Tasmanian devils have a reputation as a fearsome animal – most of the time this is undeserved. When it comes to the mating season, however, it’s a fair judgement. Between February and April, mating can be incredibly aggressive, with male and female devils prone to biting one another both during and after the act.
That could be deadly for the devils, according to new research published online in the journal Behavioral Ecology.
DFTD is highly unusual for a cancer because it can transfer between individual devils and grow in its new host.
The fact that devils regularly bite one another around the mouth means tumour cells can easily transfer from an infected devil to an open wound on a healthy devil. This makes the buildup of wounds in devils extremely important to our understanding of this disease.
When devils mate
In our study, we examined the accumulation of bite wounds in a population of wild devils in northwest Tasmania.
We found males were much more likely than females to pick up high numbers of bite wounds. But these wounds appear to be related to the amount of time males spent in mating season interactions with females, as opposed to fights with other males (as we had previously thought).
In the mating season, after male devils have mated with females, they spend an extended period either confining the female in a den, or closely following her to make sure other males are unable to mate with her.
During our study we found this behaviour could go on for up to two weeks in the wild. The process is known as “mate guarding” and is relatively common in the animal kingdom.
We found the longer males spent engaging in mate guarding behaviour, the more bite wounds they received. This would seem to put successful males, who mate with a high number of females, in the firing line when it comes to acquiring DFTD.
But no pattern of sex bias in DFTD prevalence has ever been observed in the wild.
So how does this fit with our study on the increased vulnerability in males?
A crucial unknown in the DFTD transmission process involves directionality – which way the deadly disease is passed on by a devil. There are two possibilities:
an infected devil bites an uninfected animal, transferring tumour cells (from its teeth or saliva) directly into the wound it causes
an uninfected devil bites into tumours on an infected animal, and cells transfer into an open wound inside the biter’s mouth.
The reality is likely to involve a combination of the two.
Our results indicate that most disease transmission occurs during extended mating season interactions, when females appear to be causing high numbers of wounds to their mates.
If DFTD can transfer in either direction during these encounters, then both the males receiving the wounds and the females causing them would be equally at risk of acquiring the disease.
Future of the devil
We have highlighted mating season encounters between the sexes as crucial transmission points for the spread of DFTD. The behaviour of male devils appears to be driving patterns that support transmission of the disease.
This information is important for potential disease management options, as it pinpoints males in good condition – who are likely to be reproductively successful – as targets for management interventions, such as vaccinations.
Most importantly, these results add one more piece to the puzzle of rapid evolution in the Tasmanian devil, in response to the strong evolutionary pressure DFTD is placing on this iconic species. With almost 100% mortality once devils reach breeding age, any advantage an individual devil might have to survive a little longer and reproduce should – over time – spread through the population.
DFTD is spread through biting so we can expect strong evolutionary pressure for devils to become less aggressive towards each other over time.
With these new results, we can now pinpoint for the first time who (healthy, successful males) and when (guarding females after mating) the intense selection pressure on aggressive behaviour in devils will operate.
Ultimately, devils will solve the DFTD problem themselves by evolving resistance, tolerance and changing their behaviour. One of the best things we can do is let evolution take its course, giving a helping hand along the way via well guided management actions.
David Hamilton, PhD Candidate in Zoology, University of Tasmania; Elissa Cameron, Professor of Wildlife Ecology, University of Tasmania; Menna Elizabeth Jones, Associate Professor in Zoology, University of Tasmania, and Rodrigo Hamede, Post Doctoral Research Fellow, Conservation Biology and Wildlife Management, University of Tasmania
The Tasmanian devil – despite its name – once roamed the mainland of Australia. Returning the devil to the mainland may not only help its threatened status but could help control invasive predators such as feral cats and foxes.
But now we’ve explored the idea from a palaeontological view. We looked at the fossil record of mainland devils, in a paper published online and in print soon in the journal Biological Conservation.
The fossil record helps us better understand how the devils co-existed on mainland Australia with other wildlife. It also helps us see how these iconic animals may possibly interact with small and medium-sized animals if reintroduced to the mainland in the future.
Back in the wild
Ecologists have reintroduced several apex predators to environments where they were once driven to localised extinction. This has helped restore past ecosystems by providing a clearer ecological balance.
One of the best-known examples is the reintroduction of wolves to Yellowstone National Park in the United States, to check the overgrazing and destruction of habitat by elk.
By reintroducing Tasmanian devils into mainland Australia, can we possibly help restore ecological systems that support devils along with small to medium-sized native mammals?
Native and exotic predators
But these iconic Australian predators were still able to survive in Tasmania. The island was created 10,000 years ago by rising sea levels, well before the arrival of dingoes on mainland Australia.
Dingoes have now been eradicated across much of mainland Australia, particularly within the seclusion zone of the dingo fence in the southeast of the continent. The 5,400km fence stretches eastwards across South Australia into New South Wales and to southeast Queensland.
Exotic predators such as foxes and cats now thrive across many parts of Australia, and have devastating impacts on small to medium-sized Australian mammals.
But until recently they have not been able to gain a foothold in Tasmania. Many ecologists believe the presence of the devil has prevented these other animals making their destructive mark on the ecology of Tasmania.
Sadly the situation is changing as a result of the deadly devil facial tumour disease, an infectious cancer that has destroyed many populations of Tasmanian devils. Estimates range up to 90% of some population groups now wiped out.
As a result, feral cats are now moving into former devil habitats and hunting native species on Tasmania.
A fossil window to the past
So what does the fossil record tell us about the past life of the Tasmanian devil in mainland Australia?
The Willandra Lakes World Heritage Area, in southeast Australia, provides an extraordinary archaeological and palaeoecological record of Ice Age Australia.
In the past, skeletal remains buried within the landscape were commonly fossilised. Evidence of small animals that dug burrows (such as burrowing bettongs) and the predators that pursued them in their burrows, are exceptionally well preserved.
Our excavations reveal how devils and other small-to-medium sized mammals and reptiles interacted over more than 20,000 years in this area. Even during the peak arid phase, known as the Last Glacial Maximum, it seems that devils and their prey successfully co-existed.
The fossil record shows that the range of habitats occupied by devils in the past was far more diverse than today, with populations being found across environments from the central arid core to the northern tropics.
This suggests that devils today should, theoretically, be able to reoccupy a similarly extensive range of habitats.
Better the devil you know
Some ecologists suggest dingoes should be reintroduced into Australian habitats in order to reduce the impact of cats and foxes on native mammals.
One problem is that dingoes also prey on livestock. This is the reason the dingo fence was constructed during the 1880s.
But devils are not active predators of cattle and sheep. So reintroducing a predator that has a much longer evolutionary history with other native mammals in this country would likely receive far less opposition from pastoralists.
A reintroduction of devils back to the mainland may be a new approach to consider for controlling the relentless, destructive march of exotic predators and restore crucial elements of Australia’s biodiversity.
It still needs to be demonstrated that devils can suppress the activities of cats and foxes on the mainland, as they seem to have done in Tasmania. Experiments with devils in a range of different settings would help to establish this.
A new research approach involving palaeontologists, conservation biologists and policy makers may help us understand how we can restore biodiversity function in Australia.
The decline of Tasmanian devils is having an unusual knock-on effect: animal carcasses would once have been gobbled up in short order by devils are now taking many days longer to disappear.
We made the discovery, published today in the journal Proceedings of the Royal Society B, by placing carcasses in a range of locations and watching what happened. We found that reduced scavenging by devils results in extra food for less efficient scavengers, such as feral cats.
Tasmanian devils have struggled for two decades against a typically fatal transmissible cancer, called devil facial tumour disease. The disease has caused devil populations to plummet by about 80% on average, and by up to 95% in some areas.
Scavengers are carnivores that feed on dead animals (carrion). Almost all carnivores scavenge to a greater or lesser degree, but the devil is Tasmania’s dominant scavenger. Since the extinction of the Tasmanian tiger, it is also the island’s top predator.
A scavenging experiment
In our study, we put out carcasses of the Tasmanian pademelon (a small wallaby weighing roughly 5kg) in a variety of places, ranging from disease-free areas with large devil populations, to long-diseased areas where devil numbers are very low. We then used motion-sensor cameras to record all scavenger species that fed on the carcasses.
Unsurprisingly, much less carrion was consumed by devils in areas where devil populations have declined. This has increased the availability of carrion for other species, such as the invasive feral cat, spotted-tailed quoll, and forest raven. All of these species significantly increased their scavenging in places with fewer devils.
The responses of native scavengers (quolls and ravens) were subtly different to those of feral cats. The amount of feeding by quolls and ravens depended simply on how much of each carcass had already been consumed by devils. Ravens and quolls are smaller and less efficient than devils at consuming carcasses, so they get the chance to feed only when devils have not already monopolised a carcass.
In contrast, feral cats tended to scavenge only at sites where devils were at very low abundance. This suggests that healthy devil populations create a “landscape of fear” that causes cats to avoid carcasses altogether in areas where they are likely to encounter a devil. It seems that the life of a feral cat is now less scary in the absence of devils.
By looking at 20 years of bird surveys from BirdLife Australia, we also found that the odds of encountering a raven in Tasmania have more than doubled from 1998 to 2017. However, we were unable to directly link this with devil declines. It is likely the raven population is growing in response to a range of factors that includes land-use change and agricultural intensification, as well as reduced competition with devils.
Other studies have shown that cats have also become more abundant in areas where devils have declined. This highlights the potential for devils to act as a natural biological control on cats. Cats are a major threat to small native animals and are implicated in most Australian mammal extinctions.
Although smaller scavengers consumed more carrion as devils declined, they were unable to consume them as rapidly as devils. This has resulted in the accumulation of carcasses that would previously have been quickly and completely eaten by devils.
In places with plenty of devils, carcasses were completely eaten within an average of five days, compared with 13 days in places where devil facial tumour disease is rife. That means carcasses last much longer where devils are rare.
Around 2 million medium-sized animals are killed by vehicles or culled in Tasmania each year, and most are simply left to decompose where they fall. With devils consuming much less carrion, it is likely that carcasses are accumulating across Tasmania. It is unclear how much of a disease risk they pose to wildlife and livestock.
Large carnivores are declining throughout the world, with knock-on effects such as increasing abundance of smaller predators. In recent years, some large carnivores have begun returning to their former ranges, bringing hope that their lost ecological roles may be restored.
Carnivores are declining for many reasons, but an underlying cause is that humans do not necessarily appreciate their pivotal role in the health of entire ecosystems. One way to change this is to recognise the beneficial services they provide.
Our research highlights one of these benefits. It supports arguments that we should help the devil population recover, not just for their own sake but for other species too, including those threatened by feral cats.
The devil seems to be solving the disease problem itself, rapidly evolving resistance to facial tumours. Any management plan will need to help this process, and not hinder it. Potentially, returning devils to mainland Australia could provide similar benefit to wildlife threatened by feral predators.
Calum Cunningham, PhD candidate, University of Tasmania, University of Tasmania; Christopher Johnson, Professor of Wildlife Conservation and ARC Australian Professorial Fellow, University of Tasmania; Menna Elizabeth Jones, Associate professor, University of Tasmania, and Tracey Hollings, Senior Scientist, Ecological Modelling at Arthur Rylah Institute for Environmental Research, and Honorary Research Fellow, University of Melbourne
Konstans Wells, Griffith University; Andrew Storfer, Washington State University; Douglas Kerlin, Griffith University; Hamish McCallum, Griffith University; Menna Elizabeth Jones, University of Tasmania; Paul Hohenlohe, University of Idaho, and Rodrigo Hamede, University of Tasmania
Tasmanian devils in their prime are most likely to become infected with deadly facial tumour disease (DFTD), our research shows.
Instead, it’s the devils that enjoy the highest survival and breeding success who eventually succumb to the fatal disease.
DFTD has had a devastating effect on devil populations in Tasmania, with the marsupial carnivore placed on the endangered list in 2009.
So what is it that makes the fitter devils more prone to infection?
The devils in detail
DFTD is unique in that it is one of only a few known cases of transmissible cancer, where the deadly tumours do not originate from the host body.
The disease is transmitted into an individual when devils bite each other.
To track DFTD in a population, over ten years we repeatedly surveyed more than 500 wild devils, visiting the same field site at least four times per year.
This allowed us to study both survival and reproduction of the devils in the context of infection dynamics and tumour growth.
Our results add to our understanding of how DFTD spreads through devil populations, and reveal more details of how disease-induced evolution in devil populations (such as resistance to the disease) may be occurring.
We suggest the way disease is transmitted plays a key role in who gets infected.
It is the dominant devils who are more likely to engage in aggressive behaviour, such as during mating. This puts them at higher risk of biting an infected individual and thus becoming infected themselves.
So it’s the devils who are otherwise very fit (in the evolutionary sense) that the disease takes out. These are the ones that have the highest survival and reproduction rates, before being killed by the cancer.
Impact on devil populations
So what does this say about the future survival of devil populations in Tasmania’s wild?
Too often, a dramatic-looking disease such as DFTD leaves the impression that it must have detrimental effect on the overall population growth.
But this is not necessarily the case if diseased individuals had a chance to reproduce before they got infected.
In the graphic (above) we can see that some devils may not reproduce because either (A) of their social status, or (B) if they get an infection early in life and rapid tumour growth results in death.
In contrast, devils who get the disease late in life © may have already reproduced earlier. In (D) devils may still get infected, but if the tumour grows slowly they may still have chance to reproduce before death.
As for healthy and dominant devils who don’t get the disease (E), they may reproduce several times in their life.
Such details can be vital to understand the spread of DFTD and the outcome for Tasmanian devil populations.
It is the complex interplay of devil demography and disease dynamics that ultimately determines whether DFTD is a conservation threat for devils.
Our results also show a recent decline in the likelihood that devils become infected in this population. This could indicate some evolving resistance of devils to the cancer, as was recently shown by researchers from our team.
Alternatively, the decline in infection rate could have resulted from a reduction in the number of socially dominant devils, if these are responsible for most transmissions of the disease.
If adult devils with high fitness are those that become infected, the potential for selection for resistant animals would be limited.
This is because these individuals still contribute more offspring (and their genetic constitution) to future generations than those not infected and with little engagement in reproduction.
Our findings could have an impact on some of the conservation strategies for devils, such as vaccination or translocation of devils to other areas.
For example, a targeted vaccination of socially dominant individuals would be more efficient than randomly picking individuals for vaccination.
If devil individuals from captive insurance populations were to be released into wild populations, the consequences for disease spread and population viability would be unpredictable without a better understanding of the role of social behaviour in disease transmission.
If introduced individuals distract existing social structure and more frequently engage in biting behaviour, they may favour the spread of DFTD.
If devils develop resistance to DFTD, the introduction of individuals from captive populations may dilute the natural selection process.
Our study suggest that DFTD appears to be selectively spread and does not affect all individuals in a population. Understanding disease transmission pathways is a prerequisite to aid conservation efforts to stop the spread of unwanted diseases.
Konstans Wells, Research Fellow in Ecology, Griffith University; Andrew Storfer, Professor & Associate Director, School of Biological Sciences, Washington State University; Douglas Kerlin, Postdoctoral Reseach Fellow, Environmental Futures Research Institute, Griffith University; Hamish McCallum, Professor, Griffith School of Environment and Acting Dean of Research, Griffith Sciences, Griffith University; Menna Elizabeth Jones, Associate professor, University of Tasmania; Paul Hohenlohe, , University of Idaho, and Rodrigo Hamede, Post Doctoral Research Fellow, Conservation Biology and Wildlife Management, University of Tasmania