“I’m not dead!” Opossum plays possum.

Figure 1: Didelphis virginiana Virginia Opossum

Perhaps you have been told that, when facing a bear, it is best to pretend to be dead so that the bear loses interest in you. To do that, you have to remain really still and have very little physiological response in order to make the bear believe that you are really dead. But I sometimes wonder how people can actually keep their cool when facing such a ferocious animal; your heartbeat will probably be so loud that it’ll give you away! Things will be a lot easier if we have the ability to feign death when we need to. The Virginia Opossum, or Didelphis virginiana, has precisely this unusual ability (Figure 1).

The opossum is well-known for its ability to feign death when it is threatened by predators (Figure 2). When physically attacked, the opossum will freeze, curl up and fall on its side. Its mouth will open up with the sides drawn back, exposing the teeth, its tongue may hang out from the side of the mouth and its eyes will remain open (Francq, 1969). Together with these behaviors, the opossum will often salivate, urinate and defecate. It will also discharge a foul-smelling substance from its anal glands (Feldhamer, Thompson, & Chapman, 2003). By then, most predators will be disgusted and leave the opossum alone. Hurray for the opossum!

Figure 2: Opossum playing possum

Is the animal actually dead during the feigning period? Studies using electrocephalogram (EEG) to measure the electrical activity of the opossum’s brain showed no significant differences between the feign death state and normal conscious state, suggesting that the opossum is actually fully conscious when it is all frozen and stiff (Norton et al, 1964)! In this state, though, the opossums do not respond to other environmental disturbances, such as being picked up or probed.

Sounds like an awesome ability to have? That’s not all to the story. For one, if feigning death does not make the predators disgusted enough to back off, the ‘dead’ opossum will have no other way from escaping real death!

Feigning death is not an ability unique to the opossum. In fact, a wide range of animals, from birds to snakes, have this ability to protect them when in danger. Watch this video to find out more: http://www.bbc.co.uk/programmes/p00z2mcb

References

Book
Feldhamer, G. A., Thompson, B. C., Chapman, J. A., 2003. Wild Mammals of North America: Biology, Management and Conservation. Baltimore: Johns Hopkins University Press.

Journal
Francq, E. N., 1969. Behavioural Aspects of Feigned Death in the Opossum Didelphis marsupialis. American Midland Naturalist, 81(2), 556-568.

Gabrielson, G. W., & Smith, E. N., 1985. Physiological responses associated with feigned death in the American opossum. Acta Physiologica Scandinavica, 123(4), 393-398.

Norton, A. C., Beran, A. V., & Misrahy, G. A., 1964. Electroencephalograph during “feigned sleep” in the opossum. Nature, 204, 162-163.

Video
“Playing Possum,” by Keith Scholey, BBC One: Weird Nature, 27 September 2012. URL: http://www.bbc.co.uk/programmes/p00z2mcb (accessed 8 April 2013)

Image
“Didelphis virginiana Virginia Opossum,” by David Hoffman, Flickr, 10 April 2007. URL: http://www.flickr.com/photos/23326361@N04/2971229691/ (accessed 8 April 2013)

“Opossum playing possum,” by Tony Alter, Flickr, 28 October 2011. URL: http://www.flickr.com/photos/78428166@N00/6289417559/ (accessed 8 April 2013)