The second Ultimate Vert Bio Challenge is a warm up for Halloween, about one of the most terrifying, albeit amazing, creatures in nature: Cobras! These reptiles found their place in the animal kingdom hall of fame due to snake charming, a very ancient and popular performance in African and Asian countries, in which a flute player pretends to hypnotize a cobra. What snake charmers actually do is take advantage of the defensive behavior called hooding, which cobras naturally perform by standing vertically, flaring the neck laterally and compressing it dorsoventrally. But, precisely, what adaptations in the skeleton and musculature of the cobras allow them to perform such a scarring defensive hooding display? When comparing X-rays of king cobras displaying hooding to cobras in a relaxed state, one is able to see how, in order to flare the hood, these snakes can rotate the ribs in two planes, frontal and transverse. The rotating movement of the ribs allow these bones to protract (move towards the head), and elevate (flatten and move dorsally), anchoring the muscles associated to the hood. Rib rotation is initiated by contraction of two muscles in the head, followed by contraction of intercostal muscles to support the protracted and elevated ribs. How long cobras can keep up with the defensive display depends on the amount of visual stimuli, or how threatened they feel, as well as intra- and inter- specific variation. However, there is evidence from laboratory observations that they are able to maintain the hood flared for at least 10 min, and up to 80 min!
If you think hooding is enough to make cobras one of the most frightful creatures out there, you probably haven’t seen a video of a cobra hooding and growling at the same time. Yes, growling. Super laud nasty scary growling. Check out the video bellow:
Most snakes are able to produce hissing-like vocalizations at a frequency of 7,500 Hz, whereas cobras’ vocalizations lie at much lower frequencies, around 700 Hz, which is what characterizes them as growlers. The production of low frequency sound is possible due to the presence of a structure called tracheal diverticula. These are sacs associated to the trachea, which work as low frequency resonating chambers for the air flushed down the respiratory passageway. Interestingly, the only snake that has tracheal diverticula and is also able to growl, is the cobra’s favorite snack, the mangrove rat snake. This is considered to be a case of vocal Batesian mimicry, in which the mangrove rat snake mimics the vocalization of the more threatening cobras. The venom of mangrove rat snake is not toxic to humans, whereas cobras can inject up to 7 ml of venom in a single bite, and can kill a person in less than half an hour. We’re aware that cobras are predated by honey badgers (because they just don’t care), but I wonder what was the actual evolutive pressure through time to select for such a nasty defensive apparatus! Any thoughts?