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Predation plays an important role in ecological communities by affecting prey behavior such as foraging and by physical removal of individual prey. In regard to foraging, animals such as desert rodents often balance conflicting demands for food and safety. This has been studied in the field by indirectly manipulating predatory risk through the alteration of cues associated with increased risk such as cover or illumination. It has also been studied by directly manipulating the presence of predators in aviaries. Here, we report on experiments in which we directly manipulated actual predatory risk to desert rodents in the field. We conducted a series of experiments in the field using a trained barn owl (Tyto alba) to investigate how two species of coexisting gerbils (Gerbillus allenbyi and G. pyramidum) respond to various cues of predatory risk in their natural environment. The gerbils responded to risk of predation, in the form of owl flights and owl hunger calls, by reducing their activity in the risky plot relative to the control plot. The strongest response was to owl flights and the weakest to recorded hunger calls of owls. Furthermore, when risk of predation was relatively high, as in the case with barn owl flights, both gerbil species mostly limited their activity to the safer bush microhabitat. The response of the gerbils to risk of predation disappeared very quickly following removal of the treatment, and the gerbils returned to normal levels of activity within the same night. The gerbils did not respond to experimental cues (alarm clock), the presence of the investigators, the presence of a quiet owl, and recorded \"white noise\". Using trained barn owls, we were able to effectively manipulate actual risk of predation to gerbils in natural habitats and to quantify how gerbils alter their behavior in order to balance conflicting demands of food and safety. The method allows assessment of aspects of behavior, population interactions, and community characteristics involving predation in natural habitats.

We have tested two hypotheses which may explain two different patterns which underlie coexistence in two species of desert gerbils (Gerbillus allenbyi and G. pyramidum). The two patterns are temporal partitioning of foraging activity and shared preference habitat selection. When sympatric, G. pyramidum uses the early part of the night most heavily while G. allenbyi is most active in the late part of the night. Although both species prefer the same habitat type (semistabilized sand dune), in the presence of G. pyramidum, G. allenbyi utilizes mainly the secondary habitat type (stabilized sand dune). The hypotheses are 1) one species is dominant (G. pyramidum) while the other (G. allenbyi) is an energetically more efficient forager, and 2) one species has the ability to quickly harvest seeds in dense resource patches and the other species can harvest seeds profitable at lower resource densities. We tested the hypotheses in an experiment conducted in four enclosed plots. Two additional unfenced plots served as controls. Two of the enclosed plots contained mixed populations of both species while the other two fenced plots contained G. allenbyi only. In contrast to patterns of activity of G. allenbyi with G. pyramidum present, G. allenbyi is also very active early in the night even on the semistabilized dunes when G. pyramidum is absent. The results of the experiment supported the hypothesis that interference is the key factor to understanding the coexistence of the two species. Both the temporal and spatial patterns are the result of the dominant G. pyramidum species excluding the energetically efficient G. allenbyi from the preferred time of activity and habitat type.

Mechanistic approaches to understanding species coexistence and community structure have recently proved highly successful. We apply this approach to a sand dune community containing two common species of gerbils, Gerbillus allenbyi and G. pyramidum. Previous work suggested that coexistence is based on temporal variability in seed resource density on a daily time scale and a tradeoff of foraging efficiency at high versus low resource abundances. We tested the predictions of this mechanism by quantifying rodent foraging activity and its timing during the night over 6 nights using sand tracking plots, sand tracking strips, and seed trays. We demonstrated temporal partitioning with all three methods. First, we demonstrated that resource patches are depleted gradually throughout the night, a necessary assumption for temporal partition to allow coexistence. Also, the first forage in a resource patch and the average forage for G. pyramidum is significantly earlier in the night than for G. allenbyi, and the last forage for G. allenbyi is significantly later than for G. pyramidum. Species differ in how the intensity of patch use changes during the night. G. pyramidum visits more patches earlier in the night than does G. allenbyi and its intensity of patch use based on sand tracking scores is also greater earlier in the night. Finally, the average resource patch (seed tray) visited by G. pyramidum is richer than that visited by G. allenbyi. These data demonstrate that G. pyramidum is profiting most from patches when resource densities are high, and it biases its foraging towards those times. We also demonstrated that resource patch giving-up densities for G. allenbyi are lower than for G. pyramidum, indicating that G. allenbyi is a more efficient forager at low resource abundances; it biases its foraging towards these times. The result is temporal partitioning. We note three tradeoffs which may underlie this mechanism of coexistence and discuss evidence for each.

We used a behavioral bioassay, in the form of foraging behavior of Gerbillus allenbyi, and the ideal free distribution to estimate the costs associated with risk of predation. Experiments were conducted in two pairs of 2-ha field enclosures. Risk of predation was introduced to one 1-ha subplot of each pair of enclosures either by simulating the light of a full moon or by causing trained owls to fly over our experimental subplots. We used a titration method to estimate the energetic costs associated with risk of predation. We added seeds to the 1-ha subplot of each enclosure that experienced the risk of predation. The adjacent subplot served as a control. Without seed addition the subplot without risk of predation had more foraging activity: the foragers were avoiding the risk of predation. As the amount of seeds in the treatment increased, the gerbils responded smoothly and quantitatively and shifted their foraging activity back to the subplot with the seeds (and risk). At addition rates of 4.24-8.47 g seeds per individual per ha per night (simulated-moonlight treatment) and 4.24-5.64 g seeds per individual per ha per night (owl-flights treatment) the seed addition compensated for the extra risk. At these rates the foraging activity on the two subplots of each enclosure was the same. Our results suggested that, when challenged by the threat of predation, G. allenbyi individuals spent at least 25% of their foraging time being vigilant.

Oliceridine (TRV130), a novel μ-receptor G-protein pathway selective (μ-GPS) modulator, was designed to improve the therapeutic window of conventional opioids by activating G-protein signaling while causing low β-arrestin recruitment to the μ receptor. This randomized, double-blind, patient-controlled analgesia Phase IIb study was conducted to investigate the efficacy, safety, and tolerability of oliceridine compared with morphine and placebo in patients with moderate to severe pain following abdominoplasty (NCT02335294; oliceridine is an investigational agent not yet approved by the US Food and Drug Administration). Patients were randomized to receive postoperative regimens of intravenous oliceridine (loading/patient-controlled demand doses [mg/mg]: 1.5/0.10 [regimen A]; 1.5/0.35 [regimen B]), morphine (4.0/1.0), or placebo with treatment initiated within 4 hours of surgery and continued as needed for 24 hours. Two hundred patients were treated (n=39, n=39, n=83, and n=39 in the oliceridine regimen A, oliceridine regimen B, morphine, and placebo groups, respectively). Patients were predominantly female (n=198 [99%]) and had a mean age of 38.2 years, weight of 71.2 kg, and baseline pain score of 7.7 (on 11-point numeric pain rating scale). Patients receiving the oliceridine regimens had reductions in average pain scores (model-based change in time-weighted average versus placebo over 24 hours) of 2.3 and 2.1 points, respectively ( =0.0001 and =0.0005 versus placebo); patients receiving morphine had a similar reduction (2.1 points; <0.0001 versus placebo). A lower prevalence of adverse events (AEs) related to nausea, vomiting, and respiratory function was observed with the oliceridine regimens than with morphine ( <0.05). Other AEs with oliceridine were generally dose-related and similar in nature to those observed with conventional opioids; no serious AEs were reported with oliceridine. These results suggest that oliceridine may provide effective, rapid analgesia in patients with moderate to severe postoperative pain, with an acceptable safety/tolerability profile and potentially wider therapeutic window than morphine.

We investigated the effects of increasing rodent (Acomys cahirinus and Gerbillus dasyurus) predation efficiency on their population density and the population of their prey, the desert snail (Trochoidae seetzenii). The study was carried out on a rocky hillslope in the Central Negev Desert, Israel. Rodent predation on snails is limited by the number of shelters for rodents in the area. In most natural conditions, more shelters are found on the upper than on the lower slope. We introduced artificial shelters along the slope. We then censused the number of rodents, snails, and of eaten snails for five years. In the presence of artificial shelters the predation efficiency of rodents (number of snail eaten per rodent) increased. The number of rodents also increased and the number of snails decreased on the experimental plots relative to control plots. We suggest that the persistence of the snail-rodent system is due to spatial variation in the relative importance of direct prey-predator relations and the indirect effects of snail immigration to the slope and presence of protective shelters for the rodents. Our study supports recent work indicating that in a prey-predator system, in addition to direct predation, the heterogeneity of the environment and predation risk while foraging, affect both the behavior and the density of the predator and the prey.

1. It has been shown that the two common granivorous gerbil species Gerbillus allenbyi and G. pyramidum that coexist in the sand dunes of the Israeli Negev show temporal partitioning in their time of activity. The bigger species, G. pyramidum, aggressively displaces the smaller species from early night-time. We examined the change in the activity pattern of G. allenbyi in pure and mixed populations in two 1-ha field enclosures. 2. We confirmed the temporal pattern reported by Ziv et al. (1993) and Kotler et al. (1993). 3. We also measured how much energy it takes (in g of millet seeds) to compensate for the costs, associated with interference and exploitation competition, by adding millet seeds to 18 seed trays/enclosure. We added 1, 3, 5, 7 or 9 g of seeds/seed-tray. In each seed-tray we mixed the seeds in 2 L of sand. 4. It took 3-5 g of seeds/seed-tray (1.8-3 g seeds/day/ha/individual) to completely overcome the competition by G. pyramidum. This is the cost of the interference and exploitation competition from G. pyramidum (in the currency of millet seeds). 5. The result suggests that there is a trade-off between interference competition and food to which gerbils respond behaviourally.

We used trained barn owls to introduce a controlled predation threat to two species of gerbil, Gerbillus allenbyi and G. pyramidum in a system of 2-ha, sandy-substrate field enclosures in the Negev Desert, Israel. Using the principles of optimal density-dependent habitat selection, we estimated several coefficients of population interaction focusing on G. allenbyi. G. allenbyi exhibits strong intraspecific competition. In the absence of owls, G. pyramidum competes with it (α = - 0.35). We estimated the slope of the G. allenbyi victim isocline to be - 0.60. The competitive effect of G. pyramidum disappeared in the presence of owl, although the intraspecific competition remained. Our results indicated that in the presence of owls, the threat of predation overwhelms the cost of interspecific competition.

Since 1963, nonlinear predation theory has predicted that, at low population densities, victim species may well be mutualistic rather than competitive. Theory identifies this mutualism as a principal source of dynamic instability in the interaction. Using gerbils and trained barn owls, we conducted the first (to our knowledge) field tests of the theory's prediction of mutualism. The behavior of the gerbils confirms its existence.

We trained barn owls to fly over 2-ha field enclosures containing populations of Gerbillus allenbyi. Each 2-ha plot was divided into two equal parts by a fence with gates allowing easy passage of the gerbils. We varied the number of gerbils in the enclosure and the number of owl flights on each side of the dividing fence. Gerbil foraging activity responded within two hours to the difference in number of owl flights over the subplots. The greater the difference, the more the gerbils shifted their foraging to the subplot with fewer flights. Gerbils did not reduce their total foraging effort but redistributed it between subplots. In the absence of owl flights, gerbils divided their foraging effort equally between the matched subplots. Thus, they appear to seek an ideal free distribution in the two halves of each plot. Therefore, based on the gerbils' distribution of activity in the presence of different numbers of owl flights over the two subplots we estimated the shapes and slopes of the gerbil victim isoclines in the middle and right hand side of the 'gerbil-activity vs number of owl flights' state space. The isoclines were parallel straight lines with slope equal to -0.57. We believe this to be the first estimate of a victim isocline for a population of vertebrates in the field. Using the isodar method of Morris, we also determined that the fitness cost of each additional gerbil, i.e., per capita intraspecific competition among the G. allenbyi, is constant. It does not depend on either owl flight frequencies or G. allenbyi densities. The isodar also shows that the fitness cost of added owl flights does not vary with G. allenbyi population densities.