Like something out of a zombie movie, species that were once thought extinct seem to be rising from the dead. Between February 21 and March 4 2019, three notable rediscoveries were announced – the Fernandina Island Galápagos tortoise (Chelonoidis phantasticus), which was last seen in 1906; Wallace’s giant bee (Megachile pluto), which had supposedly disappeared in 1980; and the Formosan clouded leopard (Neofelis nebulosa brachyura), which disappeared after the last sighting in 1983 and was officially declared extinct in 2013.
These rediscoveries suggest we may know very little about some of the world’s rarest species, but they also raise the question of how species are declared extinct in the first place. The IUCN Red List collates a global register of threatened species and measures their relative risks of extinction. The Red List has a set of criteria to determine the threat status of a species, which are only listed as “Extinct” when…
… there is no reasonable doubt that the last individual has died.
According to the Red List, this requires…
… exhaustive surveys in known and/or expected habitat, at appropriate times… throughout its historic range [which] have failed to record an individual. Surveys should be over a time frame appropriate to the taxon’s life cycle and life form.
Given all the evidence – or rather, lack of evidence – that’s needed, it’s surprising that any species is ever declared extinct. The criteria show that to understand whether a species is extinct, we need to know what it was doing in the past.
Sightings at a certain time and in a certain place make up our knowledge of a species’ survival, but when a species becomes rare, sightings are increasingly infrequent so that people start to wonder whether the species still exists.
People often use the time since the last sighting as a measure of likelihood when deciding if a species has died out, but the last sighting is rarely the last individual of the species or the actual date of extinction.
Instead, the species may persist for years without being seen, but the length of time since the last sighting strongly influences assumptions as to whether a species has gone extinct or not.
But what is a sighting? It can come in a variety of forms, from direct observation of a live individual in the flesh or in photographs, indirect evidence such as foot prints, scratches and faeces, and oral accounts from interviews with eyewitnesses.
But these different lines of evidence aren’t all worth the same – a bird in the hand is worth more than a roomful of recollections from people who saw it in the past. Trying to determine what are true sightings and what are false complicates the declaration of extinction.
The idea of a species being “rediscovered” can confuse things further. Rediscovery implies that something was lost or forgotten but the term often gives the impression that a species has returned from the dead – hence the term “lazarus species”. This misinterpretation of lost or forgotten species means the default assumption is extinction for any species that hasn’t been seen for a number of years.
So, what does this mean for the three recently “rediscovered” species?
While a living specimen of the Fernandina Island Galápagos tortoise had not been seen since 1906, indirect observations of tortoise faeces, footprints and tortoise-like bite marks out of prickly pear cacti had been made as recently as 2013.
The uncertainty around the quality of these later observations and the long time since the last living sighting probably contributed to it being declared “Critically Endangered (Possibly Extinct)” in 2015. In the natural world, a species is presumed extinct until proven living.
Wallace’s giant bee may not have been recorded in the last 38 years but it was never actually declared extinct according to the IUCN Red List. In fact, for many years it languished under the criteria of Data Deficient and was only recently assessed as Vulnerable.
So, while this is an exciting find for something that hadn’t been seen for so long, its rediscovery shows how little is known about many rare species in the wild, rather than how scarce they are.
The Formosan clouded leopard, meanwhile, was actually listed as Extinct. The last sighting of the species was in 1983, based on interviews with 70 hunters, and extensive camera trapping during the 2000s failed to detect its presence. It was officially declared extinct in 2013.
While the giant tortoise and bee were proclaimed alive after living specimens were found, the clouded leopard’s rediscovery is more uncertain. Based on sightings on two separate occasions by two sets of wildlife rangers, the evidence is compelling. But whether the Formosan Clouded Leopard has really risen from the dead will require considerably more effort to prove.
Few sights and sounds are as emblematic of the North American southwest as a defensive rattlesnake, reared up, buzzing, and ready to strike. The message is loud and clear, “Back off! If you don’t hurt me, I won’t hurt you.” Any intruders who fail to heed the warning can expect to fall victim to a venomous bite.
But the consequences of that bite are surprisingly unpredictable. Snake venoms are complex cocktails made up of dozens of individual toxins that attack different parts of the target’s body. The composition of these cocktails is highly variable, even within single species. Biologists have come to assume that most of this variation reflects adaptation to what prey the snakes eat in the wild. But our study of the Mohave rattlesnake (Crotalus scutulatus, also known as the Mojave rattlesnake) has uncovered an intriguing exception to this rule.
A 20-minute drive can take you from a population of this rattlesnake species with a highly lethal neurotoxic venom, causing paralysis and shock, to one with a haemotoxic venom, causing swelling, bruising, blistering and bleeding. The neurotoxic venom (known as venom A) can be more than ten times as lethal as the haemotoxic venom (venom B), at least to lab mice.
The Mohave rattlesnake is not alone in having different venoms like this – several other rattlesnake species display the same variation. But why do we see these differences? Snake venom evolved to subdue and kill prey. One venom may be better at killing one prey species, while another may be more toxic to different prey. Natural selection should favour different venoms in snakes eating different prey – it’s a classic example of evolution through natural selection.
This idea that snake venom varies due to adaptation to eating different prey has become widely accepted among herpetologists and toxinologists. Some have found correlations between venom and prey. Others have shown prey-specific lethality of venoms, or identified toxins fine-tuned for killing the snakes’ natural prey. The venom of some snakes even changes along with their diet as they grow.
We expected the Mohave rattlesnake to be a prime example of this phenomenon. The extreme differences in venom composition, toxicity and mode of action (whether it is neurotoxic or haemotoxic) seem an obvious target for natural selection for different prey. And yet, when we correlated differences in venom composition with regional diet, we were shocked to find there is no link.
In the absence of adaptation to local diet, we expected to see a connection between gene flow (transfer of genetic material between populations) and venom composition. Populations with ample gene flow would be expected to have more similar venoms than populations that are genetically less connected. But once again, we drew a blank – there is no link between gene flow and venom. This finding, together with the geographic segregation of the two populations with different venoms, suggests that instead there is strong local selection for venom type.
The next step in our research was to test for links between venom and the physical environment. Finally, we found some associations. The haemotoxic venom is found in rattlesnakes which live in an area which experiences warmer temperatures and more consistently low rainfall compared to where the rattlesnakes with the neurotoxic venom are found. But even this finding is deeply puzzling.
It has been suggested that, as well as killing prey, venom may also help digestion. Rattlesnakes eat large prey in one piece, and then have to digest it in a race against decay. A venom that starts predigesting the prey from the inside could help, especially in cooler climates where digestion is more difficult.
But the rattlesnakes with haemotoxic venom B, which better aids digestion, are found in warmer places, while snakes from cooler upland deserts invariably produce the non-digestive, neurotoxic venom A. Yet again, none of the conventional explanations make sense.
Clearly, the selective forces behind the extreme venom variation in the Mohave rattlesnake are complex and subtle. A link to diet may yet be found, perhaps through different kinds of venom resistance in key prey species, or prey dynamics affected by local climate. In any case, our results reopen the discussion on the drivers of venom composition, and caution against the simplistic assumption that all venom variation is driven by the species composition of regional diets.
From a human perspective, variation in venom composition is the bane of anyone working on snakebite treatments, or antidote development. It can lead to unexpected symptoms, and antivenoms may not work against some populations of a species they supposedly cover. Anyone living within the range of the Mohave rattlesnake can rest easy though – the available antivenoms cover both main venom types.
Globally, however, our study underlines the unpredictability of venom variation, and shows again that there are no shortcuts to understanding it. Those developing antivenoms need to identify regional venom variants and carry out extensive testing to ensure that their products are effective against all intended venoms.
Planning a trip to the tropics? You might end up bringing home more than just a tan and a towel.
Our latest research looked at mosquitoes that travel as secret stowaways on flights returning to Australia and New Zealand from popular holiday destinations.
We found mosquito stowaways mostly enter Australia from Southeast Asia, and enter New Zealand from the Pacific Islands. Worse still, most of these stowaways are resistant to a wide range of insecticides, and could spread disease and be difficult to control in their new homes.
Undetected insects and other small creatures are transported by accident when people travel, and can cause enormous damage when they invade new locations.
Of all stowaway species, few have been as destructive as mosquitoes. Over the past 500 years, mosquitoes such as the yellow fever mosquito (Aedes aegypti) and Asian tiger mosquito (Aedes albopictus) have spread throughout the world’s tropical and subtropical regions.
Dengue spread by Aedes aegypti mosquitoes now affects tens to hundreds of millions of people every year.
Explainer: what is dengue fever?
Adults in your luggage
You probably won’t see Aedes mosquitoes buzzing about the cabin on your next inbound flight from the tropics. They are usually transported with cargo, either as adults or occasionally as eggs (that can hatch once in contact with water).
It only takes a few Aedes stowaways to start a new invasion. In Australia, they’ve been caught at international airports and seaports, and in recent years there has been a large increase in detections.
In our new paper, we set out to determine where stowaway Aedes aegypti collected in Australia and New Zealand were coming from. This hasn’t previously been possible.
Usually, mosquitoes are only collected after they have “disembarked” from their boat or plane. Government authorities monitor these stowaways by setting traps around airports or seaports that can capture adult mosquitoes. Using this method alone, they’re not able to tell which plane they came on.
But our approach added another layer: we looked at the DNA of collected mosquitoes. We knew from our previous work that the DNA from any two mosquitoes from the same location (such as Vietnam, for example) would be more similar than the DNA from two mosquitoes from different locations (such as Vietnam and Brazil).
So we built a DNA reference databank of Aedes aegypti collected from around the world, and compared the DNA of the Aedes aegypti stowaways to this reference databank. We could then work out whether a stowaway mosquito came from a particular location.
We identified the country of origin of most of the Aedes aegypti stowaways. The majority of these mosquitoes detected in Australia are likely to have come from flights originating in Bali.
Now we can work with these countries to build smarter systems for stopping the movement of stowaways.
As the project continues, we will keep adding new collections of Aedes aegypti to our reference databank. This will make it easier to identify the origin of future stowaways.
New mosquitoes are a problem
As Aedes aegypti has existed in Australia since the 19th century, the value of this research may seem hard to grasp. Why worry about invasions by a species that’s already here? There are two key reasons.
Currently, Aedes aegypti is only found in northern Australia. It is not found in any of Australia’s capital cities where the majority of Australians live. If Aedes aegypti established a population in a capital city, such as Brisbane, there would be more chance of the dengue virus being spread in Australia.
The other key reason is because of insecticide resistance. In places where people use lots of insecticide to control Aedes aegypti, the mosquitoes develop resistance to these chemicals. This resistance generally comes from one or more DNA mutations, which are passed from parents to their offspring.
Importantly, none of these mutations are currently found in Australian Aedes aegpyti. The danger is that mosquitoes from overseas could introduce these resistance mutations into Australian Aedes aegpyti populations. This would make it harder to control them with insecticides if there is a dengue outbreak in the future.
In our study, we found that every Aedes aegpyti stowaway that had come from overseas had at least one insecticide resistance mutation. Most mosquitoes had multiple mutations, which should make them resistant to multiple types of insecticides. Ironically, these include the same types of insecticides used on planes to stop the movement of stowaways.
Other species to watch
We can now start tracking other stowaway species using the same methods. The Asian tiger mosquito (Aedes albopictus) hasn’t been found on mainland Australia, but has invaded the Torres Strait Islands and may reach the Cape York Peninsula soon.
Worse still, it is even better than Aedes aegypti at stowing away, as Aedes albopictus eggs can handle a wider range of temperatures.
A future invasion of Aedes albopictus could take place through an airport or seaport in any major Australian city. Although it is not as effective as Aedes aegypti at spreading dengue, this mosquito is aggressive and has a painful bite. This has given it the nickname “the barbecue stopper”.
Beyond mosquitoes, our DNA-based approach can also be applied to other pests. This should be particularly important for protecting Australia’s A$45 billion dollar agricultural export market as international movement of people and goods continues to increase.
Tom Schmidt, Research fellow, University of Melbourne; Andrew Weeks, Senior Research Fellow, University of Melbourne, and Ary Hoffmann, Professor, School of BioSciences and Bio21 Institute, University of Melbourne