How Australian Snakes Evolved to be so Venomous
THE EVOLUTION OF THE HIGHLY TOXIC VENOM FOUND IN AUSTRALIAN ELAPIDS
Australian fauna is infamous for the toxicity of its venomous snakes, but little is known about the evolutionary circumstances leading to such a characteristic. With a limited representation in the fossil record, it is difficult to track the evolution of these snakes back through time (Johnson, 1955). Like much of the biota of Australia, its venomous snakes are unique both in progeny and ecology. I propose the key in the evolutionary path of these snakes is held in the biogeographical isolation experienced on the Australian continent coupled with ecological factors surrounding the prey these snakes rely on.
Colubridae is the largest snake family in the world, but in Australia the family is extremely underrepresented only being found in the north and east coast of the continent (Hodgson and Wickramaratna, 2006). Australia’s serpent fauna instead is made up mainly of the venomous snake family Elapidae , being characterized by their small head size and closed-canal fangs on the front of their upper jaw (Shea, Shine and Covacevich, 1993). The Colubridae family is presumably only a recent arrival around fifteen million years ago with the collision between Australia and Southeast Asia (Ehmann, 1993) and in its absence Elapidae has diversified and filled niches all across the continent in relative isolation. There are 83 elapids present in Australia, including almost all of the venomous snakes present (Shea, Shine and Covacevich, 1993). Recent studies suggest that the Australian members of the family Elapidae are more closely related to members of the sea snake family Hydrophiidae than their terrestrial counterparts on other continents (Shea, Shine and Covacevich, 1993). There is some evidence suggesting that these sea snakes have radiated out from the terrestrial elapids in Australia, perhaps giving them the same evolutionary history resulting in their fierce venom (Shea, Shine and Covacevich, 1993). Unfortunately there are little fossil remains suggesting the origin of elapids in Australia, but they are believed to have either evolved in Australia or arrived from another area while the continent was connected to Gondwana. Some recent studies suggest the group’s divergence occurred much later, around 20 million years ago (Shea, Shine, and Covacevich, 1993).
Australian elapids are generally active hunters searching for their small prey. Some employ a strike and release method of capture while other species use a strike and constrict method to prevent prey escape (Shea, Shine and Covacevich, 1993). The venom delivered in their bite varies in amount and composition from species to species, but holds an arsenal of highly lethal chemicals including neurotoxins, myotoxins, procoagulants, anticoagulants, haemolysins, and phospholipases (Shea, Shine and Covacevich, 1993). A single species of Australian elapid can contain over 100 unique protein sequences having various functions (Pierre et. al., 2007).
Venom can have many benefits to the fitness of a snake, but is also highly energy expensive to produce. A study by McCue (2006) on pitvipers found resting metabolic rate to be elevated by 11 percent relating to venom replenishment for 72 hours following venom extraction. The question then is why would Australian snakes evolve such highly potent venom considering the energy required to produce it? In general, venom is extremely valuable because it allows snakes to bring down prey items much larger than without toxins. A venomous bite may also be especially useful in juvenile individuals allowing them to bring down larger prey items than they otherwise would be able to at such an age (Minton and Minton, 1981). Larger prey allows the snake to quickly reach maturity or to acquire a large energy store for long periods without food. The advantages of venom may even go beyond predatory success in these snakes. Many snakes use their venom to begin digestion internally within the prey, reducing the energy and time required to the process their food once ingested (Gans, 1961). Furthermore many snakes use a strike and release tactic, injecting the venom and leaving the prey to be subdued, which protects the snake from retaliation (Shine and Covacevich, 2006). Finally there is a defensive advantage detouring any potential predator away with the threat of a deadly bite.
All of these advantages of venom would explain the occurrence of venom in Australian elapids, but what advantage is there in developing such extreme toxicity? With the energy budget in mind, perhaps more potent venom can actually conserve resources by allowing the snake to use less of its venom to bring down a prey item. Furthermore, the stronger and more rapid acting the venom the less likely a prey item will escape. Predatory success is crucial in the harsh arid zones throughout Australia where food supplies are limited. Beyond these obvious considerations, it is unclear why a snake would need venom many times more potent than a lethal strength. Snakes are exposed to similar harsh circumstances around the world, yet Australia remains a central point of extreme toxicity.
The effectiveness of some Australian snake venom in actually subduing their prey in some cases has even come to question. When used against mammals, the snake’s venom is unsurprisingly highly effective. This is an interesting phenomenon however, as most mammalian prey items like mice would have only arrived 15 million years ago when Australian and Southeast Asian biota collided. It is possible however, that this venom could have evolved in conjunction with a change in diet to small mammals, evident in the specialization of some snakes for mammalian prey such as Oxyuranus sp (Shine and Covacevich, 2006). In the case of Oxyuranus highly toxic venom is extremely advantageous because it allows them to take down large mammalian prey such as rats, while limiting the risk of injury with venom that kills the prey quickly and allows them to strike and release (Shine and Covacevich, 2006).Most Australian snakes however do not predate solely on mammals and are still highly toxic. For these snakes their toxic venom is still usually effective against anurans, geckos, and frogs, which are typical prey items (Minton and Minton, 1981). Interestingly enough one of their most common prey, skinks, was found to be highly resistant to the venom in a study by Minton and Minton (1981). Their study suggests that snakes of the genus Pseudonaja for example that feed primarily on skinks and agamids, are little affected by their highly toxic venom (Minton and Minton, 1981). With this in mind, why would one of the most venomous genuses in Australia prey on reptiles that do not succumb to their toxins? The answer may simply be that with 370 species of skink (family Scincidae) in Australia, they represent too significant of a food source for the snakes not to take advantage of despite their resistance (Hutchinson, 1993). Looking from a coevolutionary standpoint perhaps skinks’ and other common prey items ability to resist venom is not coincidental, but a direct cause of heavy predation by venomous snakes. A study by Heatwole and Poran (1995) concluded that resistance in eel species (Gymnothorax hepaticus and G. undulatus) to venom of Laticauda colubrine, an Australian sea snake known to specialize in eels species, is likely the result of coevolution. Scincidae is thought to be present in Australia at least since the early Tertiary period (Hutchinson, 1993), leaving perhaps 65 million years for coevolution to occur with elapid species. Furthermore, for forty million years Australia was surrounded by ocean, during which there was relatively no immigration of new species or populations. With little outside interference, the predatory snakes are locked in a direct battle with their prey. In response to the evolving venom in snakes many prey species, specifically skinks, developed resistance to that venom. As resistance rose in these species, the toxicity of the snake’s venom had to further evolve in potency to have its desired effects. For further evidence of the coevolution and altering of the Australian predatory snakes we can look to elapid species that employ both venomous bites and constriction feeding mechanisms. Elapids such as Pseudonaja and Notechi sp. use constriction on frogs and lizards to subdue the prey after striking, while not with small mammals (Shine and Schwaner 1985). This strategy seemingly is an adaptation to prevent the threat of prey escape in the more resistant species, while keeping a safer distance from the species capable of a dangerous retaliation.
It seems likely that an extreme toxicity developed terrestrially among lizard eating elapids then expanded outward into other niches such as the predation of small mammals. In Australian marine snakes a similar coevolution may have occurred, as was the case with Gymnothorax sp., or their toxicity may have developed terrestrially before the divergence of the two families.
Snake venom has developed as a means to bring down prey that could not otherwise be overpowered. Snakes of the family Elapidae have perfected this feeding mechanism having species located all over the world. Like most Australian fauna, the Australian elapids are found to be distinct among their global counterparts. The continent’s drift through isolation has allowed for unique evolutionary circumstances where in the absence of Colubridae, the venomous family Elapidae has come to dominance and the following preditor-prey coevolution, a community of extremely toxic snakes has evolved.
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