It’s well known that deadly snakes strike very swiftly, and it is easy to infer that if you’re unlucky enough to be bitten, the moment of contact will be as simple as it is sudden: a lightning-quick penetration that precipitates an emergency. It may be unsettling therefore to view the results of very recent research that has captured, on ultra-slow motion video, the attack styles of 36 very venomous serpents.
The study was led by researchers from the School of Biological Sciences at Monash University, in snaky Australia, but it did not restrict itself to Antipodean reptiles. Instead, the Australians collaborated with researchers at Venomworld, a venom collection center on the eastern outskirts of Paris. Venomworld houses dozens of species across the big three families of poisonous snakes: vipers, elapids and colubrids.
Vipers include rattlesnakes such as the western diamondback, which is believed to inflict more bites on humans than any other snake in the United States. Among the elapids are cobras, and in Australia death adders and tiger snakes. Members of both families deliver venom from fangs rooted at the front of their jaws. Unlike those of an elapid, the large fangs of a viper are hinged.
Colubrids, by contrast, tend to be rear-fanged, with a mangrove snake (also called the gold-ringed cat snake), widespread in south-east Asia, among two examples of this class whose biting style was examined.
The study was innovative for the very high frame rate – more than 1,000 fps – achievable by the cameras that recorded the animals’ behavior, and for shooting from two cameras simultaneously, set in different positions, from which software could generate 3D representations. It was also by far the biggest study of this type to compare snakes across different families.
The snakes were induced to strike at cylinders of medical gel, which had been painted with two dots (to resemble eyes), warmed to mammal body temperature, and placed at one end of a snake hook. Strikes were provoked by advancing and suddenly withdrawing the gel, and sometimes by tapping the snake on its tail. Multiple strikes from each species of snake were recorded.
Arguably the most disturbing feature of the results is the evidence that venomous snake bites tend not to be simple. Rather, each snake family displayed characteristic ways of maximizing the effects of a contact.
Vipers used their folding fangs to contrive serial penetrations from a single strike. About half of the 31 viperid species often augmented a first penetration, in which case “one or both of the fangs were removed from the gel and repenetrated at a more favorable angle,” the study reported.
Deinagkistrodon acutus, commonly known as the Chinese moccasin, a viper.
“When the snake only just reached its prey and needed a better grip, the fang was removed and that side of the jaw was moved forward, ‘walking’ the fang further onto the prey so that it could be repenetrated in a better position,” the study said.
The fixed fangs of elapids denied them this option. Instead, they resorted to multiple injections of venom at the initial site. “After the first bite, they slightly released and bit again a few times,” the researchers said. “This repeated biting action is likely a result of the muscles tensing repeatedly to squeeze the venom gland and inject more venom.”
Aspidelaps lubricus, commonly known as the Cape coral cobra, an elapid.
Poisonous colubrids had developed a third technique that created the biggest wounds. “After prey contact, they closed their mouths and started to alternatingly drag their maxilla [jaws] across the prey surface, resulting in two crescent-shaped cuts. This large wound likely ensures good venom transfer into the prey,” the researchers said.
The study was also able to compare the elapsed time of snake strikes, from initiation to contact, across species, finding that vipers were the most rapid, on average, but that some elapids matched the speediest of them.
Prior studies had established that humans and other mammals have startle reflexes that initiate body movement in the range 60–395 milliseconds after a stimulus. In this study, 84% of the vipers recorded reached their prey within 90 ms of initiating a strike, and 55% did so within a mere 60 ms.
Among elapids, elapsed time for the rough-scaled death adder, a denizen of northern Australian deserts, was only 30 ms, while the African forest cobra reached its target in 73 ms, on average.
Several deadly species of snake are widespread in Australia, even in the temperate south where I’ve grown up. You might meet one when hiking in the countryside, on a rural property or even in a suburban backyard, especially in late spring when many emerge from winter hideouts and go hunting.
The official policy drummed into residents is to avoid alarming or attempting to kill snakes, since their interactions with humans are wholly defensive. The study results reinforce that advice: if a venomous snake is within striking range you are probably powerless to deflect an attack, because it will reach you before you can move.
Nevertheless, surprise encounters occur and about 3,000 Australians each year are bitten. Typically, only two of those bites will be fatal. While the availability of anti-venom treatment contributes to the survival rate, the complexity observed in the study helps explain why some snakebites are much more deadly than others.
The study was published on 23 October in the Journal of Experimental Biology.
Source: Monash University
