Increase In Nearest-neighbor Distance Research Articles

The impact of stress on social behavior in adult zebrafish (Danio rerio).

Stress has adverse effects on social behavior that is mediated by dopamine circuits in the midbrain. The purpose of this research is to examine the effect of chronic stress and dopamine signals on social behavior in zebrafish (Danio rerio). Chronic stress was induced chemically with low dosage of ethanol (0.25% for 5 days), and psychosocially with isolation (3-5 days) or overcrowding (5 days). Dopamine activity was decreased by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) exposure. Social behavior was observed by introducing one treated zebrafish to a group of four control zebrafish and measuring the nearest neighbor distance (NND). Swimming ability was analyzed by measuring total swim distance and average velocity. Analysis of swim ability showed that treatment had no adverse effect upon locomotor functioning. However, stress and MPTP affected social behavior similarly. In all stress conditions, there was a significant increase in NND (7.4±3.9-9.1±4.4 cm). MPTP also caused an increase in NND (8.9±2.7 cm), but MPTP/isolation treatment did not amplify the effect (8.9±5.5 cm). One possible explanation is that chronic stress causes a change in dopamine activity and decreases social behavior, providing insight into the function of dopamine in social behavior.

Vision in two cyprinid fish: implications for collective behavior.

Many species of fish rely on their visual systems to interact with conspecifics and these interactions can lead to collective behavior. Individual-based models have been used to predict collective interactions; however, these models generally make simplistic assumptions about the sensory systems that are applied without proper empirical testing to different species. This could limit our ability to predict (and test empirically) collective behavior in species with very different sensory requirements. In this study, we characterized components of the visual system in two species of cyprinid fish known to engage in visually dependent collective interactions (zebrafish Danio rerio and golden shiner Notemigonus crysoleucas) and derived quantitative predictions about the positioning of individuals within schools. We found that both species had relatively narrow binocular and blind fields and wide visual coverage. However, golden shiners had more visual coverage in the vertical plane (binocular field extending behind the head) and higher visual acuity than zebrafish. The centers of acute vision (areae) of both species projected in the fronto-dorsal region of the visual field, but those of the zebrafish projected more dorsally than those of the golden shiner. Based on this visual sensory information, we predicted that: (a) predator detection time could be increased by >1,000% in zebrafish and >100% in golden shiners with an increase in nearest neighbor distance, (b) zebrafish schools would have a higher roughness value (surface area/volume ratio) than those of golden shiners, (c) and that nearest neighbor distance would vary from 8 to 20 cm to visually resolve conspecific striping patterns in both species. Overall, considering between-species differences in the sensory system of species exhibiting collective behavior could change the predictions about the positioning of individuals in the group as well as the shape of the school, which can have implications for group cohesion. We suggest that more effort should be invested in assessing the role of the sensory system in shaping local interactions driving collective behavior.

Microhabitat availability and seedling recruitment ofLobelia urens: a rare plant species at its geographical limit

AbstractIn Britain,Lobelia urens(L.) (the heath lobelia) occurs in rough grassland, is rare and only found in southern England, where it is at the northern limit of its range. Emergence and survival ofL. urenswas investigated at six locations in two geographically distinct sites experiencing spring, autumn or no grazing in two consecutive years. Four factors were evaluated qualitatively, as a means of characterizing microhabitats for germination and survival: all permutations of higher plant cover, bryophytes, plant litter and surface depressions. The potential effect of adjacent plants on recruitment was also assessed using the nearest neighbour distance (NND). Grazing created depressions, removed plant litter and increased the proportion of sites with higher plant cover. It also resulted in a more open sward with higher NNDs. None of these changes stimulated recruitment. Instead, small increases in the frequency of some rare or very rare microhabitat types were vital in making grazed rough grassland more suitable for emergence. Reduced litter loads and a greater quantity of moss were two key responses. Overall, survival ofL. urenswas less than 1% and was particularly favoured by moss and an increase in NNDs. Shading from higher plants, with or without plant litter, decreased emergence, but the precise role of litter was complex and most probably related to its quantity. In an experimental seed bed, only empty depressions favoured emergence. The microhabitat relationships ofL. urenswere unusually consistent among locations and consecutive years.L. urensrequires high soil surface temperatures but also adequate water for large scale recruitment, and such conditions are encouraged by grazing. The particular problems of experimental design and statistical analysis of data from recruitment experiments are also discussed.

Comparing Peak Asymmetries in EXAFS using RMC and Cumulant Expansion Data Analysis

EXAFS measurements on RbBr at the Br absorption edge and different temperatures were carried out. Conventional analysis of the data using the Daresbury EXCURV92 code shows with increasing temperature a decrease in nearest neighbour distance. Also large differences are found in the coordination numbers for k and k' weighted EXAFS with increasing temperature. Using cumulant expansion when fitting the data results in a small increase in nearest neighbour distance with temperature, as expected from thermal data.