The cane toad may be the poster animal for invasive species. Native to South America, it has been introduced to many other ecosystems in the hope it would chow down on agricultural pests. Instead, the toad has become a pest itself, most notably in Australia. Free from the predators and parasites in its native range, the toad’s poison glands have turned out to be a hazard for most species that try to eat it where it has been introduced.
But that doesn’t mean that it’s completely free of the risk of predation. Australian cane toad tadpoles have been observed feeding on their fellow cane toad offspring. This cannibalism seems to be an evolutionary response to the lack of competing species in its invasive range, causing cane toads to turn on their remaining competition: each other. And the toad has already turned to an additional evolutionary response to try to limit the danger of cannibalism.
Only Competing With Themselves
From an evolutionary perspective, cannibalism can make sense as a way to limit the competition posed by other members of your species. But the research team at the University of Sydney that has tracked the cane toad’s cannibalism suggests that the species’ successful invasion of Australia has accentuated this evolutionary pressure—something that may also occur with other invasive predators. One of the marks of an invasive species is its abundance in its new range, at which point competition for limited resources becomes more likely. Cannibalism not only limits this competition but provides nutritional resources as well.
With the Australian population reaching about 10 times the density of the population in the cane toad’s native range, there’s plenty of opportunity for inter-toad competition. And that competition has been documented at early stages in the toad’s development. Recently hatched toads spend several days developing into tadpoles and, during this time, often get eaten by older, more mature tadpoles. In a heavily populated body of water, clutches of eggs laid after mature tadpoles are present may be completely wiped out before they can live past the hatchling stage.
Tadpoles eating tadpoles can occur in South America. But it happens much more often in Australia. So the researchers decided to see if cannibalism was producing biological differences between the native and invasive populations.
To do so, they obtained toads from both native and invasive populations and tracked the behavior of the offspring. To start, the researchers simply placed fertilized eggs in a container with a single tadpole. This showed that the Australian cane toads had become aggressive cannibals, as eggs placed in with them were over 2.5 times more likely to be cannibalized before producing a tadpole.
While many changes can produce this sort of difference, the researchers demonstrated that the Australian tadpoles were more likely to seek out recently hatched cane toads. When given a choice of moving into empty containers and one containing cane toad hatchlings, the invasive Australian toads were nearly 30 times more likely to go into the container with hatchlings.
By the time the hatchlings reach the tadpole stage and are too large to eat, their fellow tadpoles lose interest. There’s some indication that the earlier attraction is based on toxins put into the fertilized egg by the mother.
The Best Defense
High levels of predation tend to produce evolutionary responses to limit vulnerability, and cannibalism is no different. The researchers found that Australian toads were simply spending less of their developmental time in the vulnerable hatchling stage in order to avoid some of the impact of cannibalism.
This occurred via two different mechanisms. One of these was specifically dependent upon the presence of tadpoles. In other words, when the threat was present, development accelerated. But a separate acceleration was present regardless of whether tadpoles were present. While South American cane toads spent a total of about five days at the hatchling stage, Australian populations only spent three days. So the pressure of cannibalism had cut hatchling development time by nearly half.
Rotifers are microscopic freshwater-dwelling multicellular organisms. They’re already known to withstand freezing (even in liquid nitrogen), boiling, desiccation, and radiation, and the group has persisted for millions of years without having sex. The humble yet remarkably hardy bdelloid rotifer has now surprised researchers yet again—a recent study unearthed 24,000-year-old Siberian permafrost and found living (or at least revivable) rotifers there. Surviving 24,000 years in a deep freeze is a new record for the species.
Rotifers aren’t the only living organisms to emerge from permafrost or ice. The same researchers behind this latest discovery had previously found roughly 40,000-year-old viable roundworms in the region’s permafrost. Ancient moss, seeds, viruses, and bacteria have all shown impressive longevity on ice, prompting legitimate concern about whether any potentially harmful pathogens may also be released as glaciers and permafrost melt.
Given that bdelloids are generally only a threat to bacteria, algae, and detritus, however, there’s not much need for concern regarding this particular discovery. But as key players in the bottom of the food chain, newly reemerged rotifers indicate that maybe we should think about how species that haven’t been seen for millennia might reintegrate into modern ecosystems.
The Soil Cryology Lab in Pushchino, Russia, has been digging up Siberian permafrost in search of ancient organisms for roughly a decade. The group estimates the age of the organisms it finds by radiocarbon dating the surrounding soil samples (evidence has shown that there is no vertical movement through layers of permafrost). For example, last year, the researchers reported a “frozen zoo” of 35 viable protists (nucleus-containing organisms that are neither animal, plant, nor fungus) that they calculated ranged from hundreds to tens of thousands of years old.
In their most recent discovery, the cryology researchers found the living bdelloids after culturing the soil samples for about one month. Among rotifer classes, bdelloids have the fairly unusual ability to reproduce parthenogenetically—i.e., by cloning—and so the original specimens had already begun to do so. Although the clones made identifying the ancient parent challenging, this did greatly facilitate further investigation of the characteristics and behavior of the unfrozen strain.
Throughout all of the above permafrost studies, there is always the concern of sample contamination by modern-day organisms. Besides using techniques designed to prevent this, the team also addressed this issue by looking at the DNA present in the soil samples, confirming that contamination was highly unlikely. Phylogenetic analysis furthermore showed that the species didn’t match any known modern rotifers, although there is a closely related species found in Belgium.
The team was naturally interested in better understanding the freezing process and gaining insight into just how these rotifers survived for so long. As a first step, the researchers subsequently froze a selection of the cloned rotifers at -15° C for one week and captured videos of the rotifers reviving.
The researchers found that not all of the clones survived. Surprisingly, the clones generally weren’t much more freeze-tolerant than contemporary rotifers from Iceland, Alaska, Europe, North America, and even the Asian and African tropics. They were a little more freeze-tolerant than their closest genetic relative, but the difference was marginal.
The researchers did find that the rotifers could survive a relatively slow freezing process ( around 45 minutes). This is noteworthy because it was gradual enough that ice crystals formed inside of the animals’ cells—a development that is usually catastrophic for living organisms. In fact, protective mechanisms against this are highly sought after by anyone in the business of cryopreservation, making this latest finding especially enticing from that perspective.
Although the authors aren’t quite in that business, they do plan additional experiments to better understand cryptobiosis—the state of almost completely arrested metabolism that made the rotifers’ survival possible. As for research into cryopreservation of larger organisms, the authors suggest that this becomes trickier as the organism in question becomes more complex. That said, rotifers are among the most complicated cryopreserved species so far—complete with organs such as a brain and a gut.
The limestone caves and rock shelters of Indonesia’s southern Sulawesi island hold the oldest traces of human art and storytelling, dating back more than 40,000 years. Paintings adorn the walls of at least 300 sites in the karst hills of Maros-Pangkep, with more almost certainly waiting to be rediscovered. But archaeologists say humanity’s oldest art is crumbling before their very eyes.
“We have recorded rapid loss of hand-sized spall flakes from these ancient art panels over a single season (less than five months),” said archaeologist Rustan Lebe of Makassar’s culture heritage department.
The culprit is salt. As water flows through a limestone cave system, it carries minerals from the local bedrock, and the minerals eventually end up in the limestone. At the limestone’s surface, those minerals oxidize into a case-hardened rocky crust. Nearly all of the oldest rock art in Maros-Pangkep—like the oldest drawing in the world that depicts an actual object—is painted in red or mulberry-purple pigment on that hard outer layer. The rock is resistant to most weathering, providing a durable canvas for humanity’s oldest artwork.
But beneath the surface, trouble is brewing. Flowing water deposits minerals in the void spaces beneath the mineralized outer crust, and some of those minerals crystallize into mineral salts. As those crystals form, grow, and shrink, they push against the outer layer of mineralized limestone. Eventually, the rocky canvas where people first drew images of their world 40,000 years ago falls apart in hand-sized flakes.
To help understand the extent of the problem and confirm that salt is to blame, Griffith University archaeologist Jillian Huntley and her colleagues collected flakes from the walls and ceilings of 11 caves in the area, including Leang Timpuseng, home of the oldest hand stencil. They found mineral salts like halite and calcium sulfate on the back sides of flakes from three of the sites. And all 11 sites showed high levels of sulfur, which is a key ingredient in many of the destructive salts that worry rock-art conservators.
Exfoliation isn’t a new process, but archaeologists and site custodians in Maros-Pangkep say they have watched the process speed up over the last few decades. Some of the local people who manage and protect the rock-art sites have done so for generations, and they report “more panel loss from exfoliation over recent decades than at any other time in living memory,” wrote Huntley and her colleagues.
That’s no coincidence, according to Huntley and her colleagues.
Here’s how the process works: heavy monsoon rains drench Indonesia and the surrounding region from November to March, leaving behind water in cave systems, flooded rice fields, and brackish aquaculture ponds along the coast. The water carries a load of dissolved salts and their mineral ingredients—things like table salt or halite, along with gypsum, sodium sulfate, magnesium sulfate, and calcium chloride.
When the water begins to evaporate, the salt it carried stays behind as crystals, which expand and contract along with changes in temperature and humidity. Some geological salts, like the ones mentioned above, can expand up to three times their original size when heated, and they can put an impressive amount of pressure on the surrounding rock. The result is similar to the freeze-thaw cycles that enable water ice to crack rocks and concrete.
The whole cycle is more active and more pronounced when temperatures rise and the local weather swings from extremely wet to extremely dry every few months. And that’s precisely the conditions Indonesia is experiencing as the climate gets warmer and extreme weather events become more frequent. More and more over the last few decades, severe monsoon flooding is followed by periods of intense drought.
People struggle, rocks crack, and a little more of humanity’s deepest connection to itself fades away.
“We are in a race against time,” said rock-art expert Adhi Agus Oktaviana of Indonesia’s National Research Center for Archaeology (ARKENAS). “Our teams continue to survey the area, finding new artworks every year. Almost without exception, the paintings are exfoliating and in advanced stages of decay.”