Desert ant increase the visibility of their nest entrances in the absence of landmarks

Desert ants have outstanding navigational skills. They live in the saltpans of North Africa, an extremely inhospitable environment. To find food for their nest mates, foraging ants have to walk far into the desert. Once they have found food, for example a dead insect, their actual problem begins: How do they find their way back to their nest as quickly as possible in the extremely hot and barren environment? "The desert ant Cataglyphis fortis stands out due its remarkable ability to successfully navigate and forage in even the harshest environments, making it an excellent subject for studying the intricacies of navigation. With an innate navigation mechanism called path integration, these ants use both a sun compass and a step counter to measure the distances they cover. In addition, they possess the ability to learn and utilize visible and olfactory cues. We believe that this extremely harsh habitat has led, during evolution, to a navigation system of unsurpassed precision," said Marilia Freire, the study's lead author, summarizing what is known so far about the amazing orientation skills of these small animals.

The scientists had noticed during previous studies in Tunisia that the nests in the center of the saltpans, where there are hardly any visible landmarks, had high mounds at the nest entrances. In contrast, nest hills near the shrub-covered edges of the saltpans were lower or barely noticeable. So the research team has wondered for some time if these visible differences serve a purpose in helping the ants better find their way home. "It's always hard to tell whether an animal does something on purpose or not. The high nest mounds in the middle of the saltpans could have been a side effect of differences in soil structure or wind conditions. However, crucial for our study was the idea to remove the mounds and to provide some nests with artificial landmarks and others not, and to observe what would happen," Markus Knaden, head of the Project Group Odor-guided Behavior in the Department of Evolutionary Neuroethology, explains the goal of the study.

For their experiments, the researchers followed the ants with a GPS device. This allowed them to track the ants on their way to the saltpan and back home. "We observed that desert ants are capable of traveling much greater distances than previously reported. The farthest distance a single animal traveled was more than two kilometers. However, we also observed an unexpectedly high mortality rate. About 20% of foraging ants do not find their way back home after extremely long runs and died in front of our eyes, which explains the enormous selection pressure for even better orientation," says Marilia Freire.

Experiments in which ants could be tracked with particular accuracy during the last meters to the nest, thanks to a grid painted on the floor, showed that the nest hills are important visual cues. If they were removed, fewer ants found their way back to the nest, while their nest mates simultaneously began to rebuild nest mounds as quickly as possible. If, on the other hand, the scientists placed artificial landmarks in the form of small black cylinders near the nest entrances whose mounds they had previously removed, the ants did not invest in building new ones. Apparently, the cylinders were sufficient for orientation.

In ant nests, labor is divided. Ants that go foraging are usually older and more experienced nest members, while younger ants are busy building. Therefore, there must be some kind of information flow between the two groups. The researchers do not yet know exactly how this is achieved. "One possibility would be that ants in the nest somehow notice that fewer foragers return home, and as a result, hill-building activities at the nest entrance are increased," says Marilia Freire.

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South Africa's desert-like interior may have been more inviting to our human ancestors

Lining the Cape of South Africa and its southern coast are long chains of caves that nearly 200,000 years ago were surrounded by a lush landscape and plentiful food.

During a glacial phase that lasted between 195,000 to 123,000 years ago, these caves served as refuge to a group of humans that some researchers think were the only people to survive this ice age, called Marine Isotope Stage Six, or MIS6. And in this coastal region, a lot of archaeological research has taken place. Of less interest to archaeologists has been the interior of South Africa, which was thought to be an uninhabited, inhospitable place during at least two waves of ice ages, MIS3 and 2.

Now, a study has shown that the region might have been more fertile and temperate during these two glacial periods than previously thought, and that the region likely played host to human populations living around a series of paleolakes. The study, led by University of Michigan archaeologist Brian Stewart, provides a more comprehensive timeline of the age and stages of these lakes, and shows human fingerprints across the region. The research, funded by the National Geographic Society, is published in the journal PNAS.

"There's this perennial assumption that human population centers were always along the coast and that the interior, especially the southern interior of the Karoo Desert, were largely depopulated for long stretches of time," Stewart said. "The funny thing is that one just has to go into the interior and walk around and notice that there's archaeology everywhere."

But to flag the region as worthy of archaeological attention, the researchers needed to show that humans could have actually lived there. The research team, an international group including researchers from South Africa, the United Kingdom and France, examined a series of super flat areas of land ringed by higher ground. They showed that these regions, called "pans" in Afrikaans, are ancient lake beds, while the areas of higher elevation that encircle them are erosional landforms and sedimentary deposits left over from their ancient shorelines.

This suggests that these time periods were not as dry in this region as previously thought: there would have needed to have been sustained rainfall and humidity to keep these lakes full. The more temperate climate required to sustain lakes also meant the landscape was also able to sustain both vegetation and animals required to support human populations.

Reconstructing the paleolakes

The researchers used a variety of technologies to date and reconstruct these ancient lakes, shorelines and lake bottom deposits, and to recreate the landscape of the region.

These include radiocarbon dating and a technique called luminescence, which measures the radioactivity of tiny crystals of quartz or feldspar that haven't seen the light of day since they were covered by sediment tens of thousands of years earlier. While buried, electrons from radioactive elements common to all sediments get trapped in these crystal matrices at a constant rate. By measuring how many electrons accumulated in the sample and comparing that to the degree of background radioactivity, the researchers can predict its age.

The researchers used these techniques to date columns of sedimentary lakeshore and lake bottom deposits, called lacustrine deposits, found surrounding and within a series of three pans dotted across a region some 100,000 square miles, about the size of Texas. They also aged the shells of freshwater mollusks found scattered throughout the region and embedded within the sedimentary lake deposits.

An aquatic gastropod, Tomichia ventricosa, found at a pan called Swartkolkvloer, was embedded in a column of lacustrine deposits. Together, the deposits and shells were radiocarbon dated to two time periods: approximately 39-55 thousand years before present, and approximately 31-34 thousand years before present.

At another pan called Grootvloer, the researchers found a freshwater mollusk called Unio caffer, which required "perennial freshwater" and the presence of fish to reproduce. These shells and lacustrine deposits in this pan were dated to between approximately 20-22 thousand years before present.

The researchers were also able to predict the area of land the lakes encompassed, as well as how deep they were, based on the height of the lacustrine deposits ringing their shorelines. For example, they determined that the paleolake at Swartkolkvloer was about 83 miles square and 59 feet deep, while the paleolake at Grootvloer was likely about 17 miles square and 62 feet deep. Another pan called Alexanderfontein, some 300 miles northeast near the town of Kimberley, held a lake about 13.5 miles square and 48 feet deep.

Based on these parameters, the researchers knew the lakes would have required a climate drastically different from the one that exists in South Africa's interior today. Using a hydrological model, they determined that evaporation rates within the region were between 20% and 25% lower than what they are today, while precipitation would have been more than 200% higher at Swartkolkvloer and 88% higher at Alexanderfontein.

"In various parts of the interior, between about 60 to about 12,000 years ago, we can see that there were enduring phases of pretty large bodies of water kicking around what used to be thought of as an arid and inhospitable environment," Stewart said. "This was a period that used to be thought of as notoriously dry and freezing cold in the winter. But we've got this evidence for these large lakes existing during this time period."

The researchers say current day regions that have similar climates and rates of precipitation and evaporation are likely northern Botswana, Zambia and Zimbabwe. Based on this estimation and data found from a contemporaneous site called Equus Cave, the researchers think large grazing and browsing mammals would have thrived in this region.

A complex, prehistoric people

Scattered alongside the lakes are human artifacts such as triangular points, bladelets, weathered ostrich shells and other assemblages. Much of these are from the Middle Stone Age, which ranged from about 280,000 years ago to about 25,000 years ago, and the Later Stone Age, which lasted from 25,000 years ago right up to European contact in the 15th century.

"There's Middle Stone Age archaeology just everywhere," Stewart said. "You can't drive anywhere and open the car door and not step on it. It's astounding how much there is."

Stewart says their findings could also expand the region where archaeologists think humans became more behaviorally complex. Over the past decades, researchers have found evidence that humans living in coastal South Africa started making the leap into complex thought, showing some of the earliest signs of behavioral complexity. It was thought that both climate and nutrient stability -- including the rich omega fatty acids gleaned from seafood -- allowed humans here to become behaviorally modern.

The group's work may challenge this idea by suggesting that favorable conditions for hunter-gatherers were not confined to the coastline but probably characterized many regions as climates changed, including the interior, current-day desert.

"Ours is a basin-wide study with much more macro scale implications. It also involves some of these areas that are just inland of this coastal and mountain belt that's dominated the archaeological narratives for so long," Stewart said. "Just inland of this area is a region that has been, for a long time, portrayed as hostile, and it just simply appears not to have been the case for big chunks of time -- with the caveat that we need information on temperature depressions to understand how humans dealt with that."

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With rapidly increasing heat and drought, can plants adapt?

At a time when climate change is making many areas of the planet hotter and drier, it's sobering to think that deserts are relatively new biomes that have grown considerably over the past 30 million years. Widespread arid regions, like the deserts that today cover much of western North America, began to emerge only within the past 5 to 7 million years.

Understanding how plants that invaded these harsh deserts biomes were able to survive could help predict how ecosystems will fare in a drier future.

An intensive study of a group of plants that first invaded emerging deserts millions of years ago concludes that these pioneers -- rock daisies -- did not come unequipped to deal with heat, scorching sun and lack of water. They had developed adaptations to such stresses while living on dry, exposed rock outcroppings within older, more moist areas and even tropical forests, all of which made it easier for them to invade expanding arid areas.

The study by University of California, Berkeley, researcher Isaac Lichter-Marck is the first to provide evidence to resolve a long-standing evolutionary debate: Did iconic desert plants, like the stately saguaro cacti, the flaming ocotillos and the Seussian agaves, adapt to arid conditions only after they invaded deserts. Or did they come preadapted to the stresses of desert living?

The question has relevance today, Lichter-Marck said, because accelerating aridity due to climate change is challenging plants to adapt much more quickly than they have in the past. Already, about one-fifth of Earth's land surface is desert. If adaptation to arid conditions was only possible for plants that had already evolved to deal with such stresses, then many today may not be equipped with an adequate genetic tool kit to survive.

"If you think about aridity only as a stimulus to plant evolution, then in many cases people could say these plants are survivors, they are adaptable, and they will be fine. They will take advantage of these new conditions, and they will thrive," said Lichter-Marck, who is also a National Science Foundation postdoctoral research fellow at UCLA.

But the history of rock daisies suggests that "when the deserts emerged, those plants that had the necessary preadaptations to take advantage of new conditions were the ones that thrived," he said. "Adding more aridification to the system doesn't necessarily mean more rapid adaptive evolution will occur. There's a limited source of lineages that can take advantage of new levels of aridity, and that is important for understanding the effect of climate change on biodiversity."

Lichter-Marck and Bruce Baldwin, UC Berkeley professor of integrative biology, curator of the Jepson Herbarium and chief editor of The Jepson Desert Manual: Vascular Plants of Southeastern California (2002), published their study about the evolution of rock daisies in North American deserts this week in the journal Proceedings of the National Academy of Sciences.

Seven years roaming the desert

Botanists realized long ago that when plants invaded desert areas, they quickly diversified to fill the many niches created by this new type of habitat.

"Even as recently as 1 million to 1.5 million years ago, it would have been difficult to find widespread desert habitats like we see today in North America, which is kind of surprising because now deserts and arid habitats are the most widespread biome on earth," Lichter-Marck said. "But during the late Miocene Epoch, dry habitats spread, and the world's lineages of desert plants, especially the succulent lineages like the cacti, the agaves and the ice plants -- as well as many other drought tolerant lineages -- underwent a synchronous rapid diversification."

Paleontologists pointed out, however, that fossilized plants that thrived tens of millions of years before the proliferation of deserts had characteristics similar to those of desert plants today. Some scientists, like the late paleoecologist Daniel Axelrod of UCLA and UC Davis, argued that this meant the plants that thrived in the desert today evolved earlier and were preadapted -- or exapted -- to survive desert conditions by growing in dry microsites, such as rock outcrops, rain shadows or mountaintops. Others, like UC Berkeley's Ledyard Stebbins, an evolutionary biologist who helped found the UC Davis Department of Genetics, argued that aridity itself spurred plants to diversify and develop traits to withstand dryness, heat, intense sunlight and strong winds.

Despite the similarities between rocky outcrops and deserts, it has been hard to prove that desert plants descended from plants already adapted to the stresses of aridity, in part because fossils rarely form in dry habitats and cannot tell us much about the habitat in which these ancient plants were growing.

To Lichter-Marck and Baldwin, rock daisies, which are classified in the tribe Perityleae in the sunflower family, seemed like a good group in which to explore the connection. Some species live on dry, exposed rock in tropical areas of Mexico -- what might be considered "micro-deserts" -- while others have fully adapted to desert areas, such as the Mojave in California and the Great Basin, Chihuahuan and Sonoran deserts that cover most of western North America.

"Plants that live on rock outcrops face many of the same challenges as those living in a dry, desert habitat," Lichter-Marck said. "Rock outcrops tend to be exposed to UV light, wind and dry, desiccating conditions, as well as heat and frost. They also tend to be more exposed to herbivores.

"The ways that plants deal with them are diverse, but they usually involve some kind of specialized root morphology that helps them to anchor in rock outcrops, as well as deal with the heightened arid conditions. And they tend to have smaller leaves, or leaves with a dense covering of hairs that help buffer them against drought and block sunlight, including UV light. They also tend to have heightened chemical defenses against herbivores, because it takes a lot of energy to regenerate after being munched."

For his Ph.D. thesis in the Department of Integrative Biology and at the Jepson Herbarium, Lichter-Marck, a Southern California native, roamed the deserts of Arizona, California, Texas and Mexico for months at a time in a pickup truck, accompanied by his blue heeler, Rio, to collect hundreds of specimens of rock daises. Some rock daisies are among the most dramatic bloomers in spring, carpeting the desert with colorful blossoms. Many, however, are limited to small geographic regions where they grow only on vertical rock faces or sky island mountain ranges, making them hazardous to collect. Lichter-Marck is an experienced mountaineer, an important skillset for field work in rough terrain.

He later sequenced the DNA of these specimens -- 73 of the 84 recognized species of rock daisy -- and catalogued their life histories, such as where they grew, what type of root system they had, and whether they were annual or perennial, an herb or a shrub. He then compared them to fossilized daisies to develop a rough timeline of the evolution of these characteristics and the lineage's eventual shift into deserts.

This allowed him to conclude that most rock daises -- in particular, the genus Laphamia, which was the first to move into deserts and is the largest rock daisy genus -- had adapted to the stress of heat, aridity, wind and sun by virtue of their growth on cliffs before invading deserts.

"This is a clear empirical demonstration of what was originally Axelrod's hypothesis -- of a desert plant group originating in dry microclimates prior to the widespread emergence of desert habitats," said Lichter-Marck. "What this means is that the strategies for drought tolerance that are so characteristic of desert vegetation might not actually represent responses to the dry conditions found in deserts. Instead, they could be traits that evolved earlier in association with much older and more stable dry microclimates, such as rock outcrops in tropical settings."

Preadaptation may be the key to the success of many desert plants, including cacti, which are known to inhabit rock outcrops or grow as epiphytes in the canopies of trees within tropical areas, though these large lineages would require a much more extended analysis, he said.

Rock daisies, many of which live in specialized habitats that make them vulnerable to extinction, highlight the importance of conserving seemingly niche species.

"A lot of the rock daisies are very specialized and tend to be very narrow in their distribution and might be seen as less significant to the survival of the ecosystem as a whole. In evolutionary biology and in conservation biology, specialized organisms with narrow geographic ranges are often considered vulnerable lineages and have sometimes even been called evolutionary dead ends," he said. "An important implication here is that a group of ecological specialists growing on scattered cliffs in tropical habitats started this major radiation in the desert. So, it actually shows that specialists are not just these vulnerable lineages on the edge of extinction. They might actually be really important sources for innovation in evolution."

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Tumultuous migration on the edge of the Hot Neptune Desert

All kinds of exoplanets orbit very close to their star. Some look like the Earth, others like Jupiter. Very few, however, are similar to Neptune. Why this anomaly in the distribution of exoplanets? Researchers from the University of Geneva (UNIGE) and the National Centre of Competence in Research (NCCR) PlanetS have observed a sample of planets located at the edge of this Hot Neptune Desert to understand its creation. Using a technique combining the two main methods of studying exoplanets (radial velocities and transits), they were able to establish that a part of these exoplanets has migrated in a turbulent way near their star, which pushed them out of the orbital plane where they were formed. These results are published in the specialized journal Astronomy & Astrophysics.

Since the discovery of the first exoplanet in 1995, researchers have detected more than 5'000 planets in our galactic neighborhood, most of them orbiting very close to their star. If the diversity of these new worlds ranges from gas giants the size of Jupiter or Saturn to smaller planets the size of Mercury, including rocky planets larger than the Earth, gas planets the size of Neptune seem to be missing. Astronomers call this empty ''box'' in the distribution of close-in planets the Hot Neptune Desert.

''The distribution of planets close to their star is shaped by a complex interaction between atmospheric and dynamical processes, i.e. the motions of the planets over time,'' comments Vincent Bourrier, assistant professor in the Department of Astronomy at the UNIGE Faculty of Science. ''Today we have several hypotheses to explain this desert but nothing is certain yet and the mystery remains''. Did these planets lose their atmosphere entirely, eroded by the intense radiation of their star? Did they migrate from their birthplace to the outer parts of the system by a different mechanism than other types of planets, preventing them from reaching the same close orbits?

Disrupted migration

In a recent work, a team of scientists from the UNIGE brings some answers by looking at the orbital architecture of the planets located at the edge of this desert. By surveying fourteen planets around this area, ranging from small planets to gas giants, the astronomers were interested in the way their orbits are oriented with respect to the axis of rotation of their star. This information makes it possible to distinguish the processes of soft migration (the planets move in the equatorial plane of their star where they were formed) from the processes of disruptive migration (the planets migrate and are pushed out of the plane where they were formed).

The researchers were able to show that most of the planets in their sample have an orbit misaligned with the stellar equator. ''We found that three-quarters of these planets have a polar orbit (they rotate above the poles of their star), which is a larger fraction than for planets further away from the desert. This reflects the role of disruptive migration processes in the formation of the desert,'' summarizes Vincent Bourrier, first author.

Two methods combined

To achieve these results, the scientists used the radial velocity method and the transit method, which are employed to study exoplanets. ''Analyzing the radial velocities during the transit of a planet allows us to determine if it orbits around the stellar equator, around the poles, or if the system is in an intermediate configuration, because different architectures will produce different signatures,'' explains Omar Attia, a doctoral student in the Department of Astronomy at the UNIGE Faculty of Science and second author of the study. These two methods were combined with data obtained with the HARPS and HARPS-North spectrographs, created at UNIGE and located on the 3.6m telescope of ESO (European Southern Observatory) and TNG (Telescopio Nazionale Galileo).

The path to understand all of the mechanisms involved in the formation of the Hot Neptune Desert is still long. It will be necessary in particular to explore with this technique the smallest planets at the edge of the desert, today difficult to access even with instruments of last generation such as the spectrograph ESPRESSO, built by the UNIGE and installed on the largest European telescopes. It will be necessary to wait for the commissioning of the ELT, the 39-meter super telescope of ESO, planned for 2027.

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Deserts 'breathe' water vapor, study shows

Deserts may seem lifeless and inert, but they are very much alive. Sand dunes, in particular, grow and move -- and according to a decades long research project, they also breathe humid air.

The findings show for the first time how water vapor penetrates powders and grains, and could have wide-ranging applications far beyond the desert -- in pharmaceutical research, agriculture and food processing, as well as planetary exploration.

The team's paper published in the Journal of Geophysical Research-Earth Surface.

Wanting to measure matter with greater sensitivity, lead author Michel Louge, professor of mechanical and aerospace engineering at Cornell University, developed a new form of instrumentation called capacitance probes, which use multiple sensors to record everything from solid concentration to velocity to water content, all with unprecedented spatial resolution.

In the early 2000s, Louge began collaborating with Ahmed Ould el-Moctar from University of Nantes, France, to use the probes to study the moisture content in sand dunes to better understand the process by which agricultural lands turn to desert -- an interest that has only become more urgent with the rise of global climate change.

The probe eventually revealed just how porous sand is, with a tiny amount of air seeping through it. Previous research hinted this type of seepage existed in sand dunes, but no one had been able to prove it until now.

"The wind flows over the dune and as a result creates imbalances in the local pressure, which literally forces air to go into the sand and out of the sand. So, the sand is breathing, like an organism breathes," Louge said.

That "breathing" is what allows microbes to persist deep inside hyper-arid sand dunes, despite the high temperature. For much of the last decade, Louge has been collaborating with Anthony Hay, associate professor of microbiology at Cornell, to study how microbes can help stabilize the dunes and prevent them from encroaching into roads and infrastructure.

Louge and his team also determined that desert surfaces exchange less moisture with the atmosphere than expected, and that water evaporation from individual sand grains behaves like a slow chemical reaction.

The bulk of their data was gathered in 2011, but it still took Louge and his collaborators another decade to make sense of some of the findings, such as identifying disturbances at the surface level that force evanescent, or nonlinear, waves of humidity to propagate downward through the dunes very quickly.

The researchers anticipate their probe will have a number of applications -- from studying the way soils imbibe or drain water in agriculture, to calibrating satellite observations over deserts, to exploring extraterrestrial environments that may hold trace amounts of water. That wouldn't be the first time Louge's research made its way into space.

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Researcher unearths an ice age in the African desert

Drumlins, hills formed in places once covered by glaciers, were discovered in Namibia by WVU's Graham Andrews.

A field trip to Namibia to study volcanic rocks led to an unexpected discovery by West Virginia University geologists Graham Andrews and Sarah Brown.

While exploring the desert country in southern Africa, they stumbled upon a peculiar land formation -- flat desert scattered with hundreds of long, steep hills. They quickly realized the bumpy landscape was shaped by drumlins, a type of hill often found in places once covered in glaciers, an abnormal characteristic for desert landscapes.

"We quickly realized what we were looking at because we both grew up in areas of the world that had been under glaciers, me in Northern Ireland and Sarah in northern Illinois," said Andrews, an assistant professor of geology. "It's not like anything we see in West Virginia where we're used to flat areas and then gorges and steep-sided valleys down into hollows."

After returning home from the trip, Andrews began researching the origins of the Namibian drumlins, only to learn they had never been studied.

"The last rocks we were shown on the trip are from a time period when southern Africa was covered by ice," Andrews said. "People obviously knew that part of the world had been covered in ice at one time, but no one had ever mentioned anything about how the drumlins formed or that they were even there at all."

Andrews teamed up with WVU geology senior Andy McGrady to use morphometrics, or measurements of shapes, to determine if the drumlins showed any patterns that would reflect regular behaviors as the ice carved them.

While normal glaciers have sequential patterns of growing and melting, they do not move much, Andrews explained. However, they determined that the drumlins featured large grooves, which showed that the ice had to be moving at a fast pace to carve the grooves.

These grooves demonstrated the first evidence of an ice stream in southern Africa in the late Paleozoic Age, which occurred about 300 million years ago.

"The ice carved big, long grooves in the rock as it moved," Andrews said. "It wasn't just that there was ice there, but there was an ice stream. It was an area where the ice was really moving fast."

McGrady used freely available information from Google Earth and Google Maps to measure their length, width and height.

"This work is very important because not much has been published on these glacial features in Namibia," said McGrady, a senior geology student from Hamlin. "It's interesting to think that this was pioneer work in a sense, that this is one of the first papers to cover the characteristics of these features and gives some insight into how they were formed."

Their findings also confirm that southern Africa was located over the South Pole during this period.

"These features provide yet another tie between southern Africa and south America to show they were once joined," Andrews said.

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