American Scientist

In this roundup, managing editor Stacey Lutkoski summarizes notable recent developments in scientific research, selected from reports compiled in the free electronic newsletter Sigma Xi SmartBrief.

Scuba-Diving Reptiles

Some lizards can breathe underwater by forming an air bubble over their snouts. Evolutionary biologist Christopher K. Boccia of the University of Toronto led a team that investigated how members of the Anolis lizard genus—also called anoles—manage to stay submerged for several minutes at a time. The team captured 32 species of both semiaquatic and nonaquatic anoles in Costa Rica in order to observe how the animals behaved in water. By rebreathing air trapped in a bubble around their snouts, semiaquatic anoles were able to stay underwater for up to 18 minutes. The hydrophobic skin common among all Anolis species is the key to the bubble formation. When an anole is submerged, a thin film of air is trapped between the water and its skin. As the lizard exhales, that film traps the air in a bubble over its nose; when it inhales, the bubble deflates and the film retracts around its body. Some nonaquatic anoles can also form a bubble and rebreathe, though they cannot do so consistently. The replication of this rebreathing behavior across all of the semiaquatic and some nonaquatic Anolis species that the team studied is an example of macroevolutionary convergence among anoles.

Boccia, C. K., et al. Repeated evolution of underwater rebreathing in diving Anolis lizards. Current Biology 31:2947–2954.e4 (July 12).

Smoke-Filled Clouds Don’t Rain

Wildfires may hamper clouds’ ability to form precipitation, which leads to dry conditions that are more likely to trigger wildfires, thus creating a feedback loop. Atmospheric scientists studied cumulus clouds over the western United States during the 2018 wildfire season. (See “Artists as Archeologists of Wildfire,” pages 346–350.) They found that smoke-filled clouds could hold five times as many droplets as clouds without smoke, but that each droplet was about half the normal size. These small droplets were less likely to collide and accumulate into masses large enough to form rain. Similar results have been found in the Amazon, but these U.S. findings show that the phenomenon is not unique to that environment. Global climate change has resulted in an increased number of high-intensity wildfires worldwide, and this self-perpetuating cycle could be contributing to the severity of the problem.

Twohy, C. H., et al. Biomass burning smoke and its influence on clouds over the western U.S. Geophysical Research Letters doi:10.1029/2021GL094224 (July 26).

A Glimpse Inside the Red Planet

Data from seismic activity on Mars are giving researchers a peek into the inner workings of the planet. These findings are the first seismic mapping of the interior of any planet other than Earth and are an important step in understanding the formation of rocky planets. The NASA lander InSight (Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport) has been gathering data from “marsquakes” since 2018. In a trio of articles published in Science, a bevy of experts unpack the numbers to expose the planet’s composition. They looked at data from 35 quakes that ranged from magnitude 3.0 to magnitude 4.0 on the Richter scale, which are large by Martian standards but would barely be felt on Earth. Most of the marsquakes originated in the crust, but 10 subcrustal seismic events provide clues about the planet’s inner layers. The researchers determined that Mars’s core starts about 1,560 kilometers beneath the surface, or nearly halfway to the center. Given the planet’s known mass, this larger-than-expected core must also be less dense than previously thought. Between the core and the crust is a relatively thin mantle—about half as thick as that of Earth. The data indicate that the Martian core is liquid, though further research is required to determine whether there is a solid inner core similar to that of Earth. InSight’s success has resulted in a two-year extension of its mission.

Khan, A., et al. Upper mantle structure of Mars from InSight seismic data. Science doi:10.1126/science.abf2966 (July 23).
Knapmeyer-Endrun, B., et al. Thickness and structure of the martian crust from InSight seismic data. Science doi:10.1126/science.abf8966 (July 23).
Stähler, S. C., et al. Seismic detection of the martian core. Science doi:10.1126/science.abi7730 (July 23).

Dams Increase Risk of Malaria

Reservoirs formed by small dams are a breeding ground for mosquitoes, resulting in a significant number of malaria incidences. A team of public health and water management experts looked at malaria rates in the population of 14.7 million people living within 5 kilometers of dam reservoirs in four sub-Saharan river basins: the Limpopo, the Omo-Turkana, the Volta, and the Zambezi. The regions’ 258 large dams (those with a height of 15 meters or greater, or that hold more than 3 million cubic meters of water) represent major infrastructure projects, whereas the 4,907 small dams are often community constructions built without input from hydrologists or engineers. Those experts are trained to design dams that are less likely to attract mosquitoes by, for example, avoiding shallow slopes, which generally correspond to poor drainage. The researchers found that between 0.9 million and 1.7 million malaria cases per year in those four river basins could be attributed to living in close proximity to a dam, and between 77 percent and 85 percent of those incidences were due to small dams. People often live near small dams because they rely on the captured water to irrigate fields and water livestock. These findings can help public health and water management professionals focus their malaria mitigation efforts.

Kibret, S., M. McCartney, J. Lautze, L. Nhamo, and G. Yan. The impact of large and small dams on malaria transmission in four basins in Africa. Scientific Reports doi:10.1038/s41598-021-92924-3 (June 25).