Snakes use an astonishing variety of behaviours to deter or startle a potential predator. From convolving, gaping, hissing and rattling to exuding musk, feigning death and defecating, all of these strategies are aimed to show a predator that it’s just not worth it and rather leave the snake alone. One of the most spectacular defensive displays comes from the genus Tropidophis. These snakes are commonly referred to as pygmy boas but aren’t really boas at all! They are actually more closely related to American pipe snakes and together make up the superfamily Amerophidia. Locally, they are also called “Thunder Snakes”, because you see them more often after bigger storms, which is a fantastic common name if you ask me.

Thunder snakes display what is called cephalic autohaemorrhaging, which describes their ability to deliberately eject blood from an orifice of the head. Autohaemorrhaging is known from many insect groups but in reptiles, this behaviour is rare and has only been reported in few species. You might have seen or heard of the American horned lizards (Phrynosoma) squirting blood from their eyes but in snakes, this behavioural display is unheard of.

So we knew that these snakes autohaemorrhage but visual documentation and detailed information on the process were not available. Furthermore, there is still debated about whether this behaviour is a targeted antipredator response or just a byproduct of elevated stress. Therefore, we aimed to document this behaviour to answer some questions about the process and potentially attribute it to a specific function.

Because autohaemorrhaging in these snakes has been reported to occur as a response to handling and pressure, we induced this behavioural display by gently applying pressure to the body of the snake and I want to emphasise that the pressure was absolutely minimal. Afterwards, the snake seemed in perfect health and was returned to the same rock under which it was found, as you can see in the video to the left.

Here is the video of the autohaemorrhaging in a thunder snake.

How nuts is the speed at which the eyes flood with blood and fully clear again?
It only took seconds for the eyes to become red and entirely clear up again.

It has been argued whether cephalic autohaemorrhaging in Tropidophis is a targeted antipredator behaviour or simply the byproduct of increased stress. We observed cephalic autohaemorrhaging alongside other defensive behaviours like musking, defecating and convolving. The expelled blood could exacerbate the snakes’ other off-putting behaviours and convince an active predator that the snake is no good to eat.

To get a more conclusive answer we need to assess the components of the blood to look for deterring compounds similar to those added by many insects as well as conducting predatory experiments to understand how this behaviour could deter a predator. Furthermore, an understanding of the physiological processes involved in the incredibly fast onset and stop of the haemorrhaging could even find applications in wound treatments across medical fields!

You can find out more about this spectacular display in our paper: and by checking out my virtual poster for the Animal Behaviour Live 2020 conference below.