What do I have in common with great white sharks, black widow spiders, snakes and opossums? Thanatosis, or tonic immobility. What is this? Well most people know this phenomenon as ‘playing possum’. I first discovered that I shared this trait with some of the most fearsome of animals when I fell off the back of a truck and was catapulted down a hill. As soon as I felt my body begin to leave the back of the pick-up, I went limp and ‘blacked out’. I have no recollection about my trip down the hill, but observers remarked that it was spectacular. Everyone, including myself, was shocked when I ‘woke up’ to find that I had not broken a single bone, nor so much as sprained a joint. The only evidence of my journey was a minor scrape on one knee. A second incident further convinced me that I had a special knack for ‘playing possum’. I used to volunteer for an orangutan and chimpanzee sanctuary. One day, a young chimpanzee ran over to me and jumped in my lap, hugging me. A few seconds later, he was startled and screamed, reaching for my neck with his teeth. All it took was for me to hear him scream and I folded like a deck of cards. Literally. I went limp, ‘blacked out’, and woke up once he was off of me. Upon waking, I found that I had received a shallow bite to the neck, but was otherwise uninjured. It struck me that if I had struggled against him my injuries might have been more severe. On the other hand, I was concerned that my inability to remain active during a threat could be problematic. I worried that if I needed to act I would be unable to do so because I always seemed to pass out! A third and final incident resolved this concern for me. Several years ago I was in a near fatal car accident. I say near fatal because, but for my actions, I don’t think I would have survived. Upon impact I was thrust into a sea of trees. I distinctly remember steering the car to avoid the trees. I was successful only briefly and when I realized impact with a tree was inevitable, I placed my hands in my lap, put my head down, and you guessed it, ‘blacked out’. I walked (okay crawled) out of the car with no serious physical injuries, no whiplash, etc. So it would seem that when action is possible and likely to allow me to successfully avoid a threat, I act. When it is unavoidable, however, it seems my strategy is to remove myself from the situation, metaphorically speaking. This got me wondering what goes on in animals and if this trait is adaptive?
As I already mentioned, the official term for ‘playing possum’ is called thanatosis, which loosely translates to-apparent death. I suppose it got its nickname because 1) it is much more entertaining to say ‘playing possum’ than thanatosis and 2) because people are more likely to see an opossum do it, than say, a shark. But I am only speculating. Why do animals feign death? A number of ideas have been suggested including:
1. predators don’t like to eat already dead prey (generally)
2. it is a form of physical defense (though obviously passive) from predators, from mates that are harassing you, and even from hostile members of your own species
3. you can blend in better with the background if you play dead
4. since most predators like to chase their food, if you don’t move, there is no chase
5. prey that play dead may indicate that they are dangerous to eat and will taste bad
Let’s talk about spiders first. There have been movies made about them, drawing parallels to humans, specifically warning men of the dangers associated with ‘black widows’, or women that kill their husbands. Sexual cannibalism will probably be a separate topic altogether, but it is relevant here because males in some spider species have come up with a way to avoid being eaten by their love interest. They play dead. In many species the male presents the female with a gift. This gift is referred to as a ‘nuptial’ gift and usually consists of food. Maybe he is hoping that if he gives her enough food, she will forgo eating him as her meal! That may not work for many species though, since females will eat the male before he has a chance to mate with her. It is as if she decides he’s good enough to eat but not good enough to mate with! To deal with this mating dilemma, males will enter thanatosis. In the nursery web spider, the males have an elaborate strategy. First the male raises his body to nearly vertical and presents the female with a nuptial gift and conveniently ‘hides’ his abdomen (body) behind this gift to protect himself. When the female approaches, sometimes she goes for the gift. Sometimes, however, she bypasses the gift and goes for the male. It is at this exact moment that the male enters a motionless state. He does this by still holding the gift in his chelicerae, usually the two mouthparts found in the front, while extending his legs backwards, stretched out and completely motionless. The female can go so far as to grab onto the gift and drag it and the male along. If, however, the female begins to consume the gift, the male ‘magically’ pops back to life and initiates copulation while she is otherwise…ahem…occupied. Don’t think he relaxes for one second, though. The male, ever cautious, usually maintains contact with one leg on the nuptial gift possibly to keep track of what the female is up to. This strategy seems to work in the male’s favor as all males that ‘played dead’ were able to mate. For the ones that didn’t, well you can just imagine their fate!
In black-widow spiders, considered the most venomous of all spiders, thanatosis, or tonic immobility, is used for protection, even by the females who have much larger venom sacks than males. A small preliminary study reported that by tapping females on the back firmly with the hard end of a paintbrush, black widows will feign death. So why play possum when you have the means to attack back? One thought is that it may be costly to fight back. You have to use energy, or in the case of black widow, venom, which is energetically costly to produce. Depending on the threat, it may be more advantageous to conserve your energy and wait it out. This strategy works well if the particular predator in question only likes to eat live prey.
Venomous spiders are not alone in this behavior. Snakes do this too. When threatened, many species will play dead. This may not be their first tactic though. The hog-nosed snake for instance may rear up, looking very similar to a cobra. If all else fails, however, it will flop over onto its back with its mouth open and its tongue drooping out of its mouth and then it will release a foul smelling fluid in an attempt to convince the predator that it is dead, rotting, and will not appeal to the predator’s palate.
An interesting question then is: do top predators ever ‘play possum’? Yes they do. In the group that includes sharks, rays and skates, (Elasmobranchs) it manifests itself with the individual usually inverted. For this group of species, once immobility occurs it can last anywhere from under a minute to several hours. The list of sharks that display this behavior continues to grow and includes several species of dogfish shark, the lemon shark, the sandbar shark, the swellshark, the leopard shark, the blacktip reef shark, the whitetip reef shark, and the Caribbean reef shark. How exactly can we determine whether or not a shark feigns death? If you guessed that it somehow involves handling the shark and turning it upside down, you would be correct. Don’t you want to be part of that research team? Basically the shark is caught and then gently, but quickly inverted. Apparently sharks find this threatening and will go limp. You think?
Okay, but what about the mightiest of sharks, the top of the food chain, the most feared creature in the ocean (unnecessarily, I might add): the Great White? Yes, even they will exhibit this behavior. It was briefly seen on a National Geographic special “The Whale That Ate Jaws". This technique, though used by the shark to protect itself, could prove deadly. Why? Because some sharks, including great whites and hammerheads, need to keep moving to breath. If they remained in the hypnotic state too long, they would simply suffocate. There are some shark enthusiasts who seem to enjoy inducing this in wild sharks and not only is this a bad idea, but they fail to understand that this response is an extreme stress/trauma response. There is no excuse for any person to deliberately traumatize an animal for the entertainment of themselves or others.
What these examples reveal is that, for many species, tonic immobility is an extreme reaction to a life threatening and fearful situation. Not only that, but for some it is employed only when all other defense reactions fail. I can see now, how in the context of my experiences, this is true. In the first two incidences, there was no opportunity for any other reaction. In the accident, it was only after all else failed that I entered this state. This response has been found in all taxa, including humans, except jawless fish. In humans, like in other animals, it is not a learned response that one can develop. Meaning, it is an automatic response, not a coping strategy. Perhaps I am not such an outlier after all.
References:
Bilde, T et al. 2006. Death feigning in the face of sexual cannibalism Biology Letters, 2:23–25.
Cassill, D. L., Vo, K. & Becker, B. 2008 Young fire ant workers feign death and survive aggressive neighbors. Naturwissenschaften 95, 617–624.
Miyatake, T., Katayama, K., Takeda, Y., Nakashima, A., Sugita, A. & Mizumoto, M. 2004 Is death-feigning adaptive? Heritable variation in fitness difference of death-feigning behaviour. Proc. R. Soc. Lond. B 271, 2293–2296.
No comments:
Post a Comment