To unravel the myth of the 'aggressive' snake we have to look at the world from their perspective.
Snakes have a terrible public relations problem. While snake apologists frequently point out that many cultures have positive stories and myths involving snakes, it remains true that in very many cultures a snake is an animal to be feared and avoided at all costs, or if not avoided, destroyed. In popular literature and newspaper reports snakes are routinely referred to as aggressive – an “aggressive attack”, “snakes more aggressive in this warm weather”. So unfair. The truth is that, while snakes are sometimes compelled to be defensive, they are not aggressive as far as humans are concerned. To understand what is going on, it is essential that we see things from the snake’s point of view.
Snakes have enemies. There are animals out there would be happy to eat three snake meals a day, venomous or otherwise. These predators can deal with the fact that their prey has a venomous bite, and the deadliest snakes in the world are readily turned into lunch by their well-adapted enemies.
- The simplest examples are predators that are just so big and strong compared to the snake that any bite fails to penetrate and/or the snake is just overwhelmed by the brutality of the attack. Across Australia, Asia and Africa, monitor lizards (goannas to most Australians) are one such predatory group, large active predators with bone-reinforced scales, shark-like teeth and crushing jaws. Tests have shown that at least in many Australian species, the lizards are not immune to the snake’s venom. It’s like they don’t care; their attacks are so ferocious that the snake is unable to defend itself.
- Many snake enemies are birds, which can use their mastery of the air to attack from above, a direction from which attacks are difficult to defend. Birds of prey and other hunters such as kingfishers (notably Australia’s kookaburra) can drop straight down on to a snake’s vulnerable head before it is even aware of their presence. Not only that, birds can avoid a lot of close contact, allowing them to avoid being bitten while simultaneously striking with talons or beak, keeping the body, further protected by the mat of feathers, away from counterattacks.
- Immunity. Some of the most dangerous snake predators are more or less immune to the venom of the snakes they feed on. Members of several mammalian carnivore families that have snakes as part of their diets are at least partially protected by immunity. Indian mongooses are famously resistant to cobra bites, honey badgers are resistant to puff adder bites, and even domestic cats can withstand much higher doses of venom than can other species, like dogs, or humans, that have not evolved to routinely eat snakes. In southern North America, non-venomous kingsnakes prey on venomous rattlesnakes, even allowing the rattlesnake to bite, because when this happens, the rattlesnake has to emerge from the safety of its defensive coils. As it does so, the unaffected kingsnake can take the opportunity of grabbing the rattlesnake near the head, drag it out of its coil and wrap around it to constrict its breathing and kill it.
Throughout what follows, keep these horror stories in mind and that the guiding idea is that snakes (like all of us) have had to evolve, as far as possible, to avoid unpleasant interactions with other animals.
So if they really are out to get you, the best defence is if your enemies have no idea you are there. Snakes are able to avoid detection to a degree that is almost unmatched by any other major vertebrate group. They follow three primary rules that tend to make them invisible, all stemming from their various evolutionary specialisations.
- Maintain a low profile. The long thin body shape allows a snake to occupy spaces that would not be available to an animal of the their body weight if they had the usual head-body-legs arrangement that most other land vertebrates have. Limbless locomotion enables snakes to glide through undergrowth with minimal disturbance and to enter and curl up in small narrow spaces.
- Chill. Snakes are solar powered – their body warmth that allows energetic movement comes from the environment, and when inactive they can allow the body temperature to drop so that a minimum amount of energy is used for maintenance. The end result is that a snake does not have to feed as often as an animal like a bird or mammal that needs a high calorie input to maintain its body temperature.
- Eat few large meals. The remarkable – OK, astonishing – capacity of a snake to swallow whole prey items that are much larger than its head means that a single predatory success can keep a snake a going for a month or more. That is a whole month during which the snake does not have to move around looking for food and risking exposure to its predators. It can find a safe place and just stay there for days.
Being caught in crossfire can leave you just as dead as if you were the target. While snakes are in the worst danger from the specialised snake predators, they may also be attacked opportunistically by other general predators that might have a go at anything potentially edible, and these second-tier predators may not have the sorts of defences that the specialists have. This means that they can be killed or injured by a defensive bite. Equally many other species that don’t necessarily think of snakes as food might be injured if bitten by a snake through accidental contact, and consequently benefit from the evolution of an aversion to wiggly sticks. Narrow escapes seem to have led to animals of all sorts evolving an automatic wariness around snake-like animals.
If you have developed a rep as a bad critter to fool with, it is in your interests to remind everyone of this, leading to the fact that some sort of threat display, a demonstration of seeming murderous intent, is widespread among snakes both venomous and non-venomous. Across snakes, just about all medium to large species, and even many small ones, may act as if they are deranged killers when they are surprised or unable to escape. Snake threat displays can include a mix of ingredients including rearing up and threatening to bite, flattening or puffing up the body to make it look bigger, revealing bright colours in the skin, menacing noises, such as hissing, sudden movements such as lashing out with the head or thrashing the tail or body. Some snakes have evolved anatomy and behaviour that seem entirely directed to enhancing threat displays – the rattle of a rattlesnake, the hooded neck of a cobra. To repeat, a threat display is part of the repertoire of non-venomous or trivially venomous species not just the seriously dangerous ones. It is the threat that is simulated by display, putting the fear of danger into the enemy, that is the real defence here, not the delivery of a venomous bite. The aim for the terrified snake is to avoid contact and get the enemy to back off, increasing the distance between it and the snake. Once the space opens up, the typical next move for the snake is to bolt for cover, using its head start. If the enemy does not retreat and the snake can’t get away then out of sheer desperation it may bite. Or perhaps the snake was concealed but someone has stood or put a hand on it, stimulating an immediate reflex bite on whatever was pressing down on its body. This is when that tragic interaction happens and a human is injured or killed.
Snake venom is for feeding (mostly). To understand, let’s step back a bit and consider how venom should act if used as a defence. A venom defence should ensure that the animal being attacked was ideally, not eaten, and escaped unharmed from the encounter. For this to be the case, defensive venom has to act instantly and very effectively – cause such pain or incapacity that the attack from the predator ceases immediately. It is not so important that the predator actually dies, only that for the time being it becomes incapable of continuing its attack. The details of snake bite effects on humans show that in most cases snake venom performs poorly as a defence. Immediate snake bite effects can be painful, but often not especially so. In some cases, including Australian species such as brown snakes, people can even be unaware that they have been bitten, so minor are the local symptoms of pain or swelling. Often symptoms develop incrementally with some local pain then increasing pain, inflammation, swelling and the beginnings of systemic symptoms such as nausea or shock. This gradual build-up of distress is far from the immediate “put me down” response required from an effective defence, such as comes from handling a wasp or bee.
Instead of defence, venom is an offensive weapon, sculpted by evolutionary change to be maximally effective on the main prey species. Many studies on the variations that occur in snake venom show that the changes can be readily explained in terms of the way they affect prey species of that snake species, or even that particular snake population. It can even change in a snake’s lifetime, from juvenile to adult. If we think of Australian brown snakes, their venom is typical of snake venom generally in including components with different effects. They have several types of neurotoxins, that block the release or reception of the signals between nerves or nerves and muscles, leading to paralysis. They also have components that affect blood clotting, or cause leakage of fluid from cells, leading to physiological shock. The proportion of these components changes during a brown snake’s life time. Young brown snakes eat lizards, which are killed through the effect of venoms with a heavy bias towards nerve-blocking neurotoxins, but older snakes switch to mammal prey, and the venom balance switches to emphasise components that affect the circulation, immobilising prey through shock, to which the mammalian prey is more susceptible. In short, venom activity matches prey, not predators.
There is one exception that helps to demonstrate the truth of the above general picture. A few species of cobra have venom that has clearly been evolutionarily modified for defence and they help to demonstrate what this looks like. These are the so-called spitting cobras, snakes that can spray (not actually spit) venom towards an attacker. This defence has evolved twice, once in some African species, and again in some Asian species. In both groups one of their neurotoxins (known by its chemical shortcut as PLA2) is produced in greatly increased amounts. This protein has a direct effect on mammalian pain receptors – it massively stimulates them. The snakes defend themselves by rearing up and compressing the venom glands with the mouth open, spraying the venom towards an enemy’s face, possible because the holes at the tip of their fangs are modified to point forwards instead of down. If the venom hits the eyes it cause immediate and crippling pain and inflammation and can lead to at least temporary blindness. It is clearly a defence – the effects are instant and disabling causing any threat to immediately come to a halt.
Snakes live lives that maximise their safety and minimise their exposure to possible predation. Their venom, evolved for prey-getting, can harm other animals but often takes too long to work to be an effective life saver for the snake. Still, venomousness in snakes seems to have affected the evolution of the animals they live with, such that emphasising their snakiness through a display can be used as defence even in snakes that are not venomous.
If the snake in front of you is putting on a threat display, it is running out of options. It is afraid to flee or does not know where to go. It is behaving defensively, not aggressively. If you encounter one of these performances, just take the hint, step away from the action, give a polite round of applause for a fine show, and allow the snake to exit the stage without further fuss. Everyone will be better off.