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Humans ‘can instinctively tell if a snake is venomous’

Humans are biologically conditioned to distinguish venomous snakes from non-threatening species, a new study suggests.  

In experiments, volunteers were shown alternating photos of venomous and non-venomous snakes.

Photos of venomous snakes elicited stronger fear responses, such as sweating and increased heart rate, than images of non-venomous ones. 

Even though the participants weren’t sure which species were venomous and which weren’t, they displayed innate biological reactions to the venomous ones. 

The researchers suggest that this might be a skill we evolved to trigger our ‘fight or flight’ responses in the face of potential danger. 

Differences in snake appearance may also trigger one of two distinct emotions: fear in the face of big, thick and scaled snakes, and disgust in the presence of thinner, slippery-looking snakes. 

‘Snakes have been important ambush predators of both primates and human hunter-gatherers throughout their co-evolutionary history,’ say the study authors, from Charles University in Prague and the National Institute of Mental Health in Klecany.

‘Humans demonstrate a remarkable ability to discriminate between dangerous viperids and harmless fossorial snakes, which is also reflected in distinct autonomous body responses.’ 

Ever since their appearance, primates and early hunter-gatherers have been subject to deadly risks from snakes. 

As a consequence, humans have developed improved visual abilities for detecting snakes and other stimuli representing an threat.

Fear-inducing snakes generally have a large body size, conspicuous scales with contrasting patterns, and bright coloration. 

Our fear response to them involves activation of the sympathetic nervous system, which initiates the ‘fight or flight’ reaction characterised by heart rate acceleration and increased blood pressure.

The main purpose of this is to mobilise energy reserves and prepare the body for rapid action, such as making a quick getaway. 

For this study, researchers focused on physiological responses of human subjects elicited by 20 species of snakes belonging to two distinct groups.

The team measured sweat response and heart rate in 143 human subjects exposed to pictures of 10 venomous viperid species and 10 non-venomous fossorial species, as well as 20 photographs of tree leaves for ‘an emotionally neutral’ control photo.   

Sweat response was measured using dry sensors attached to participants’ hands, while heart rate was measured with electrodes attached to the skin under the collarbone.  

The pictures of fear-eliciting snakes, disgust-eliciting snakes and leaves were presented on a computer screen individually in an alternating order, each presented for five seconds and separated with a black screen.

The venomous snake species presented to the subjects included the Sahara sand viper, Sochurek’s saw-scaled viper and Gaboon viper. 

Non-venomous species included the Eurasian blind snake, northern rubber boa and brahminy blindsnake. 

Venomous snakes triggered a stronger physiological response than repulsive non-venomous snakes, the team found. 

On a seven-point Likert scale of fear, with one being ‘no fear’ and seven ‘extreme fear’) the venomous snakes scored 5.15 on average while the non-venomous ones scored much lower 3.24. 

Participants who reported a high fear of snakes before the experiments who were confronted with images of the venomous viperid snakes showed stronger, longer-lasting and more frequent changes in sweating and higher heart rate compared to low-fear subjects.

Interestingly, more intense exposure to venomous snakes – when participants were presented to venomous snake images in a block of 10 – did not lead to a stronger emotional response than when presented single images.        

Due to the varied appearance of snakes, some may be perceived as not frightening but highly disgusting, the team concluded. 

This is interesting because fear and disgust are two different, genetically fixed responses to potentially life-threatening situations. 

While fear is elicited by the presence of a predator, such as a snake, or other imminent threat posing a direct risk of physical harm, disgust has originally developed as a food-rejection emotion. 

Its main function is to prevent the transmission of illness or disease through ingestion of contaminated object.

Therefore, it triggers disease-avoidance behaviour as another facet of a human’s ‘behavioural immune system’.

The team conclude by highlighting the importance of paying attention not only to the venom itself, but also to the particular species delivering the bite, due to the changes it may have on our biological reaction.  

‘It should become an integral part of studies quantifying the effects of envenomation, including studies on animal models,’ they say in their paper, published in PLOS ONE.

Despite progresses in anti-venom, even today, snakebite remains a huge health concern around the world. 

There are 3,709 snake species currently recognised and around 35 per cent of them use venom to kill their prey.

The number of wider reptile species capable of producing toxins in their saliva may be up as high as 2,000 and out of these, 250 are listed by the World Health Organisation as being medically important.

Every year, somewhere between 4.5 million and 5.4 million people are bitten by snakes worldwide and the estimated death toll ranges from 81,000 to 138,000, with another 400,000 victims suffering major disabilities such as amputations.

Snakebites have been recently claimed the world’s biggest and grossly underestimated hidden health crisis.     

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