Vertebrate Biology

Mastering scariness: the mechanisms behind hooding and growling in cobras

Snake charming is a very popular and ancient performance in Africa and Asia, which takes advantage of the natural defensive behavior of cobras of forming a hood.

Snake charming is a very popular and ancient performance found in Africa and Asia, in which flute players takes advantage of the natural defensive behavior of cobras of forming a hood. Picture from: http://cdn.fansided.com/wp-content/blogs.dir/75/files/2013/07/snake-charmer.jpg

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!

Young BA, Kardong KV. 2010. The functional morphology of hooding in cobras.J Exp Biol. 213, 1521-8.

Young BA, Kardong KV. 2010. The functional morphology of hooding in cobras.J Exp Biol. 213, 1521-8.

I wouldn't hold a king cobra for a million dollars...wait, maybe for that money I would..but I definitely wouldn't smile while doing it like this guy does.  Picture from: http-//static.panoramio.com/photos/large/100885070.jpg

I wouldn’t hold a king cobra for a million dollars…wait, maybe for that money I would..but I definitely wouldn’t smile while doing it like this guy does. Picture from: http-//static.panoramio.com/photos/large/100885070.jpg

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?

Just to prove that King cobras can also look cute! Picture from: http://www.snaketype.com/wp-content/uploads/king_cobra_200-623x200.jpg

Just to prove that King cobras can also look cute! Picture from: http://www.snaketype.com/wp-content/uploads/king_cobra_200-623×200.jpg

 

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Zombies of the ocean: the mechanism behind shark tonic immobility

Shark in tonic immobility state.

Shark in tonic immobility state.

My true passion in science is ecology and evolution of host-parasite systems. However,vertebrate evolution was what really caught my attention when I first started to study biology. Just based on the number of fans the movie Jurassic Park has, I’m sure I’m not alone with my fascination by vertebrate biology and evolution. Luckily, I got the chance to TA the Vertebrate Biology Lab at UMSL, which is an anatomy lab that I try to teach in an evolutionary, ecological and behavioral context. This Fall, I’ve decided to spice things up, and proposed to the students what I called the “Ultimate Vert Bio Challenge”. The idea here is to get our brains around some of the coolest, but, complex and most times under studied, facts involving vertebrates. In this first challenge, students had to try to explain the mechanism involved on shark tonic immobility (TI), a very popular topic referred to as ‘shark hypnosis’ or ‘zombie sharks’ in the media, and recently featured on Discovery Channel’s shark week (see video bellow).

Tonic immobility is assumed to be a behavioral strategy of preys – but, what does it mean when a predator presents the same type of behavior? Figure from the book Epossumondas Plays Possum, by Salley and Stevens.

Tonic immobility is assumed to be a behavioral strategy of preys – but, what does it mean when a predator presents the same type of behavior? Figure from the book Epossumondas Plays Possum, by Salley and Stevens.

TI is a behavioral strategy found in several species of vertebrates, such as  rabbits, chickens, hummingbirds, opossums, lizards, humans, and even in invertebrates, such as the red-flour beetle. In terrestrial vertebrates, TI is characterized as an unlearned and reversible behavior, in which the animal involuntarily enters a dead-like state characterized by motor inhibition. It is a behavioral display commonly associated with stress and fear responses to predators – hence a very widespread strategy among prey species. If TI is a response to predation, why the heck sharks, one of the sea’s top predators, can also be induced into a TI state? The TI mechanism is somewhat understood in terrestrial vertebrates: it involves activity of the hypothalamic-adrenal-axis, production of corticosteroids and muscle contraction. In contrast, in sharks and other elasmobranchs, TI is characterized by muscle relaxation. It is known that sharks experience physiological stress when in TI, due to high levels of carbon dioxide in the blood caused by inefficient ventilation while immobilized and turned upside down. However, the precise mechanism of TI in sharks has yet to be determined.

To get some insights on the possible mechanistic pathway of this phenomena, I got in touch with Dr Stephen Kajiura, the PI of the Elasmobranch Research Laboratory, at the Florida Atlantic University. Dr Kajiura mentioned that the consensus is that we just don’t know what the precise mechanism is. When I asked him to speculate what he believes the mechanism could be, he stated: “Since it (TI) works when the animal is flipped upside down, I would suspect that the mode of action is initiated by the vestibular system.  Another option is that the position causes blood flow to the brain to be compromised causing the animal to pass out.  In the wild, these animals are only likely to be flipped upside-down when being mated and it would probably be adaptive to be somewhat passive during that procedure to avoid being damaged by the mate’s teeth.” Another fact frequently pictured in shark TI videos are divers rubbing the animal’s snout with metal gloves, to stimulate the shark’s Ampullae of Lorenzini (AOL), an electro-receptive sensory system. This often misleads us to believe that AOL disruption is somehow the mechanism behind TI. Dr Kajiura explains that “it is possible to flip the sharks in the absence of any metal glove and get the same result.  AOL detect changes in electric fields so the shark may be momentarily confused by the metal glove, which might help to get it flipped upside-down, but remaining in TI is accomplished without any metal.  Again, we flip sharks with just our bare hands and get the same result so AOL are not likely the mechanism“. What is your hypothesis about the mechanism responsible for turning sharks into zombies?

Thanks to Dr Stephen Kajiura for kindly answering my questions so promptly!!