Many sessile colonial organisms intensively compete with conspecifics for growing space. This competition can result in either cooperative fusion or aggressive rejection between colonies, and some species have evolved highly polymorphic genetic systems that mediate the outcome of these interactions. Here we demonstrate the potential for interactions among close kin as the basis for the evolutionary maintenance of a genetically polymorphic allorecognition system in the colonial hydroid Hydractinia symbiolongicarpus, which lives on gastropod shells occupied by hermit crabs. Fusion between hydroids in the laboratory is restricted mainly to encounters between full siblings, whereas other encounters result in aggressive rejection. Natural selection acting on the costs or benefits of fusion between colonies could be responsible for the present maintenance of such a highly specific behavioral response, but only if encounters between fusible colonies still occur in contemporary populations. The large size of these hydroid populations and the mobility of the crabs should limit the potential for interactions among closely related hydroids on the same shell. However, RAPD polymorphisms among a large sample of hydroids from a population off the coast of Massachusetts indicate that genetically similar colonies are often found together on the same shell. Some genetic distances between colonies on the same shell were low relative to genetic distances between colonies on different shells or genetic distances between known full siblings from laboratory matings. We conservatively estimate that 2-18% of co-occurring colonies may be full sibling pairs. These observations suggest that encounters between genetically similar hydroids are common, despite the mobile nature of their habitat, and these encounters may provide frequent opportunities for natural selection to influence the evolution of cooperative and agonistic behaviors and their polymorphic genetic basis.

We present several case studies from the western United States where faults are mapped on the basis of geomorphic and structural evidence that is equally likely to indicate landsliding. In some examples, faults have obscured evidence of landslides that utilized fault planes as rupture surfaces. In the Southern California examples, late Pleistocene or Holocene faults are mapped solely based on linear scarps. Such faults are often better explained by landsliding. Similarly, both landslides and faults have been proposed to explain prominent scarps and grabens in the Saddle Mountains of Washington. We note that both faulting and landsliding have been invoked by consultants and reviewers to explain offset Quaternary colluvium in observation pits and linear scarps in a subdivision in central Utah. Several subparallel linear scarps in granitic rock on a ridge top in the Southern California desert have also been mapped as faults. Recent studies, however, show that the features more likely indicate incipient landsliding that grades laterally into fully developed landslides. The Hebgen Lake, Montana, earthquake of 1959 produced landsliding as well as tectonic ground rupture. We suggest that an arcuate scarp that formed north of the primary ground rupture zone, previously interpreted as a fault, was likely produced by reactivation of a 6-mi-wide (9.7 km) landslide. We include a final case study where a combination of normal and thrust faulting mimics landsliding near St. George, Utah. Failure to correctly differentiate between landslides and faults leads to incorrect evaluation of a site's stability as well as incorrect evaluation of seismic hazard and ultimately impacts public health and safety.

Significance Arginine–vasopressin (AVP) acting on V1a receptors (Avpr1as) represents a key signaling mechanism in a brain circuit that increases the expression of social communication and aggression. We produced Syrian hamsters that completely lack Avpr1as ( Avpr1a knockout [KO] hamsters) using the CRISPR-Cas9 system to more fully examine the role of Avpr1a in the expression of social behaviors. We confirmed the absence of Avpr1as in these hamsters by demonstrating 1) a complete lack of Avpr1a-specific receptor binding throughout the brain, 2) a behavioral insensitivity to centrally administered AVP, and 3) an absence of the well-known blood-pressure response produced by activating Avpr1as. Unexpectedly, however, Avpr1a KO hamsters displayed more social communication behavior and aggression toward same-sex conspecifics than did their wild-type (WT) littermates.

Long-runout landslides are well-known and notorious geologic hazards in many mountainous parts of the world. Commonly encompassing enormous volumes of debris, these rapid mass movements place populations at risk through both direct impacts and indirect hazards, such as downstream flooding. Despite their evident risks, the mechanics of these large-scale landslides remain both enigmatic and controversial. In this work, we illuminate the inner workings of one exceptionally well-exposed and well-preserved long-runout landslide of late Pleistocene age located in Eureka Valley, east-central California, Death Valley National Park. The landslide originated in the detachment of more than 5 million m3 of Cambrian bedrock from a rugged northwest-facing outcrop in the northern Last Chance Range. Its relatively compact scale, well-preserved morphology, varied lithologic composition, and strategic dissection by erosional processes render it an exceptional laboratory for the study of the long-runout phenomenon in a dry environment. The landslide in Eureka Valley resembles, in miniature, morphologically similar “Blackhawk-like” landslides on Earth, Mars, and minor planet Ceres, including the well-known but much larger Blackhawk landslide of southern California. Like these other landslides, the landslide in Eureka Valley consists of a lobate, distally raised main lobe bounded by raised lateral levees. Like other terrestrial examples, it is principally composed of pervasively fractured, clast-supported breccia. Based on the geologic characteristics of the landslide and its inferred kinematics, a two-part emplacement mechanism is advanced: (1) a clast-breakage mechanism (cataclasis) active in the bedrock canyon areas and (2) sliding on a substrate of saturated sediments encountered and liquefied by the main lobe of the landslide as it exited the main source canyon. Mechanisms previously hypothesized to explain the high-speed runout and morphology of the landslide and its Blackhawk-like analogs are demonstrably inconsistent with the geology, geomorphology, and mineralogy of the subject deposit and its depositional environment.

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