Only fish have gills, right? Wrong. Meet Hydrophis cyanocinctus, a snake that can breathe through the top of its own head.
The 3m species, which is native to Australian and Asian coastal waters, can draw in oxygen with the help of a unique set of blood vessels below the skin in its snout and forehead.
The network of blood vessels works very similarly to a fish’s gills, and represents a newly discovered addition to the extraordinary range of adaptations that sea snakes use to thrive below the waves.
In evolutionary terms, sea snakes are relative newcomers to aquatic life, having evolved from land-based snakes only about 16 million years ago. This is much more recent than marine mammals such as whales and dugongs, which arose around 50 million years ago.
The roughly 60 known species of sea snakes have nevertheless developed an impressive array of adaptations to marine life. These include salt glands under the tongue, nostrils that face upwards and can be sealed by valves, paddle-like tails to facilitate swimming, and the ability to absorb oxygen and eliminate carbon dioxide through their skin.
Some sea snakes have even evolved light sensors on the tips of their tails, possibly as a way to avoid having them nibbled off by predators when partially hidden in crevices.
Just when we thought we had uncovered all the strange things sea snakes do, we discovered something new. As we report today in the journal Royal Society Open Science, the annulated sea snake Hydrophis cyanocinctus effectively has a set of gills on its forehead.
The first sign of something unusual was an odd hole (in anatomical terms, a “foramen”, the Latin word for “hole”) in the roof of this species’ skull.
This hole is reminiscent of the “pineal foramen” found in several lizard species, which contains a tiny light-sensitive organ called the pineal eye. Could sea snakes also have a pineal eye?
No trace of such a foramen has ever been found in a modern snake. In fact, snakes are thought to have lost the pineal foramen at least 100 million years ago, which is the age of the oldest reasonably complete fossil snakes.
However, because some sea snakes have light-sensitive organs in their tails, we couldn’t rule out the possibility of a light-sensitive organ reappearing in its ancestral position in the skull – snakes did evolve from lizards, after all.
We decided to investigate this unexpected foramen in H. cyanocinctus more closely. We obtained some live specimens from Vietnam, where sea snakes are commonly sold as food in fish markets, and generated images of the soft tissues around the foramen using a combination of traditional and computer-assisted methods.
These images revealed that this snake does not have a pineal eye. What actually goes through the mysterious hole in its skull is a large blood vessel (sometimes paired). This blood vessel then travels forward and branches into a complex network of veins and sinuses immediately under the skin of the forehead and snout.
We then examined other snakes, both terrestrial and marine, using the same methods, and realised that this network of blood vessels in H. cyanocinctus is unique.
Did snakes evolve from ancient sea serpents?
While a network of blood vessels is expected to be present under the skin of all snakes, what is special about H. cyanocinctus is the greatly exaggerated size of the blood vessels and the fact that they converge towards a single large vein that goes into the brain.
This strange network of blood vessels makes sense when we consider that sea snakes can breathe through their skin. This happens thanks to arteries containing much lower oxygen concentrations than the surrounding seawater, which allows oxygen to diffuse through the skin and into the blood.
However, these low oxygen levels in arterial blood can cause problems, because the brain may not get the oxygen it needs. The dense network of veins on the forehead and snout of these sea snakes helps solve this problem by picking up oxygen from seawater and redistributing it to the brain while swimming underwater.
If you think that sounds similar to what fish do with their gills, you’re absolutely right. H. cyanocinctus has managed to evolve a respiratory system that works in much the same way as gills, despite the vast evolutionary distance between these two groups of species. Truly, these snakes are indeed creatures of the sea.
We are all familiar with the concept of “fake news”: stories that are factually incorrect, but succeed because their message fits well with the recipient’s prior beliefs.
We and our colleagues in conservation science warn that a form of this misinformation – so-called “feelgood conservation” – is threatening approaches for wild animal management that have been developed by decades of research.
The issue came to a head in February when major UK-based retailer Selfridges announced it would no longer sell “exotic” skins – those of reptile species such as crocodiles, lizards and snakes – in order to protect wild populations from over-exploitation.
But this decision is not supported by evidence.
Banning the use of animal skins in the fashion industry sounds straightforward and may seem commendable – wild reptiles will be left in peace, instead of being killed for the luxury leather trade.
But decades of research show that by walking away from the commercial trade in reptile skins, Selfridges may well achieve the opposite to what it intends. Curtailing commercial trade will be a disaster for some wild populations of reptiles.
How can that be true? Surely commercial harvesting is a threat to the tropical reptiles that are collected and killed for their skins?
Actually, no. You have to look past the fate of the individual animal and consider the future of the species. Commercial harvesting gives local people – often very poor people – a direct financial incentive to conserve reptile populations and the habitats upon which they depend.
If lizards, snakes and (especially) crocodiles aren’t worth money to you, why would you want to keep them around, or to protect the forests and swamps that house them?
The iconic case study that supports this principle involves saltwater crocodiles in tropical Australia – the biggest, meanest man-eaters in the billabong.
Overharvested to the point of near-extinction, the giant reptiles were finally protected in the Northern Territory in 1971. The populations started to recover, but by 1979-80, when attacks on people started to occur again, the public and politicians wanted the crocodiles culled again. It’s difficult to blame them for that. Who wants a hungry croc in the pond where your children would like to swim?
But fast-forward to now and that situation has changed completely. Saltwater crocs are back to their original abundance. Their populations bounced back. These massive reptiles are now in every river and creek – even around the city of Darwin, capital of the Northern Territory.
This spectacular conservation success story was achieved not by protecting crocs, but by making crocs a financial asset to local people.
Eggs are collected from the wild every year, landowners get paid for them, and the resulting hatchlings go to crocodile farms where they are raised, then killed to provide luxury leather items, meat and other products. Landowners have a financial interest in conserving crocodiles and their habitats because they profit from it.
The key to the success was buy-in by the community. There are undeniable negatives in having large crocodiles as neighbours – but if those crocs can contribute to the family budget, you may want to keep them around. In Australia, it has worked.
The trade in giant pythons in Indonesia, Australia’s northern neighbour, has been examined in the same way, and the conclusion is the same. The harvest is sustainable because it provides cash to local people, in a society where cash is difficult to come by.
So the evidence says commercial exploitation can conserve populations, not annihilate them.
Why then do companies make decisions that could imperil wild animals? Probably because they don’t know any better.
Media campaigns by animal-rights activists aim to convince kind-hearted urbanites that the best way to conserve animals is to stop people from harming them. This might work for some animals, but it fails miserably for wild reptiles.
We argue that if we want to keep wild populations of giant snakes and crocodiles around for our grandchildren to see (hopefully, at a safe distance), we need to abandon simplistic “feelgood conservation” and look towards evidence-based scientific management.
We need to move beyond “let’s not harm that beautiful animal” and get serious about looking at the hard evidence. And when it comes to giant reptiles, the answer is clear.
The ban announced by Selfridges is a disastrous move that could imperil some of the world’s most spectacular wild animals and alienate the people living with them.
Daniel Natusch, Honorary Research Fellow, Macquarie University; Grahame Webb, Adjunct Professor, Environment & Livelihoods, Charles Darwin University, and Rick Shine, Professor in Evolutionary Biology, University of Sydney
Few sights and sounds are as emblematic of the North American southwest as a defensive rattlesnake, reared up, buzzing, and ready to strike. The message is loud and clear, “Back off! If you don’t hurt me, I won’t hurt you.” Any intruders who fail to heed the warning can expect to fall victim to a venomous bite.
But the consequences of that bite are surprisingly unpredictable. Snake venoms are complex cocktails made up of dozens of individual toxins that attack different parts of the target’s body. The composition of these cocktails is highly variable, even within single species. Biologists have come to assume that most of this variation reflects adaptation to what prey the snakes eat in the wild. But our study of the Mohave rattlesnake (Crotalus scutulatus, also known as the Mojave rattlesnake) has uncovered an intriguing exception to this rule.
A 20-minute drive can take you from a population of this rattlesnake species with a highly lethal neurotoxic venom, causing paralysis and shock, to one with a haemotoxic venom, causing swelling, bruising, blistering and bleeding. The neurotoxic venom (known as venom A) can be more than ten times as lethal as the haemotoxic venom (venom B), at least to lab mice.
The Mohave rattlesnake is not alone in having different venoms like this – several other rattlesnake species display the same variation. But why do we see these differences? Snake venom evolved to subdue and kill prey. One venom may be better at killing one prey species, while another may be more toxic to different prey. Natural selection should favour different venoms in snakes eating different prey – it’s a classic example of evolution through natural selection.
This idea that snake venom varies due to adaptation to eating different prey has become widely accepted among herpetologists and toxinologists. Some have found correlations between venom and prey. Others have shown prey-specific lethality of venoms, or identified toxins fine-tuned for killing the snakes’ natural prey. The venom of some snakes even changes along with their diet as they grow.
We expected the Mohave rattlesnake to be a prime example of this phenomenon. The extreme differences in venom composition, toxicity and mode of action (whether it is neurotoxic or haemotoxic) seem an obvious target for natural selection for different prey. And yet, when we correlated differences in venom composition with regional diet, we were shocked to find there is no link.
In the absence of adaptation to local diet, we expected to see a connection between gene flow (transfer of genetic material between populations) and venom composition. Populations with ample gene flow would be expected to have more similar venoms than populations that are genetically less connected. But once again, we drew a blank – there is no link between gene flow and venom. This finding, together with the geographic segregation of the two populations with different venoms, suggests that instead there is strong local selection for venom type.
The next step in our research was to test for links between venom and the physical environment. Finally, we found some associations. The haemotoxic venom is found in rattlesnakes which live in an area which experiences warmer temperatures and more consistently low rainfall compared to where the rattlesnakes with the neurotoxic venom are found. But even this finding is deeply puzzling.
It has been suggested that, as well as killing prey, venom may also help digestion. Rattlesnakes eat large prey in one piece, and then have to digest it in a race against decay. A venom that starts predigesting the prey from the inside could help, especially in cooler climates where digestion is more difficult.
But the rattlesnakes with haemotoxic venom B, which better aids digestion, are found in warmer places, while snakes from cooler upland deserts invariably produce the non-digestive, neurotoxic venom A. Yet again, none of the conventional explanations make sense.
Clearly, the selective forces behind the extreme venom variation in the Mohave rattlesnake are complex and subtle. A link to diet may yet be found, perhaps through different kinds of venom resistance in key prey species, or prey dynamics affected by local climate. In any case, our results reopen the discussion on the drivers of venom composition, and caution against the simplistic assumption that all venom variation is driven by the species composition of regional diets.
From a human perspective, variation in venom composition is the bane of anyone working on snakebite treatments, or antidote development. It can lead to unexpected symptoms, and antivenoms may not work against some populations of a species they supposedly cover. Anyone living within the range of the Mohave rattlesnake can rest easy though – the available antivenoms cover both main venom types.
Globally, however, our study underlines the unpredictability of venom variation, and shows again that there are no shortcuts to understanding it. Those developing antivenoms need to identify regional venom variants and carry out extensive testing to ensure that their products are effective against all intended venoms.
It’s spring in Australia and that means reptiles are starting to move about again. Including snakes.
The venom of the eastern brown snake (Pseudonaja textilis) is, drop for drop, one of the most potent of any venoms tested on laboratory mice.
Venoms work by targeting the bitten animal with deadly chemicals. And our recent research shows toxins in the venom of eastern brown snakes change as the snakes grow from juveniles to adults. It’s the first example of a significant age-related change in venom from an Australian snake.
It’s a beautiful example of evolutionary adaption, in which the chemistry of the snake’s venom appears to change in parallel with its diet.
Venoms are typically a mixture of different toxins, each of which attacks the system of a potential prey animal or predator in a different way.
Sometimes toxins work together, each making the other more powerful, and sometimes they work completely independently, engaging in chemical warfare on multiple fronts.
Brown snake venom contains many toxins, but there is one toxin above all others that is responsible for the life-threatening effects of bites to humans. This toxin is a “haemotoxin”, which means it attacks the blood.
The haemotoxin starts clotting the blood at an extremely elevated rate, using up all of the coagulation factors, which clot the blood under normal circumstances. When all these are used up, the victim is at risk of bleeding to death.
In the worst case scenario this toxin, perhaps working with others, gives the system such a shock that people collapse within a short period of time following the bite. In this situation, immediate CPR can be the difference between life and death.
Venom is a tool that has evolved in snakes to help them secure a meal: it gives them a chance of overpowering animals that would otherwise be very difficult for them to subdue. Venom and its toxins are therefore “designed” (by evolution) to mess up the normal operations of a prey animal’s body.
The best toxins for this purpose may differ according to the specific type of prey animal (e.g. mammal or reptile), or the condition of that prey animal (e.g. whether it is active or inactive) when the snake finds it. As a result, we often find snakes that feed upon different types of animals have different toxins in their venoms.
This starts to get really interesting when you consider brown snakes, because adult brown snakes seem to have quite different diets from baby brown snakes.
Age-related shifts in venom chemistry have already been demonstrated for the venoms of a few species of pit vipers from the Americas, but not for anything even remotely related to Australian brown snakes.
This wasn’t because people hadn’t looked – several species of Australian snake had been investigated, but no evidence of a significant age-related change in venom had been found for any of them. This made sense to me, because none of those snakes dramatically change their diets throughout their lives.
Brown snakes are special – as far as we know the juveniles eat lizards almost exclusively, whereas the adults are generalists that eat a lot of mammals.
When we compared venom in adult and baby brown snakes, we did indeed find them to be different. Baby brown snake venom seems to entirely lack haemotoxins: instead, it’s almost exclusively composed of neurotoxins – toxins that attack nerve junctions.
What this suggests is that the haemotoxins that are so dangerous to humans (and lab mice) aren’t very effective against the lizards that baby brown snakes eat. We can make this dietary link with a degree of confidence because many other Australian snakes that feed exclusively on lizards have similar venom – no haemotoxins, only neurotoxins.
We don’t yet know what this means from a clinical perspective. It may be that baby brown snake venom is less dangerous to humans than adult brown snake venom, but the opposite might also be true – brown snake antivenom might be less effective against the venom of the babies.
There has been at least one fatal bite from a very small brown snake in Australia, so they must be treated with respect at any age.
As always, the best policy for snakes is to leave them alone and let them go about their business, and to teach children to do the same – snakes want no more to do with us than we want with them.
This is an article from Curious Kids, a series for children. The Conversation is asking kids to send in questions they’d like an expert to answer. All questions are welcome – serious, weird or wacky!
If a lethally poisonous snake bites another lethally poisonous snake of the same species does the bitten snake suffer healthwise or die? – Ella, age 10, Wagga Wagga.
That’s a great question.
If a venomous snake is bitten by another venomous snake of the same species, (for example during a fight or mating), then it will not be affected.
However, if a snake is bitten by a venomous snake of another species, it probably will be affected.
This is probably because snakes have evolved to be immune to venom from their own species, because bites from mates or rivals of the same species probably happen fairly often.
But a snake being regularly bitten by another snake from a different species? It’s unlikely that would happen very often, so snakes haven’t really had a chance to develop immunity to venom from other species.
Many people believe that snakes are immune to their own venom so that they don’t get harmed when eating an animal it has just injected full of venom.
But in fact, they don’t need to be immune. Scientists have found that special digestive chemicals in the stomachs of most vertebrates (animals with backbones) break down snake venom very quickly. So the snake’s stomach can quickly deal with the venom in the animal it just ate before it has a chance to harm the snake.
People that have snakes as pets often see this. If one venomous snake bites a mouse and injects venom into it, for example, you can then feed that same dead mouse to another snake. The second snake won’t die.
By the way, scientists usually use the word “venomous” rather than “poisonous” when they’re talking about snakes. Many people often mix those words up. Poisons need to be ingested or swallowed to be dangerous, while venoms need to be injected via a bite or a sting.
Some snakes can inject their toxins into their prey, which makes them venomous. However, there seem to be a couple of snake species that eat frogs and can store the toxins from the frogs in their body. This makes them poisonous if the snake’s body is eaten. Over time, many other animals will have learned that it is not safe to eat those snakes, so this trick helps keep them safe.
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Despite the common belief that Australia has some of the most venomous snakes in the world, our new research shows being bitten by a snake is uncommon in Australia and dying from a snakebite is very rare.
And of the few unlucky people to collapse after venom enters their bloodstream, a bystander performing cardiopulmonary resuscitation (CPR) is the most likely thing to save them.
These are just some of the findings from 10 years of data from the Australian Snakebite Project published today in the Medical Journal of Australia.
Although many people go to hospital with a suspected snakebite, many do not turn out to have envenomation (when venom enters the bloodstream) after all.
In more than 90% of cases people are bitten by a non-venomous snake, venom is not injected when the snake bites (known as a “dry bite”) or are not even bitten by a snake (known as a “stick” bite).
Our analysis of about 1,548 cases of suspected snakebites from all around Australia, showed there were on average just under 100 snake envenomations a year, and about two deaths a year.
The most common snakebites were from brown snakes, then tiger snakes and red-bellied black snakes. Brown snakes were responsible for 40% of envenomations. Collapsing, then having a heart attack out of hospital was the most common cause of death (ten out of 23), and most deaths were from brown snakes.
Venom from a snakebite travels via the lymphatic system to the bloodstream. There, it circulates to nerves and muscles where it can cause paralysis and muscle damage. In the blood itself, the venom destroys clotting factors, which makes the blood unable to clot, increasing the risk of bleeding.
In the most severe cases, most commonly in brown snake bites, someone can collapse because they have low blood pressure (we don’t know for certain what causes the low blood pressure). In this situation, insufficient blood is pumped around the body for the brain and other vital organs.
Clearly the accurate diagnosis of snake envenomation and the timely administration of antivenom are essential to treating snakebites in hospital.
But when people collapse, CPR will keep the blood circulating to the vital organs – and is life-saving – however inexpertly a bystander performs it.
In other words, we found basic first aid before people reached hospital, of which bystander CPR is one, may be more important than any changes in how people are treated in hospital to improve people’s chance of survival. People who survived after collapsing received CPR on average within one minute of being bitten compared with 15 minutes for those who died.
Our study also showed that in most cases, people used other first-aid measures (pressure bandages and immobilising both the limb and the patient). These aim to prevent the venom travelling from the bite site, via the lymphatic system, to the bloodstream.
Our study confirmed the role of antivenom in treating snakebites and the need for it to be administered before irreversible damage is done to the nervous system and paralysis occurs.
However, we found one in four patients given antivenom had an allergic reaction to it and about one in 20 have severe anaphylaxis requiring urgent treatment.
So it is essential only patients with snake envenomation, and not just a suspected snakebite, are treated with antivenom. We found 49 patients (around 6%) were given antivenom unnecessarily, out of the total 755 patients who received it.
So we need to find ways to make sure patients get antivenom as early as possible. This requires laboratory tests that can identify patients with snake envenomation in the first couple of hours after the bite.
It is also essential anyone bitten by a snake or suspected to be bitten by a snake seeks immediate medical attention and goes to hospital by ambulance.
But the best thing is to avoid being bitten in the first place: