adaptation to deter predation exclusively from a few species of canines. When threatened by coyotes, foxes, and dogs, the horned lizard squirts blood from its eyes to hit targets up to three feet away! A total of six species of horned lizard have been proven to respond this way to canine attack, while none have responded this way to other predators, such as the grasshopper mouse or the roadrunner. So what is the deal? How do they do this and how does it work as predator defense?


Veins have one-way valves
to prevent backflow.
Drawing by Tawny Liebe.

Before we get to that, there are a few things you need to know. First of all, the circulatory system includes a network of arteries and veins. Arteries carry blood full of oxygen to body tissues while veins carry blood that lacks oxygen from the rest of the body back to the heart. This means that for the blood, in someone’s foot for instance, to get all the way back to the heart through the veins, the blood must work against gravity. When you (or this lizard or many other species) move, blood is propelled from one chamber in the vein to the next until it reaches the heart. The blood is prevented from flowing back into the previous chamber by one-way valves, causing the blood to pool.

In order to squirt blood from their eyes, horned lizards manipulate the network of veins in their head so that they build up pressure, like a volcano getting ready to blow. By constricting a pair of throat muscles unique to reptiles, they effectively close their jugular veins and increase the blood pressure in their head. This is thought to be enhanced by another pair of muscles between the jugular veins and the eyes that help increase the blood pressure in the head even further, causing the blood to move into the sinuses of the eyes. The pressure continues to build until the blood breaks through the wall of the eye socket into the eyelids where it is forced into the tear duct and erupts like a Mentos in a Coke bottle all over whatever is chomping at the poor little lizard.

As if these lizards couldn’t get any more amazing, recent studies have shown that it is actually a compound in their blood that canines don’t like. Scientists have also found that this chemical is in their circulating blood, not just the blood that is squirted from their eye, rejecting an earlier hypothesis that the chemical is picked up in the tear duct. To top off all of this awesomeness, the chemical may be acquired through its main food source- harvester ants, which are venomous. These ants aren’t actually a requirement of the horned lizard’s diet and yet they are specialized to eat them. The horned lizard’s blood plasma binds to the venom, which neutralizes its toxicity and the resulting compound may be what deters these canines.

Now that we know how horned lizards are capable of this type of defense and how they most likely make their blood so undesirable, what is it about this chemical that is so appalling to these predators? It appears that the target area of the blood is the mouth since the horned lizard only squirts the blood when the canine begins to bite down on its head. This suggests that it may be the taste of the blood that prevents the horned lizard from becoming that coyote’s tasty snack.

The horned lizard’s ability to squirt blood at canines to prevent their untimely death is truly amazing and complex and there is still much to learn about it. Their ability makes me wonder how many other cool anti-predator adaptations there are out there in the animal and even the plant kingdom! Below is a video that will allow you to appreciate the full effect of this awesome defense strategy, enjoy!

References:

Heath, J.E. 1966. Venous shunts in the cephalic sinuses of horned lizards. Physiological Zoology 39(1): 30-35.Middendorf, G.A. and Sherbrooke, W.C. 1992. Canid elicitation of blood-squirting in a horned lizard (Phrynosoma cornutum). Copeia 1992(2): 519-527.

Middendorf, G.A. and Sherbrooke, W.C. 1992. Canid elicitation of blood-squirting in a horned lizard (Phrynosoma cornutum). Copeia 1992(2): 519-527.

Middendorf, G.A. et. al. 2001. Comparison of blood squirted from the circumorbital sinus and systemic blood in a horned lizard, Phyrnosoma cornutum. The Southwestern Naturalist 46(3): 384-387.

Middendorf, G.A. and Sherbrooke, W.C. 2004. Responses of kit foxes (Vulpes macrotis) to antipredator blood-squirting and blood of Texas horned lizards (Phrynosoma cornutum). Copeia 2004(3): 652-658.

Sherbrooke, W.C. 1992. Chiricahua Mountains Research Symposium. Horny “toad” tales from the Chiricahua mountains as, told by a biologist. Southwest Parks and Monuments Association, Tuscon, AZ. 78-80.

The Real Catfish of Lake Tanganyika

Photo of Manti Te'o by Shotgun Spratling
and Neon Tommy at Wikimedia

Poor Manti Te’o may just be the most gullible schlub on the planet. For those of you that haven’t heard the story, the Notre Dame linebacker and runner-up for the 2012 Heisman Trophy led his team to the BCS National Championship Game, despite (or perhaps inspired by) the tremendous personal losses he has suffered this season. Last September, Te’o learned first of the death of his grandmother, and then within hours learned of the death of his girlfriend, Lennay Kekua. But after months of grieving and playing his heart out, Te’o began to receive phone calls from his “dead” girlfriend, telling him she missed him. Totally freaky, right? Notre Dame hired investigators to look into the undead girlfriend and they discovered that not only is Kekua not dead, she was never alive. The girl never existed. And what of Te’o’s relationship with her? According to Te’o, he never actually met her in person: Their entire long-term relationship took place online and over the phone, so he never realized that her entire persona was a fraud. He was completely and totally catfished.

He was what?

The top definition of catfish at Urban Dictionary reads:

“A catfish is someone who pretends to be someone they're not using Facebook or other social media to create false identities, particularly to pursue deceptive online romances.

Did you hear how Dave got totally catfished last month?! The fox he thought he was talking to turned out to be a pervy guy from San Diego!”

The term apparently originates with the 2010 documentary, Catfish, about a young man who falls in love with a woman on Facebook… who turns out to be someone else. Ew. But why the term catfish? A story in the movie explains that when cod are shipped from North America to Asia, their inactivity can result in mushy meat. Fishermen discovered that putting catfish in the cod tanks will keep the cod active and preserve meat quality. Like catfish for cod, the guy philosophizes, people that have deceptive identities keep idle people active. (The producers of the documentary now produce an MTV series by the same name about this online phenomenon).

But it’s not like real catfish can imitate others… Or do they?

Three poisionous Lake
Tanganyikan catfish. Figure from
Jeremy's 2010 Evolution paper.

A 2010 paper by Jeremy Wright at the University of Michigan at Ann Arbor documents the first known case of mimicry in catfish. There are several types of mimicry in the animal world. In this case, Jeremy was investigating functional Müllerian mimicry, a phenomenon in which two or more poisonous species mimic each other's predator-deterring warning signals (as opposed to Batesian mimicry, where a non-poisonous animal looks like a poisonous one). It may seem excessive to have both poison and warning coloration, but poison only helps after you’ve been bit. If your predators are smart enough to learn from experience, you can benefit from having more poisonous buddies around that look just like you so that if a predator bites just one of you it will then learn to avoid all of you. Sometimes it pays to look just like everyone else.

But just because you look like everyone else doesn’t mean that it is because you’re imitating others. I mean, maybe that’s just the way you look. So how do you know if a bunch of animals that look like one another are using functional Müllerian mimicry?

Jeremy studied a number of similarly-colored, poisonous and closely-related catfish species in the African Great Lake, Lake Tanganyika. All of these Tanganyikan catfish species (from the Synodontis genus) have dark spots on a yellowish background and dark fins with white borders. Could this be because of functional Müllerian mimicry?

Jeremy put a bunch of largemouth bass each into their own tank. Largemouth bass are predators that use their vision to find and eat most any fish that will fit in their mouths. But these bass were from Michigan, so they’d never had any experience with a poisionous, spotted Synodontis catfish. A clear barrier divided each tank in half and the bass was placed on one side of the divider, and a bite-sized fish was put on the other. The bite-sized fish was either a spotted and poisonous Synodontis multipunctata catfish, a spotted and poisonous Synodontis petricola catfish, or a not-spotted and not-poisonous minnow. He then counted how many times the bass struck the plastic divider in 5 minutes as a measure of how much that bass wanted to eat the bite-sized fish. After the 5 minutes were up, Jeremy removed the divider and watched to see if the bass ate the bite-sized fish. For each bass, he did this every day for 5 days, giving each bass the same species of bite-sized fish every day, so it could learn from its past experiences.

A naïve largemouth bass excited to eat a bitesized, but poisonous Synodontis petricola catfish.

A naïve largemouth bass gets to try to eat a bitesized, but poisonous Synodontis petricola catfish… and it doesn’t go so well for him.

A no-longer naïve largemouth bass gives his best death stare to a bitesized, but poisonous Synodontis petricola catfish. Videos provided by Jeremy Wright.

On the first day with the bite-sized fish, all the bass struck at the divider equally regardless of whether it was a spotted poisonous catfish or a minnow. But after their first bite, the bass given spotted poisonous catfish quickly lost their interest in them even though the bass given minnows continued to vigorously strike at them every day. When Jeremy later gave them a different species of bite-sized fish, those previously given a spotted poisonous catfish avoided both species of spotted poisonous catfish, but readily ate the minnows. So the bass had learned. Spotted catfish: bad! Minnows: yum! And the spotted catfish look was transferable between the two species… the hallmark of functional Müllerian mimicry. Further analysis of the venom revealed that these catfish species were all equally poisonous: Painful, but not deadly.

Online catfish like Lennay Kekua are usually like these real-life spotted poisonous catfish: painful, but not (usually) deadly. And they typically have facebook pages and twitter accounts full of sexy photos and superficial chatter. If we’re smart, we can learn to avoid them. Do you know if all your “friends” on social media sites are who they say they are?

Want to know more? Check this out:

Wright, J. (2011). CONSERVATIVE COEVOLUTION OF MÜLLERIAN MIMICRY IN A GROUP OF RIFT LAKE CATFISH Evolution, 65 (2), 395-407 DOI: 10.1111/j.1558-5646.2010.01149.x