The Contagious Cancer (A Guest Post)

By Stephanie Stanton

The Tasmanian devil, perhaps more popularly known by its animated counterpart Taz in Warner Bros.’ “Looney Toons,” is a carnivorous marsupial native to Tasmania, an island off the southern coast of Australia. Similar to Taz, the Tasmanian devil lives a violent lifestyle. While a good portion of fights don’t go beyond screaming matches, sometimes (especially during the mating season) fights escalate to full-on biting matches. Unfortunately, it is this aggressive nature that has been linked to the alarming drop in Tasmanian devils’ numbers over the last decade. However, it is not violent wounds acquired during fights that are causing this rapid decline, but rather the Devil Facial Tumor Disease (DFTD), a contagious cancer.

DFTD is a transmissible cancer that operates as its own living entity- it is genetically distinct from its host and lives on its host’s face. Most of these tumors appear on their faces. Coincidentally, this also happens to be where a majority of open wounds are acquired in this species. Because of this, it is believed that DFTD is transferred through open wounds on the skin.

A healthy Tasmanian devil in all his glory. Photo by Chen Wu at Wikimedia Commons.

This cancer has been so successful in spreading throughout the population because of the devils’ small population size and low genetic diversity. Among the genes with low genetic diversity in the population is the Major Histocompatibility Complex (MHC), a collection of genes responsible for a strong immune response in vertebrates. Without a strong immune response, it is difficult to fight off serious threats such as DFTD. Unfortunately for the devils, the tumors growing on their faces do not even register on their limited immune system’s radar- so their bodies don’t even fight back! Because of this, DFTD is in most cases fatal within six to nine months of showing clinical symptoms.

A Tasmanian devil afflicted with DFTD. Photo courtesy of Menna Jones, available at Wikimedia Commons

Three Australian scientists by the names of Rodrigo Hamede, Hamish McCullum, and Menna Jones from the University of Tasmania and Griffith University recognized the alarming decline in the Tasmanian devil population and sought to find a way to better understand and control the spread of the disease. They looked at two separate populations over four seasons, collecting data once every three months by taking counts of bites on individual devils and tracking who got DFTD, when, and on what part of their bodies. They hypothesized that because the tumor was transmissible through open wounds, then the number of open wounds could be used as an early predictor for the onset of DFTD.

And they were right…although perhaps not in the ways they thought they would be. Contrary to what common sense would have everyone believe, devils with the least amount of facial wounds were the most likely to develop the fatal cancer. How could this be?

Simply put, it appears that the disease is getting transferred from devil to devil not because their bodies are exposed to a bite from an infected individual, but because devils are biting the tumors of infected individuals, thereby creating a direct path for the tumor to enter the new host.

The scientists argued that the devils that have the fewest open wounds were better at fighting and also the most aggressive (A side effect of the cancer? Perhaps.) Tasmanian devils are likely to have cuts or scrapes in their mouths because of their aggressive eating style, providing a port for the cancer cells to invade. It was because they were biting the tumors of the infected devils that they were contracting the disease, which also explains the higher occurrence of tumors in the mouth. Less aggressive devils accumulated more injuries to the face, but as long as the cancer cells did not come into contact with open wounds, their likelihood of contracting the disease was slim.

Rodrigo, Hamish and Menna hope that their results along with further research can help reduce the effects of the disease on the shrinking Tasmanian devil population by offering potential solutions to better control its spread. Exciting research published in 2016 is also already offering hope in keeping Taz and his furry counterparts alive for future generations to enjoy.


Want to know more? Check out the original article below:

Hamede, Rodrigo K., McCullum, H., Jones, M. (2013). “Biting injuries and transmission of Tasmanian Devil facial tumour disease. Journal of Animal Ecology. DOI: 10.1111/j.1365-2656.2012.02025.x.

Suicidal Sex

A brown antechinus. Males of this species mate like crazy
for two weeks, then die. Photo by Alan at Wikimedia.

Although most species breed repeatedly over their lifetimes, a select few invest everything they’ve got in a single reproductive bout, after which they keel over and die. This strategy, called semelparity, can be beneficial in species that can have many offspring at once and that are not likely to survive long enough for a second breeding attempt anyway. It is most commonly seen in plants, invertebrates and some fish. It is a rare strategy in mammals, in part because mammalian females do not have many offspring at once and they need to live long enough to care for their young after they are born, which dying early would obviously prevent. Despite this, there are over a dozen species of mammals of which all the males die after their one and only breeding season. How could this possibly be adaptive?

Today at Accumulating Glitches, I talk about how patterns of insect abundance and competitive sperm have pushed some mammals to mate themselves to death! Check out the article here.

Loving to Death

The brown antechinus may look like a
mouse - but that is where the similarities
end. Photo by Glen Fergus at Wikimedia.

Although most animal species breed multiple times throughout their lives, a few oddballs put everything they've got into a single reproductive season, after which they promptly die. This is a rare strategy (for obvious reasons), especially in mammals. One Australian mammal, the brown antechinus, is just odd enough to pull it off.

The brown antechinus is a small insectivorous mouse-sized critter from Australia that in fact is not a mouse at all. It is a marsupial; but unlike kangaroos and koalas, females do not carry their young in a pouch, but rather let them hang off their eight teats for four months. All males die when they are 11 months old (if not sooner) after a single 2-3 week long mating season during which they do little else than mate as often as possible. The mating season leaves all the males (whether mated or not) sterile, coursing with stress hormones, immunosuppressed, and riddled with microorganisms and parasites. Shortly thereafter all the males die, balding and bleeding messes.

The reproductive strategy of putting everything you've got into a single mating season and then dying is only an advantage if you can have many offspring in that single reproductive event. Male brown antechinuses can only succeed in this suicidal mating strategy if they father many of the young of many of the females. As a result, both male and female brown antechinuses are promiscuous (mate with many individuals).

Male brown antechinuses are generally bigger than females, and DNA testing has shown us that in the wild, larger males and males with bigger testes impregnate the most females. Diana Fisher and Andrew Cockburn from Australian National University tested whether larger male brown antechinuses were more likely to get the girls because females were more likely to choose them or because they were outcompeting other males.

Diana and Andrew trapped brown antechinuses and brought them into the lab. In one test, they placed three males in separate nest boxes next to one another in an arena and allowed females to choose among them and mate with whichever one she chose. Surprisingly, when presented with this choice, females did not consistently choose the largest males. They didn't even check them all out - The females mated with whatever male happened to be in the first nest box she entered.

When the researchers put three males into a single nest box and allowed the females to mate, she almost always immediately mated with one of the three males. The next day, the researchers put the female in a nest box with either the two losers from the day before or with two randomly chosen males she did not know. On this second day, females presented with two strangers immediately mated with one male, whereas females presented with the two losers from the day before were more likely to spend more time evading both males, but often eventually mated with one of them. On the third day, the researchers put the female in a nest box with either the loser from the previous two days or with another randomly chosen stranger. Nine out of ten females paired with a stranger mated with him on this third day, whereas only one female paired with a double-loser was willing to mate with him at all. Males that successfully mated on the first day were generally the largest of the three. Loser males that mated on the second day were generally the second-largest and unsuccessful males were generally the smallest.

Interestingly, when given a choice of males one at a time, female brown antechinuses do not seem to care at all about male size. But when males are directly competing with one another, the largest male seems to get the girl. It appears that body size plays a role in the dominance interactions among the males, and that females are paying attention to how the males relate to one another. Additionally, larger males that were more successful in mating also lived longer and had fewer parasites. This could be because it is more stressful to be a loser than to be a winner. Stress increases the production of stress hormones, which in turn reduces immune function. In all of these ways, bigger males are more likely to father more young, who in turn will be more likely to grow up to be big males too... but not for long...


Want to know more? Check these out:

Fisher, D., & Cockburn, A. (2005). The large-male advantage in brown antechinuses: female choice, male dominance, and delayed male death Behavioral Ecology, 17 (2), 164-171 DOI: 10.1093/beheco/arj012

Doing it to death: suicidal sex in "marsupial mice" at The Conversation