Humans likely triggered prehistoric bird extinctions
Flightless Takahe, New Zealand.
Several species have vanished in the years since 10,000 BC, an age known as the Holocene. The largest known extinction event occurred on remote Pacific islands between 3,500 to 700 years ago, when human arrival and associated overhunting and deforestation led to the extinction of thousands of bird populations. Richard Duncan et al. (pp. 6436–6441) explore the magnitude and pattern of those extinctions, the details of which have remained elusive due to gaps in the Late Quaternary fossil bone record. Using a modeling approach that incorporates all the available information and addresses uncertainties in the fossil record, the authors quantified the loss of nonpasserine landbirds on 41 remote Pacific islands—the last habitable region of the Earth to be colonized by humans. The findings reveal that nearly two-thirds of the landbird populations on those islands vanished in the years between the arrival of the first humans and European colonization. Certain islands and bird species were particularly vulnerable to hunting and habitat destruction; these species suffered dramatic rates of extinction, the authors report. The model suggests that human colonization of the remote Pacific islands spawned the global extinction of nearly 1,000 species of nonpasserine landbirds, as well as the likely extinction of numerous nonpasserine seabirds and passerine birds. — A.G.
Factors responsible for Lake Erie’s record-breaking algal bloom
A record-breaking harmful algal bloom in western Lake Erie in 2011. Image courtesy of Todd Crail.
In the summer of 2011, Lake Erie sustained the largest harmful algal bloom in its recorded history. Anna Michalak et al. (pp. 6448–6452) explored the role of several factors that may have contributed to the bloom, including land use, agricultural practices, precipitation, temperature, wind, lake circulation, and surface runoff, and analyzed the likelihood of future blooms in the lake. The authors report that the total amount of agricultural land and crop choices in the Lake Erie watershed have changed slightly since 2008 and do not explain the 2011 bloom. By contrast, farming practices, such as fertilizer use and tillage, have changed during the last decade in ways that are conducive to increased runoff of nutrients into the lake. Furthermore, a series of intense storms and runoff events during the spring of 2011 resulted in record-breaking levels of phosphorus—a nutrient that fuels algal growth—in the lake during the late spring. After the bloom began to form, an extended period of weak circulation and warm weather further promoted its growth. The authors found that these factors are likely to continue to occur in the future due to evolving land management practices and climate change, thus increasing the chances of future blooms. — N.Z.
The combinatorial power of grammar
Nim Chimpsky. Image courtesy of Herb Terrace, Nim Project Director.
The language of young children is thought to arise from repeated imitation, similar to the signing patterns of primates. In contrast, Charles Yang (pp. 6324–6327) proposed that young children learn language through creative variations in grammar. The author used a statistical test to identify grammatical rules to study nine speech samples from toddlers learning to speak two-word phrases in American English, as well as a collection of transcribed speech samples known as the Brown corpus. The test draws on Zipf’s law, which suggests that most language is derived from a few common linguistic units. Because many nouns appear only once, the law states, these nouns can only be used with a single determiner: a word like “the” or “a” that provides context for the noun. For each speech sample, the author used the test to predict how often a single noun was expected to partner with either of two determiners. The author also devised a memory model and applied it to a public collection of 1.1 million utterances to determine whether children learn by parroting phrases from their caregivers’ speech. The findings reveal that the grammar of children between the ages of 18 months and 4 years combines linguistic units in creative ways rather than relying on the memorization of specific phrases. The author further applied the rule to the American Sign Language used by the well-known chimpanzee Nim Chimpsky, and determined that the chimp developed language through rote imitation rather than the grammar used by children. According to the author, the findings may provide a quantitative benchmark to assess children’s language development. — A.G.
Water structure at high temperature and pressure
Image sequence showing changes to the molecular structure of water with increasing temperatures and pressures.
Researchers have long tried to fully describe the dynamic microscopic structure of liquid water. Spectroscopic analyses can provide insights into the structure of a substance by revealing how its atoms are configured at bonding sites, but these techniques are not well suited to studying water under so-called supercritical conditions. Christoph Sahle et al. (pp. 6301–6306) report details about the microscopic structure of water at subcritical and supercritical pressures and temperatures, based on X-ray Raman spectroscopy (XRS), computer simulations of molecular dynamics, and a quantum mechanical model known as density functional theory. The authors identified XRS spectra for water that evolve systemically from liquid-like at ambient conditions to gas-like at the highest temperatures and pressures tested. In addition, the authors developed a structural model to interpret the experimental spectra, obtained from computer simulations of molecular dynamics. Finally, the authors found that the microscopic structure of water remains homogeneous within the parameters of the study and that the average number of hydrogen bonds per molecule drops unexpectedly to 0.6 at supercritical conditions. The findings demonstrate a method to perform theoretical investigations of unknown atomic structures, according to the authors. — T.J.
Unraveling the gecko’s grip
Tokay gecko (Gekko gecko) clinging to a leaf.
Geckos possess the ability to scale smooth surfaces. One feature of gecko adhesion continues to puzzle researchers: Despite the fact that many of these lizards live in the tropics, geckos cling poorly to wet glass. To better understand how geckos function in their natural environment, Alyssa Stark et al. (pp. 6340–6345) measured geckos’ adherence to various surfaces under wet or dry conditions. The authors fitted harnesses to six geckos. As a gecko moved across a surface, a force was applied in the opposite direction until the animal slipped, allowing measurement of the adhesive strength of an individual gecko. The authors found that the effect of wetting on adhesive strength was a function of surface hydrophobicity, conforming to a thermodynamic model of adhesion. On wet hydrophobic surfaces, similar to the waxy leaves of tropical plants, regions of contact with the gecko’s toes remained dry, and overall adhesion was not affected by the presence of water. Adhesion of geckos to PTFE (Teflon) did not conform to the model, indicating that additional factors, such as friction and surface roughness, play a role in gecko adhesion. According to the authors, the study might lead to the design of a synthetic gecko mimic that retains adhesion when wet. — C.B.