Sexual aggression key to spread of deadly tumours in Tasmanian devils



Both male and female Tasmanian devils can become very violent during sexual interactions.
Shutterstock/PARFENOV

David Hamilton, University of Tasmania; Elissa Cameron, University of Tasmania; Menna Elizabeth Jones, University of Tasmania, and Rodrigo Hamede, University of Tasmania

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.

Unfortunately, biting drives the spread of devil facial tumour disease (DFTD) a transmissible cancer that has been afflicting the species since the mid-1990s.




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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 Tasmanian devil with the Devil Facial Tumour Disease.
Menna Jones/PLOS ONE, CC BY

Disease transfer

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:

  1. an infected devil bites an uninfected animal, transferring tumour cells (from its teeth or saliva) directly into the wound it causes

  2. 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.

The species has already shown remarkably rapid shifts in their life history and genome, while some are able to mount an immune response and recover from the tumours.

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.




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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.The Conversation

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

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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Warty hammer orchids are sexual deceivers



File 20181129 170226 u1w9ow.png?ixlib=rb 1.1

The Conversation, CC BY-SA

Ryan Phillips, La Trobe University

Orchids are famed for their beautiful and alluring flowers – and the great lengths to which people will go to experience them in the wild. Among Australian orchids, evocative names such as The Butterfly Orchid, The Queen of Sheeba, and Cleopatra’s Needles conjure up images of rare and beautiful flowers.

Yet there is a rich diversity of our orchids. Some are diminutive, warty, and unpleasant-smelling, bearing little resemblance to a typical flower.




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While many orchid enthusiasts have a soft spot for these quirky members of the Australian flora, what has brought them international recognition is their flair for using some of the most bizarre reproductive strategies on Earth.



The Conversation/Ryan Phillips/Suzi Bond., CC BY

Sexual mimicry

From the very beginnings of pollination research in Australia there were signs that something unusual was going on in the Australian orchid flora.

In the 1920s Edith Coleman from Victoria made the sensational discovery that the Australian tongue and bonnet orchids (Cryptostylis) were pollinated by males of a particular species of ichneumonid wasp attempting to mate with the flower.

But this was just the beginning.

The King-in-his-carriage, Drakaea glyptodon, is the most common species of hammer orchid. Here the flower is pictured next to the female of its pollinating thynnine wasp, Zaspilothynnus trilobatus.
Rod Peakall, Author provided

We now know that while the insect species involved may vary, many of our orchid species use this strategy. Australia is the world centre for sexual deception in plants.

Perhaps the most sophisticated flower of all sexually deceptive plants is seen in the hammer orchids, a diminutive genus that only grows in southwestern Australia. Their solitary stem reaches a height of around 40cm, and each stem produces a single flower no more than 4cm in length.

Even among sexually deceptive orchids, hammer orchids stand out from the crowd. They have a single heart-shaped leaf that sits flush with the soil surface, and grow in areas of dry inhospitable sand – an unusual choice for an orchid.

The thynnine wasp Zaspilothynnus nigripes is a sexually deceived.
pollinator of the Warty hammer orchid. Here they are pictured in copula, with the
flightless female having been carried to a food source by the male.

Keith Smith, Author provided

And then there is the flower. Not only does the lip of the flower more closely resemble an insect than a petal, but it is hinged partway along. All of which starts to makes sense once you see the pollinators in action.

Like many other Australian sexually deceptive orchids, they are pollinated by thynnine wasps – a unique group in which the male picks up the flightless female and they mate in flight.

In the case of hammer orchids, the male grasps the insect-like lip and attempts to fly off with “her”. The combination of his momentum and the hinge mechanism swings him upside down and onto the orchid’s reproductive structures.

It’s not me, it’s you (you’re a flower)

So, how do you trick a wasp?

Accurate visual mimicry of the female insect does not appear to be essential, as there are some sexually deceptive orchids that are brightly coloured like a regular flower.

Instead, the key ingredient for attracting pollinators to the flower is mimicking the sex pheromone of the female insect. And boy, is this pheromone potent.

Indeed, one of the strangest fieldwork experiences I’ve had was wasps flying through my open car window while stopped at traffic lights, irresistibly drawn to make love to the hammer orchids sitting on the passenger seat!

Pollination of the Warty hammer orchid by a male of the thynnine wasp Zaspilothynnus nigripes.
Suzi Bond, Author provided

While determining the chemicals responsible for attraction of sexually deceived pollinators is a laborious process, we now know that multiple classes of chemicals are involved, several of which were new to science or had no previously known function in plants.

What’s more, we are still discovering new and unexpected cases of sexual deception in orchids that don’t conform to the insect-like appearance of many sexually deceptive orchids.

A classic example is the case of the Warty hammer orchid and the Kings spider orchid – these two species have totally different-looking flowers, yet both are pollinated by the same wasp species through sexual deception.

While the ability to attract sexually excited males without closely resembling a female insect may partly explain the evolution of sexual deception, it does not explain the benefit of evolving this strategy in the first place.

A leading hypothesis for the evolution of sexual deception is that mate-seeking males be more efficient at finding orchid flowers than food-foraging pollinators – but this remains a work in progress.

The life cycle of the Warty hammer orchid and its pollinator species,
highlighting the complex ecological requirements needed to support a population of.
the orchid.

Martin Thompson, Author provided

From a conservation point of view, pollination by sexual deception has some interesting challenges. Female animals produce sex pheromones that only attract males of their own species. This means an orchid that mimics a sex pheromone typically relies on a single pollinator species. As such, conservation of any given orchid species requires the presence of a viable population of a particular pollinator.

Further, an interesting quirk of these sexually deceptive systems is the potential for cryptic forms of the orchid: where populations of orchids that appear identical to human observers actually attract different pollinator species through shifts in pheromone chemistry. Indeed, of the ten known species of hammer orchid, three contain cryptic forms.




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Not only does this create a major challenge for managing rare species, it raises the possibility that – should these forms prove to be separate species – the true diversity of sexually deceptive orchids could be greatly underestimated.


Sign up to Beating Around the Bush, a series that profiles native plants: part gardening column, part dispatches from country, entirely Australian.The Conversation

Ryan Phillips, Senior Lecturer in Ecology, Environment & Evolution, La Trobe University

This article is republished from The Conversation under a Creative Commons license. Read the original article.