The proximity of visual landmarks impacts reaching performance

The control of goal-directed reaching movements is thought to rely upon egocentric visual information derived from the visuomotor networks of the dorsal visual pathway. However, recent research (Krigolson and Heath, 2004) suggests it is also possible to make allocentric comparisons between a visual background and a target object to facilitate reaching accuracy. Here we sought to determine if the effectiveness of these allocentric comparisons is reduced as distance between a visual background and a target object increases. To accomplish this, participants completed memory-guided reaching movements to targets presented in an otherwise empty visual background or positioned within a proximal, medial, or distal visual background. Our results indicated that the availability of a proximal or medial visual background reduced endpoint variability relative to reaches made without a visual background. Interestingly, we found that endpoint variability was not reduced when participants reached to targets framed within a distal visual background. Such findings suggest that allocentric visual information is used to facilitate reaching performance; however, the fidelity by which such cues are used appears linked to the proximity of veridical target location. Importantly, these data also suggest that information from both the dorsal and ventral visual streams can be integrated to facilitate the online control of reaching movements.     

The use of time and space by male and female gerbils exploiting a pulsed resource

Foraging theory postulates that interference is a foraging cost and affects patch exploitation and activity times. One such system contains two species of seed-eating gerbils inhabiting sandy habitats in the Negev Desert of Israel. Low population densities of the dominant species allowed us to examine the interaction between males and females of the subordinate species, Gerbillus andersoni allenbyi , as a function of interference and resource renewal. We used giving-up densities (GUDs; the amount of food left in a resource patch when a forager abandons the patch) in seed trays to quantify patch use by gerbils. By placing 6 trays at each foraging station and either presenting all 6 trays at the start of the night (pulse treatment) or presenting one tray at a station 6 times per night (renewal treatment), we were able to manipulate characteristics of resource renewal. We used radio telemetry to obtain an independent assessment of activity. Male and female G. a. allenbyi differed in their timing of activity, with males beginning earlier than females and remaining active later. This was most pronounced for the pulse treatment. For the renewal treatment, female activity in trays was more intense early in the night, but thereafter male activity was more intense. At the same time, telemetry showed that males and females did not differ in their total activity in or out of trays. This suggests that males begin their activity on the renewal treatment by exploiting the richest natural patches of seeds. Only later when these are depleted do they move to dominate the renewing seed trays. Finally, females exploited stabilized sand habitats more than did males, especially during the renewal treatment. Taken together, these findings suggest that male G. a. allenbyi interfere with foraging in females, causing temporal shifts in their use of space and resources.     

Hyperlipidemic effects of trans fatty acids are accentuated by dietary cholesterol in gerbils

Trans isomers of dietary fatty acids, generated during the commercial hydrogenation of unsaturated fats, may contribute to coronary heart disease (CHD) in humans by interfering with lipid metabolism. To examine this possibility in a fat-sensitive model, the Mongolian gerbil (Meriones unguiculatus) was used to compare the cholesterolemic and triglyceridemic potential of modest increments of trans fatty acids from partially hydrogenated soybean oil with other saturated fatty acids in the presence and absence of dietary cholesterol. Age-, dose-, and time-dependent effects were examined in weanling, 6-month-old, and 1-year-old gerbils. Although lipoprotein metabolism in weanling gerbils was initially refractory to trans fat, even as perturbations by saturated fatty acids were demonstrable, these gerbils eventually (after 16 weeks) developed a trans-induced hypercholesterolemia that was intermediate between the response to 16:0 and 12:0 + 14:0. The hepatic and plasma 18:1/18:2 cholesteryl ester (CE) ratio was depressed by trans in a manner similar to saturated fatty acids. The 6-month-old gerbils readily developed hypertriglyceridemia but not hypercholesterolemia, again revealing a decrease in the plasma 18:1/18:2 CE ratio. The 1-year-old gerbils revealed a dose-related (0, 5, 10%en as trans) elevation in total cholesterol (TC), and especially triglycerides (TG), that was accentuated by 0.04% dietary cholesterol. Increases in plasma lipids were again accompanied by a significant decrease in the mass of hepatic esterified cholesterol, particularly 18:1-cholesteryl esters. Thus, dietary trans-fatty acids induce age-, time-, and dose-dependent modulations in gerbil plasma lipids associated with decreased 18:1 cholesteryl esters. Further investigation with gerbils may reveal mechanisms by which trans fat consumption disturbs lipoprotein metabolism.     

The evolution of self-incompatibility when mates are limiting

Self-incompatibility (SI) is a genetic barrier to inbreeding that is broadly distributed in angiosperms. In finite populations of SI plants, the loss of S-allele diversity can limit plant reproduction by reducing the availability of compatible mates. Many studies have shown that small or fragmented plant populations suffer from mate limitation. The advent of molecular typing of S-alleles in many species has paved the way to address quantitatively the importance of mate limitation, and to provide greater insight into why and how SI systems breakdown frequently in nature. In this review, we highlight the ecological factors that contribute to mate limitation in SI taxa, discuss their consequences for the evolution and functioning of SI, and propose new empirical research directions.     

The use of visual cues in host evaluation by aphidiid wasps

We examined host evaluation behaviour in three species of aphid parasitoids, Ephedrus californicus Baker, Monoctonus paulensis (Ashmead), and Praon pequodorum Viereck (Hymenoptera: Aphidiidae). Mated females were provided with pairwise choices among three kinds of hosts in the laboratory: (green) pea aphid, Acyrthosiphon pisum (Harris), and a green and a pink colour morph of alfalfa aphid, Macrosiphum creelii Davis. Patterns of attack and host acceptance were species-specific. Females of E. californicus did not respond to the presence of aphids prior to making antennal contact. Variations in rates of parasitization (pea aphid>green alfalfa aphid>pink alfalfa aphid) were consistent with differences in aphid defensive behaviours; no ‘preference’ for any host type was evident when aphids were anaesthetized with carbon dioxide. In M. paulensis, the order of preference (pea aphid>green alfalfa aphid>pink alfalfa aphid) did not vary when aphids were immobilized, or presented in the dark, or both. Host movement did not influence the rate of attack by M. paulensis. In contrast, the ranked order of preference in P. pequodorum varied with circumstance. In the light, females attacked pea aphid and green alfalfa aphid with equal frequency, but parasitized significantly more of the former; both kinds of aphids were attacked and parasitized at higher rates than pink alfalfa aphid. In the dark, P. pequodorum females parasitized green and pink alfalfa aphids equally and at higher rates than pea aphids. Whereas E. californicus was more successful ovipositing in immobilized hosts, P. pequodorum females attacked and laid more eggs in normal than anaesthetized aphids. Patterns of host recognition and evaluation are compared across six species representing four genera in the family Aphidiidae.     

The rat liver mitochondrial DNA-protein complex: displaced single strands of replicative intermediates are protein coated

Mitochondrial DNA (mtDNA)-protein complexes were released from the organelles by sodium dodecyl sulfate-lysis and purified by Phenyl-Sepharose CL-4B chromatography. The mitochondrial DNA-binding protein P16 was the only detectable protein in the complex. Treatment of the complex with proteinase K, or subtilisin, revealed the presence of a protease-insensitive, submolecular domain (Mr approximately equal to 6,000) that retained the capacity to bind tenaciously to the DNA. Analysis of chemically fixed complexes by CsCl isopycnic gradient centrifugation showed that P16 was bound to a large subpopulation of mtDNA enriched in displacement loops (D-loops). Based upon the effective buoyant density of the complex in CsCl gradients and the molecular weights of P16 and mtDNA, it was estimated that a mean of 49 P16 molecules were bound per mtDNA. For this measurement, the variation in hydration of protein and DNA at different CsCl concentrations was ignored. Analysis of restriction endonuclease-digested complexes by glass fiber filters that bind only protein-associated DNA resulted in the retention of a single fragment regardless of the enzyme, or enzymes, used. In each case, the retained fragment was the D-loop-containing fragment. With direct electron microscopy, the protein was readily visualized on the displaced single strand portions of D-loops and expanding D-loops. The nucleoprotein fibers were approximately 12 nm in diameter without correcting for the thickness of tungsten coating and roughly 1/3 the length of the double strand segment of the corresponding D-loop structure. In addition, occasional molecules with the characteristics of gapped circles were seen exhibiting a nucleoprotein fibril, presumably containing the single strand gap segment, linking the ends of double strand DNA. P16 was not seen on the double strand portions in any of the complexes.     

Wind drift,compensation, and the use of landmarks by nocturnal bird migrants

In this paper we describe fall nocturnal migration at three localities in eastern New York, one adjacent to the Hudson River, the other two 30 km to the west in a topographically more uniform area. Migrants at both study areas moved southwest in winds not out of the west and were, therefore, seemingly unaffected by the river. In west winds, however, birds away from the river moved south-southeast whereas those in the vicinity of the river flew a track west of south paralleling the river. In addition, a relative increase in the number of migrants along the river compared to away was observed in west winds as birds presumably became concentrated near the river. We conclude that on most autumn nights migrants passing through this area have a preferred track direction toward the southwest and in strong winds from the west and northwest they are drifted. Upon reaching the vicinity of the Hudson River, some birds alter their headings yielding a track direction that closely parallels the river resulting in at least a partial compensation for wind drift. No alternative hypothesis is consistent with all the data.     

The Heritability of Difference Scores when Environments are Correlated

The statistical analysis of difference scores (contrasts) is a fundamental problem in all learning, feeding, and training experiments and tests, and in longitudinal studies of growth and development. Outgoing from the analogy between the mathematical models of classical psychological test theory and quantitative genetics, as well as between the parameters reliability and heritability of these models, the present paper derives the formulas of the heritability of difference scores in general cases where it is not assumed that environmental deviations on distinct tests and measurements are uncorrelated. Contrary to the assertion, made by FELDMAN and LEWONTIN , heritabilities in the broad sense can be used as ideal weighting factors in long-range personnel index selection. Longitudinal studies of twins and the cotwin method are powerful experimental designs to estimate heritabilities of differences.     

Navigation by green turtles: which strategy do displaced adults use to find Ascension Island?

Sea turtles are known to perform long-distance, oceanic migrations between disparate feeding areas and breeding sites, some of them located on isolated oceanic islands. These migrations demonstrate impressive navigational abilities, but the sensory mechanisms used are still largely unknown. Green turtles breeding at Ascension Island perform long oceanic migrations (>2200 km) between foraging areas along the Brazilian coast and the isolated island. By performing displacement experiments of female green turtles tracked by satellite telemetry in the waters around Ascension Island we investigated which strategies most probably are used by the turtles in locating the island. In the present paper we analysed the search trajectories in relation to alternative navigation strategies including the use of global geomagnetic cues, ocean currents, celestial cues and wind. The results suggest that the turtles did not use chemical information transported with ocean currents. Neither did the results indicate that the turtles use true bi-coordinate geomagnetic navigation nor did they use indirect navigation with respect to any of the available magnetic gradients (total field intensity, horizontal field intensity, vertical field intensity, inclination and declination) or celestial cues. The female green turtles successfully locating Ascension Island seemed to use a combination of searching followed by beaconing, since they searched for sensory contact with the island until they reached positions NW and N of the Island and from there presumably used cues transported by wind to locate the island during the final stages of the search.     

Career choices in the biosciences: what companies are looking for when they are filling a position

Holding a Ph.D. or other doctoral degree in the biological sciences used to allow for only one main career path. Faculty positions at either research-intensive universities or at predominantly teaching institutions defined the career path very well. The situation is considerably more complex now. Many individuals decide to pursue careers in for-profit organizations, either by personal preference, geographic needs, or simply as an expedient way to continue in biology in a meaningful way. This article arises from the need to define a better understanding of the nature of corporate job seeking for potential job applicants. The author draws upon his experience over a thirty-year scientific career in industry and academics. The distinctions he makes are helpful for job-seekers in this new environment.     

The Reliability of Tests of Hypotheses when Overdispersed Logistic-Linear Models are Fitted by Maximum Quasi-Likelihood

When overdispersed logistic-linear models are fitted by maximum quasi-likelihood hypotheses can be tested by comparing either the Wald statistic, or the quasi-likelihood score statistic, or the quasi likelihood-ratio statistic, with the approximating null X² distribution. This paper reports a simulation study of the reliability of these tests. Some factors affecting their relative reliabilities are identified. An extended quasi-likelihood ratio test is also considered.     

Giving-up densities of foraging gerbils: the effect of interspecific competition on patch use

We studied the effect of a dominant species, Gerbillus pyramidum (Egyptian sand gerbil), on the patch use of its subordinate competitor, G. andersoni allenbyi (Allenby's gerbil), to better understand interspecific competition between the two species. We used manipulated resource patches (seed trays) covered with cages with two adjustable species-specific gates (either opened or closed to the bigger-dominant species, but always opened to the subordinate one). We recorded species tracks around and on the seed trays and giving-up densities (GUDs) of seeds in the trays after each night of foraging. G. a. allenbyi depleted seed patches to a lower level whenever G. pyramidum was given the opportunity to forage on the seed trays (i.e., present on the grid). This result held regardless of whether G. pyramidum was actually present at a particular station. We suggest that competition from G. pyramidum occurs both directly by interference, in which G. a. allenbyi is forced to be active in the late part of the night, and indirectly by exploitation via resource depletion by G. pyramidum in the early part of the night. The results suggest that interspecific competition from G. pyramidum reduces seed availability and the richness of the environment for G. a. allenbyi enough to affect the marginal value of energy for G. a. allenbyi individuals and cause them to experience lower costs of predation and manifest lower GUDs.     

Reaching providers is not enough to increase IUD use: a factorial experiment of 'academic detailing' in Kenya

Although the IUD is an extremely effective and low-cost contraceptive method, its use has declined sharply in Kenya in the past 20 years. A study tested the effectiveness of an outreach intervention to family planning providers and community-based distribution (CBD) agents in promoting use of the IUD in western Kenya. Forty-five public health clinics were randomized to receive the intervention for providers only, for CBD agents only, for both providers and CBD agents, or no detailing at all. The intervention is based on pharmaceutical companies' "detailing" models and included education/motivation visits to providers and CBD programmes, as well as provision of educational and promotional materials. District health supervisors were given updates on contraceptives, including the IUD, and were trained in communication and message development prior to making their detailing visits. Detailing only modestly increased the provision of IUDs, and only when both providers and CBD agents were targeted. The two detailing visits do not appear sufficient to sustain the effect of the intervention or to address poor provider attitudes and lack of technical skills. The cost per 3.5 years of pregnancy protection was US$49.57 for the detailing intervention including the cost of the IUD, compared with US$15.19 for the commodity costs of the current standard of care--provision of the injectable contraceptive depot-medroxyprogesterone acetate (DMPA). The effectiveness of provider-based activities is amplified when concurrent demand creation activities are carried out. However, the cost of the detailing in comparison to the small number of IUDs inserted indicates that this intervention is not cost-effective.     

Reaching into thought: The minds of great apes

Edited by Anne E. Russon, Kim A. Bard, and Sue Taylor Parker. New York: Cambridge University Press. 1996. 464 pp. ISBN 0-521-47168-0. $84.95 (cloth). © 1998 Wiley-Liss, Inc.     

Age-dependent use of local and global landmarks during escape: experiments using Columbian ground squirrels

How animals utilize their space often changes during ontogeny, perhaps resulting from alternative use of orientation mechanisms. This study investigated whether landmark-based navigation mechanisms were age-dependent in Columbian ground squirrels (Spermophilus columbianus). In field tests, young (1-2 years old) and adult (3-6 years old) animals had to find an escape burrow when either local, global, or both types of landmarks were obstructed. The comparison of escape times between age groups revealed that adult squirrels found escape burrow faster than young animals if global landmarks were available. However, if only local landmarks were present, young squirrels outperformed older animals. The comparison of escape time within each age group showed that obstruction of global or local landmarks lengthened escape time of adult squirrels. In contrast, young animals had longer escapes only when local landmarks were obstructed. The results suggested that the use of different types of landmarks was age specific.