Evolution of the Fang - From Simple Tooth to High-tech Syringe

The last post explored the contents and effects of venom for a few different animals. For venom to be effective, it must have direct access to the circulatory system of the victim. In other words, it must be injected directly into the body. This post will cover one of the methods utilised by venomous animals to deliver their deadly payload.

Perhaps the most infamous method of venom delivery is through fangs.
Fangs are essentially specialised teeth used to inject venom directly into the body of another organism. They are most commonly found in reptiles, but have also evolved in arachnids such as the Funnel-Web Spider. This post will focus on the most advanced fangs seen in reptiles, which can be found on Vipers. Vipers are famous for their long fangs which act as syringes to inject large amounts of venom into their prey.

A Bush Viper, native to sub-Saharan Africa, and a great looking snake!

So how did they evolve? It is known that venom is far older than fangs themselves, and dinosaurs with a venomous bite have been identified from the early cretaceous, or approximately 120 million years ago. Note that a venomous bite does not imply the use of fangs. Even today, many lizards have a venomous bite, albeit far less potent than in snakes. A great example of venom in lizards can be seen in the monitors, or more specifically, the largest of the monitors. The Komodo Dragon.

Each kill made by a Komodo Dragon is a narrative in itself.
After tracking down a likely prey item such as a buffalo, a Komodo Dragon will charge towards it and latch onto any part of the body. It does not need to go for the windpipe like other large predators, it does not even need to overpower its prey. In fact, after a brief struggle, the Komodo Dragon releases the buffalo, leaving a nasty, but easily survivable flesh wound. After escaping with its life, it runs far away and continues with its life. In the coming days, however the buffalo becomes weaker and weaker.The bite area is now horrifically infected, and the animal is fighting for life. Another day passes and the buffalo can no longer remain standing, and within a matter of hours, it succumbs and dies. This is when the Komodo Dragon reappears. After following the scent of the animal for days, often with several other dragons, it begins to eat, sparking a feeding frenzy as more dragons arrive.
It has long been thought that the Komodo Dragon utilised bacteria in its mouth to inflict a lethal infection in its prey. However, in more recent observations, it has been noted that a bitten prey item experiences symptoms that bacterial infections do not induce, and that the death is alot faster than a bacterial infection alone could cause.
When analysing the jaw area of a Komodo Dragon, small venom glands were found. Saliva samples confirmed the presence of a venom that can prevent blood clotting, cause muscle paralysis and a decrease in blood pressure.

The potent saliva of a Komodo Dragon.

Similar tests conducted on other monitor lizards such as the Australian Perentie and the Crocodile Monitor of New Guinea indicated that they too are also very mildly venomous.
This shows that it is entirely possible to deliver venom with simple teeth, so how did the advanced hollow fangs arise?As with all evolutionary debates, it is about fitness. The more venom administered the better. A prey item that dies in a few minutes is less energy-draining for the predator than one that takes days to die, and it also reduces the amount of time the predator needs to stay out in an environment where they are not the top predator. This is perhaps the reason why snakes have evolved such potent venom in comparison to monitors, as snakes are often preyed upon, while the much larger Komodo Dragon and Perentie are not.

The question now is how did simple teeth turn into the hypodermic needles seen in some snakes today? The answer involves an intermediate stage. Many animals, during their embryonic stages of life, show physical traits which are similar to an ancestral group. A key example of this is the tail seen on human embryos during development. In other words, the embryonic stages of animals gives us an insight into some of the traits of its ancestors. Before the fangs of a viper erupt from its gum, they are not hollow. Instead, they are grooved, and as development continues and the fang erupts, this groove is enclosed to form a hollow structure. Grooved teeth can be seen today in many venomous snakes lizards, and they facilitate the transport of venom the gland in the mouth of the animal to the body of the prey.
It is hypothesised that the grooved tooth is the intermediate stage between a simple tooth and a hollow fang. In time, individuals with a deeper groove became dominant, due to their ability to administer more venom. The deep groove eventually closed over completely, and turned what was once a tooth into a hypodermic needle.

A useful diagram showing the different parts of a snake's venom delivery system.

Another adaptation to assist the injection of venom are the muscles surrounding the venom gland in snakes. When the snake bites, these muscles contract, forcing venom through the fang and into the body of the victim. The most notable group of snakes with this trait are the Vipers, including the Gaboon viper, which has the largest fangs of any snake. This ability to contract muscles around the venom gland is also how spitting cobras are able to spit their venom.

The impressive fangs of a Gaboon Viper.

In the next post, I will be venturing into a marine setting, and writing about the various forms of venomous animals found there, and their evolutionary stories. I'll leave you with a link to a documentary about the evolution of venom for anyone who would like to know a little more about this topic.

The Evolution of Nature's Deadliest Weapon


Sources:
http://www.nature.com/news/2010/101117/full/news.2010.617.html - Accessed 18Mar14
http://www.mapoflife.org/topics/topic_388_Venom-and-venom-fangs-in-snakes-lizards-and-synapsids/ - Accessed 18Mar14
http://en.wikipedia.org/wiki/Sinornithosaurus - Accessed 18Mar14
http://news.nationalgeographic.com.au/news/2009/05/090518-komodo-dragon-venom.html- Accessed 22Mar14
http://www.youtube.com/watch?v=JcEapEccNIc Accessed 26Mar14
Images:
http://sciencelakes.com/data_images/out/25/8850662-bush-viper-snake.jpg Accessed 29Mar14
http://upload.wikimedia.org/wikipedia/commons/b/b4/Bitis_gabonica_fangs.jpg -Accessed 29Mar14
http://jeanbont.pbworks.com/f/1299294821/snake%20fangs%20and%20venom.png -Accessed 29Mar14
https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgwnScSHSR-4z9m4CCzSnD-guyCAEeBiQBbu0C9oJpsND9s7aNI-uqRXkGKzCe9664_xFxR-hnAM9Bj4IyHl3daITWW4Ph_dlti-VWD2fZlG5evIFN6HvSarZV3IxTypojKkZN_PiGmaOnv/s1600/koodo+dragon+saliva.jpg -Accessed 29Mar14
 http://www.youtube.com/watch?v=JcEapEccNIc - Accessed 30 Mar14

What is venom made of?

The exact contents of venom varies between organisms, and each species has its own unique toxin cocktail. Venom is usually described by the predominant variety of toxin. The main varieties are: cytotoxic, myotoxic, haemotoxic and neurotoxic. Different types of toxins attack different systems and areas of the body. Cytotoxic venom attacks the cellular tissue around the site of envenomation, myotoxic venom attacks the muscular system, haemotoxic venom attacks the blood, and last of all, neurotoxic venom attacks the nervous system.

The venom of the tiger snake is highly neurotoxic, and affects mainly the nervous system, although it is also myotoxic and haemotoxic. The venom of a rattlesnake, on the other hand, is primarily haemotoxic, and causes the blood to coagulate, which inhibits circulation of oxygen around the body. As with the tiger snake, the venom of a rattlesnake also contains myotoxic and neurotoxic elements.

The result of adding one drop of haemotoxic venom to a blood sample. It took only a few minutes for the blood to coagulate into a thick jelly.

The fact that venom consists of these four different types of toxin is what makes it so effective as a weapon. Each constituent plays its part in killing the prey item. The neurotoxic element cripples the nervous system, and in most cases paralyses the victim, while the haemotoxin prevents the supply of oxygen to vital organs (most importantly the brain). At the same time, the myotoxin attacks the muscles, causing spasms and destruction of the muscle tissue while the cytotoxin affects the tissue around the bite, which, in time, becomes a gruesome mess of swelling, blisters and dying tissue. If you are a prey item such as a field mouse, however, it is unlikely you would be worrying about long-term damage to the bite site. Having less than two minutes to live changes perspective, even for a mouse.

Unlike the unfortunate field mouse, humans often have access to medical treatment, and antivenin for almost all venomous animals to combat the effect of envenomation. If you are stung by the stonefish, however, it may not be so simple. The pain alone of the stonefish's sting can be enough to cause the body to go into shock, and it is possible to die simply from the pain. It is not unheard of for patients to request the amputation of the affected extremity just to be rid of the pain.

There is no doubt that the toxicity of venom produced by a venomous animal is an elegant and lethal weapon, however without the means of delivering it, the organism can do no harm, just as a bullet can do no harm without the appropriate firearm. Dropping venom onto skin will not help kill the prey. It must be injected into the organism, allowing direct access to the blood and lymphatic system, which helps the spread of venom throughout the body. The animal kingdom has evolved many creative means of injecting venom into prey. The next post on this blog will investigate the different structures that organisms have developed, and how they could have evolved.




Sources:
http://en.wikipedia.org/wiki/Snake_venom - Accessed 15MAR14
http://scribol.com/environment/the-worlds-5-most-venomous-species/2 -Accessed 15MAR14
http://www.venomdoc.com/venomdoc/Reptiles.html -Accessed 16MAR14

Image:
http://img.thesun.co.uk/aidemitlum/archive/01546/Blood-thickens-frm_1546499a.jpg
-Accessed 16MAR14

A Weapon of Distinction

 Attention is always given to the large predators who overpower their prey with brute force and ferocity. Great white sharks that launch themselves meters into the air as they attack seals from below, or a 6-metre Nile crocodile throwing itself out of the water to drag a 700kg buffalo to a watery grave. These great feats of strength and aggression provide a spectacle, and people look on at these scenes through their television with a mixture of fear and awe. This blog will not be about these barbaric and ferocious methods of hunting. Instead, it will focus on the evolution of an adaption for killing that is infinitely more complex, and infinitely more devious. Venom.

Venom can be described as a formidable mixture of complex molecules that, once injected into a victim's body, can attack the blood, muscle tissue or nervous system in order to kill, paralyse or otherwise incapacitate the  unfortunate receiving organism. It is markedly different from the term "poison" in that poison is ingested or absorbed, while venom is administered directly into the body through physical structures adapted for the task such as fangs, spurs or spines.  

The venomous spines of a Stonefish.

Venom has emerged as a highly effective chemical weapon in many organisms.  To compare the use of venom to the tools used by apex predators mentioned earlier,  you could just as well compare a sundial to a Rolex watch. Yes they both tell the time, however a sundial is simple and direct, while the watch is an elegant, intricate and often mind-boggling blend of cogs, gears, and of course, no small measure of witchcraft.

 A testament to the effectiveness of venom is the fact that it has evolved independently in reptiles, insects, mammals, fish, cephalapods and Cnidarians. While the mechanism of envenomation and type of venom varies between these animals, the basic theory behind the use of venom remains the same. 
The main advantage of venom, besides its lethal effectiveness, is the fact that, unlike poison, it can be used as a defence mechanism against predators as well as a hunting aid. It would also do well to point out that its use for defence does not require being ingested by the predator in order to be effective. 


Sources:
 http://en.wikipedia.org/wiki/List_of_venomous_animals - Accessed 8MAR14

Image:
http://jaredbullock.files.wordpress.com/2012/03/stonefish1.jpg -Accessed 8MAR14