What’s eating you? – Bugs, bacteria, and zombies

The trailer for Shaun of the Dead.

Not all zombies are created equal. The most popular zombie archetype is a shambling, brain-eating member of the recently deceased, but, in recent films from 28 Days Later to Zombieland, the definition of what a zombie is or isn’t has become more complicated. Does a zombie have to be a cannibal corpse, or can a zombie be someone infected with a virus which turns them into a blood-crazed, fast-running monster?

For my own part, I have always preferred the classic George Romero zombies from the original Dawn of the Dead and Day of the Dead films (as well as my most favorite of zombie films, Shaun of the Dead). The shuffling, groaning masses not only deliver social commentary in spades – i.e. our transformation into mindless consumers inextricably drawn to shopping malls – but the prospect of slowly being closed in by a seemingly unstoppable horde is far more frightening than any sprinting zombie. Nevertheless, there is one thing that bugs me about zombie movies in the classic vein – where are all the flesh-eating insects?

Zombies are often the star antagonists of horror films, but there is another array of flesh eaters which are usually ignored. For bacteria, flies, and other organisms which typically set to work on cadavers soon after death, a zombocalypse would be a smorgasbord of epic proportions. So, in order to figure out what might happen to the living dead in the days after their horrifying resurrection, I turned to forensic science and taphonomy to see what might worry the zombies themselves.

As outlined by forensic scientist Arpad Vass in a brief overview called “Beyond the grave – understanding human decomposition“, it doesn’t take long for the human body to break down. Just four minutes after death, oxygen deprived cells begin to digest themselves and spill their contents. This is not visible at first, but after a few days fluid-filled blisters appear on the skin and large parts of the skin begin to slough off. By this time, with the onset of putrefaction and bloating of the corpse, the work of bacteria, fungi, and other microorganisms begins to become apparent.

The characteristic bad odors and greenish skin of zombies are signs of the busy activity of the microorganisms. Slowly but surely, the bacteria and fungi (such as species of Staphylococcus, Candida, Malasseria, Bacillus and Streptococcus) transform tissue into gases and liquids – unless zombies take steps to preserve themselves, their decomposition is inevitable. (Interestingly, in warm, moist environments, zombies would make their own soap through a process called saponification in which fat in the body is transformed under high pH conditions. Cadaver lather, anyone?) Eventually the internal organs will all be broken down leaving the skin as little more than a thin bag around the bones, and soon after that body will become nothing more than a skeleton. Rate of decomposition varies due to temperature and moisture, but, to provide some measure of the speed at which these transformations take place, Vass figured that – at about 50 degrees Fahrenheit – it takes about 128 days for a corpse to be entirely skeletonized.

A time-lapse video of the decomposition of a rabbit. Notice the role that insects play in its decomposition.

But what about insects? Naturalists since the time of Carolus Linnaeus have recognized that these little “death workers”, too, play important roles in decomposition, and the more insects tuck into a corpse, the faster it falls apart. Of these, an array of Diptera flies are among the most frequent arthropod visitors to corpses, and their time of arrival has been used to figure out how long a body has been dead. As stated by scientists Carlo Campobasso, Giancarlo Di Vella, and Francesco Introna in their review of flies and decomposition, among the first flies to arrive are species belonging to the Calliphoridae (blow flies), the Sarcophagidae (flesh flies), and the Muscidae (house flies), later followed by members of the Sphaeroceridae (lesser dung flies), Piophilidae (cheese flies), Fanniidae, and Phoridae. They typically lay their eggs around whatever open orifices they can find – especially on the head – with 6 to 10 days between the time they lay their eggs and the time the new adult flies emerge, and the warmer it is, the more species you will find. Beetles do their part, too. Members of the Silphidae (carrion beetles) are early visitors to corpses, while species among the Nitidulidae (sap beetles), Cleridae (checkered beetles), Scarabaeidae (scarabs), and, those most famous of all cadaver beetles, Dermestidae arrive later. (Staphylinidae (rove beetles) and Histeridae (clown beetles) can be found on corpses, too, but these often eat other insects rather than the body itself.)

While zombies would not be quite as easy to prey upon as a stone-still cadaver, I can imagine that at least some of these insects would still try to feed and breed on them, further speeding along decomposition. In fact, unburied bodies often undergo a drastic decline in mass during just a few days, thanks in large part to scavengers, and so it is not unreasonable to speculate that – as much as the living would have reason to fear zombies – bacteria, flies, and even vertebrate scavengers would give zombies something to worry about. As for mammalian and avian scavengers, these animals might be more skittish of zombies than they would be of cadavers, but I have to wonder if some of them might try to take a bite here or there, especially during the time between death and ‘zombification’ as the putrefaction which soon comes about is more of a deterrent than attractant for everything other than insect scavengers.

Despite horror film scenarios of long-lasting zombie horror, the bodies of the undead would not last very long. Between the natural breakdown of the body following death, the activities of scavengers, the colonization of microorganisms, and the habits of arthropods, any given zombie might only be able to last few a few days, weeks, or months before decomposing so much that they are unable to move. Granted, the beginning of a zombocalypse – when the public remains unaware – would probably be marked by many hapless victims being turned into zombies themselves, but given enough time the epidemic would simply rot itself away. As with H.G. Wells’ classic story about alien invasion – War of the Worlds – our greatest assets in confronting the zombie menace would not be guns or chainsaws, but the small organisms which rely on decomposing flesh to make their living.

Campobasso, C. (2001). Factors affecting decomposition and Diptera colonization Forensic Science International, 120 (1-2), 18-27 DOI: 10.1016/S0379-0738(01)00411-X
Carter, D., Yellowlees, D., & Tibbett, M. (2006). Cadaver decomposition in terrestrial ecosystems Naturwissenschaften, 94 (1), 12-24 DOI: 10.1007/s00114-006-0159-1
DeVault, T., Brisbin, Jr., I., & Rhodes, Jr., O. (2004). Factors influencing the acquisition of rodent carrion by vertebrate scavengers and decomposers Canadian Journal of Zoology, 82 (3), 502-509 DOI: 10.1139/Z04-022