Exposure affects the risk of an owl being mobbed – experimental evidence

Mobbing is a widespread anti‐predator strategy in birds, and predators are generally expected to avoid mobbing. For example, observational studies suggest that the cryptic roosting behaviour of nocturnal predators, such as many owls, may be a strategy to limit mobbing. In this paper, we present the results of the first experimental study investigating to what degree roost exposure influences the risk of being mobbed, and the intensity of a mobbing incidence once initiated. To determine these factors, we used an experimental setup with taxidermic mounts of tawny owls Strix aluco in Grib Skov forest, Denmark. The risk of an owl being mobbed during a 50 min morning survey period increased with the exposure of its roosting position, from 24% when hidden to 85% when openly exposed. The corresponding increase in the afternoon was from 6% to 36%. This suggests that an owl may minimize the mobbing rate by reducing the encounter rate with potential mobbers through its choice of roost. Once initiated, the duration of the mobbing (a proxy for the presumed cost of being mobbed) was independent of the roosting position of the mounted owl, but was positively correlated with the number of birds in the mob.     

Molecular evidence of Y-autosomal translocations in owl monkeys

Probe pDP1007, which contains highly conserved DNA sequences from the sex-determining region of the human Y chromosome, cross-hybridized with owl monkey EcoRI restriction fragments of 1.8 kb and 6.6 kb. Southern transfer analysis of owl monkey (karyotype VI)--rodent somatic cell hybrids localized the 1.8-kb fragment on the owl monkey X chromosome and the 6.6-kb fragment, which is male specific, on chromosome 14/Y. Regional in situ chromosome mapping of pDP1007 revealed specific sites of hybridization: the distal short arm of the X chromosome of karyotypes IV, VI, and VII; the small metacentric Y of karyotype IV; the C-band positive region on the short arm of chromosome 17/Y (karyotype VII); and the C-band positive region on the long arm of chromosome 14/Y (karyotype VI). These molecular findings reinforce cytological evidence that Y-chromosomal material has been transferred to autosomes 14 and 17 in owl monkeys of karyotypes VI and VII, respectively, in which there are no independently segregating Y chromosomes.     

Neighbourhood effects on the risk of an unpalatable plant being grazed

Most studies on the importance of the neighbourhood on a plant's risk ofherbivory have focused on palatable plants and how they are protected byunpalatable neighbours. This study examined the grazing intensity of arelatively unpalatable shrub, Buxus sempervirens, indifferent neighbours. Exactly 2683 plants of Buxussempervirens (including 172 controls) were sampled in 12 enclosedpastures belonging to 4 sheep farms. The enclosures were grazed at 3 differentseasons (spring, summer and autumn). Plants were divided in 4 age/hight classes(first year, < 4 cm, 4–10 cm, 10–40cm) and into 8 neighbourhoods. The first of these wascharacterisedby the absence of any plants within a radius of 5 cm around theBuxus individual and the 7 others by the identity of thedominant species in contact with the Buxus plant. Theintensity of grazing on the neighbouring plants were also recorded. At the endof one year's monitoring, 26.2% of Buxus sempervirensplants had been grazed. The proportion of plants grazed was significantlyhigherin spring than in the other two seasons. It decreased with increasing plantage.It was higher in neighbourhoods that were intensively grazed than in those withlight grazing. The proportion grazed in the absence of a neighbour plant wasintermediate between the previous two. The probability of a plant of aninvadingspecies being grazed is influenced by factors other than its life-historytraits. Some neighbourhoods consisting of unpalatable plants facilitate theestablishment of Buxus sempervirens by protecting theyoungplants from grazing, whereas other highly palatable neighbourhoods are readilygrazed by sheep, thus indirectly increasing the proportion of Buxussempervirens that are grazed. The young and short (< 4cm in height) Buxus plants, which are lessrecognisable by sheep, are most sensitive to the impact of grazing.     

Fluctuating food supply affects the clutch size of Tengmalm's owl independent of laying date

Summary In western Finland, yearly median laying dates of Tengmalm's owls varied from 14 March to 27 April during 1973–1989 and were negatively correlated with the winter densities of voles. Yearly mean clutch sizes varied from 4.0 to 6.7 and were more closely related to the spring than to the winter densities of voles. The yearly mean clutch size decreased with yearly median laying date. The 3-year vole population cycle is typical of the study area. The start of egg-laying was earliest in the peak phase of the cycle (median laying date 22 March), when vole numbers are high during egg-laying, but decline rapidly to low numbers in the next autumn or winter. In the increase phase (1 April) vole abundances are moderate at the time of laying, but increase to a peak in the next autumn or winter. In the low phase (15 April) voles are scarce in spring and in the preceding winter, starting to increase in late summer. Clutch size and female body mass were independent of laying date in the low phase, decreased slowly but significantly in the increase phase, and declined abruptly in the peak phase. These trends also held when the effects of territory quality, female age and male age were ruled out. When comparing the same laying periods, clutch sizes were significantly larger in the increase than in other phases of the cycle, but there was no difference between the peak and low phases. Supplementary feeding prior to and during egg-laying increased clutch size independent of laying date. These results agreed with the income model (the rate of energy supply during laying determines clutch size). Tengmalm's owls invest most in a clutch in the increase phase, as the reproductive value of eggs is largest because of high survival of yearlings. A high reproductive effort may be adaptive during this phase, because the availability of voles is predictable during the laying period.     

Sib‐sib communication and the risk of prey theft in the barn owl Tyto alba

Conflicts among siblings are widespread and their resolution involves complex physical and communication tools. Observations in the barn owl Tyto alba showed that siblings vocally communicate in the absence of parents to negotiate priority of access to the impending food resources that parents will bring. In the present paper, we hypothesize and provide correlative evidence that after a parent brought a food item to their progeny, sibling competition involves vocal sib‐sib communication. A food item takes a long time to be entirely consumed, and hence siblings continue to compete over prey monopolization even after parents gave a food item to a single offspring. When physical competition is pronounced and thereby the risk of prey theft is high, the individual that received a prey item consumes it in a concealed place. Concomitantly, nestlings vocalize intensely probably to indicate their motivation to siblings to not share their food item, since this vocal behaviour was particularly frequent in younger individuals for which the risk of being robbed is higher than in their older siblings. Furthermore, nestlings consumed more rapidly a food item when their siblings vocalized intensely presumably because the intensity of siblings’ vocalizations is associated with a risk of prey theft. Our correlative study suggests that sibling competition favoured the evolution of sib‐sib communication under a wide range of situations.     

Exposure to an additional alternating magnetic field affects comb building by worker hornets

Oriental hornet workers, kept in an Artificial Breeding Box (ABB) without a queen, construct within a few days brood combs of hexagonal cells with apertures facing down. These combs possess stems that fasten the former to the roof of the ABB. In an ABB with adult workers (more than 24 h after eclosion), exposed to an AC (50 Hz) magnetic field of a magnitude of B = 50-70 mGauss, the combs and cells are built differently from those of a control ABB, subjected only to the natural terrestrial magnetic field. The effects of the additional magnetic field consist of (a) 35-55% smaller number of cells and fewer eggs in each comb, (b) disrupted symmetry of building, with many deformed and imperfectly hexagonal cells, and (c) more delicate and slender comb stems.     

Variable but predictable prey availability affects predator breeding success: natural versus experimental evidence

Food supply is a major source of variation in breeding success for predators, and to what extent individuals are able to cope with temporal variability in food availability remains an outstanding question in life-history studies. We confronted the natural variation in clutch size and breeding success with results from a food supplementation experiment during egg formation, conducted over several contrasted years of natural food supply in an avian specialist predator, the Montagu's harrier Circus pygargus . This raptor mainly preys on common vole Microtus arvalis a cyclic microtine under temperate latitudes. Vole abundance together with timing of breeding accounted for most of the variance in clutch size and number of fledglings. Results from empirical and experimental data were overall in agreement. Fed pairs consistently increased clutch size compared with controls in all experimental years, whereas no effect of food supplementation on egg volume was detected. Supplemented pairs, however, did not fledge significantly more chicks than controls. The costs entailed by the increase in clutch size appear nevertheless to be limited compared with previous studies. Food supply seemed therefore to display sufficient predictability throughout a breeding season to afford individuals the opportunity to adjust their breeding effort to an optimal number of offspring, in agreement with Lack's anticipation hypothesis.     

Exposure to extremely low-frequency magnetic fields and the risk of childhood cancer: update of the epidemiological evidence

There is an ongoing scientific controversy whether the observed association between exposure to residential extremely low-frequency magnetic fields (ELF-MF) and the risk of childhood leukaemia observed in epidemiological studies is causal or due to methodological shortcomings of those studies. Recent pooled analysis confirm results from previous studies, namely an approximately two-fold risk increase at ELF-MF exposures ≥0.4 μT, and demonstrate consistency of studies across countries, with different design, different methods of exposure assessment, and different systems of power transmission and distribution. On the other hand, recent pooled analyses for childhood brain tumour show little evidence for an association with ELF-MF, also at exposures ≥0.4 μT. Overall, the assessment that ELF-MF are a possible carcinogen and may cause childhood leukaemia remains valid. Ongoing research activities, mainly experimental and few new epidemiological studies, hopefully provide additional insight to bring clarity to a research area that has remained inconclusive.     

Exposure to heat shock affects thermosensitivity of the locust flight system

The natural habitat of the migratory locust, Locusta migratoria, is likely to result in locusts being heat stressed during their normal adult life. It is known that locusts exhibit a heat-shock response: exposure to 45°C for 3 h induces thermotolerance and the expression of heat-shock proteins. We investigated the effects of exposure to heat-shock conditions on the thermosensitivity of flight rhythm generation in tethered, intact animals and in deafferented preparations. Heat shock had no effect on wingbeat frequency measured at the start of flight sequences, nor did it affect the postimaginal maturation of this parameter. During sustained flight, heat shock slowed the characteristic asymptotic reduction of wingbeat frequency. Wingbeat frequency of heat-shocked animals was less sensitive to temperature in the range 24° to 47°C than that of control animals, and the upper temperature limit, above which flight rhythms could not be produced, was 6° to 7°C higher in heat-shocked animals. These results were mirrored in the response of deafferented preparations, indicating that modifications in the properties of the flight neuromuscular system were involved in mediating the response of the intact animal. We propose that exposure to heat shock had the adaptive consequences of reducing thermosensitivity of the neural circuits in the flight system and allowing them to operate at higher temperatures. © 1996 John Wiley & Sons, Inc.     

Female attractiveness affects paternal investment: experimental evidence for male differential allocation in blue tits

Introduction

The differential allocation hypothesis (DAH) predicts that individuals should adjust their parental investment to their current mate??s quality. Although in principle the DAH holds for both sexes, male adjustment of parental investment has only been tested in a few experimental studies, revealing contradictory results. We conducted a field experiment to test whether male blue tits (Cyanistes caeruleus) allocate their parental effort in relation to female ornamentation (ultraviolet colouration of the crown), as predicted by the DAH.

Results

We reduced the UV reflectance in a sample of females and compared parental care by their mates with that of males paired to sham-manipulated control females. As predicted by the DAH our results demonstrate that males paired with UV-reduced females invested less in feeding effort but did not defend the chicks less than males paired with control females.

Conclusions

To our knowledge, this is one of the first studies providing support for male differential allocation in response to female ornamentation.     

Controlled simultaneous exposition to single mosquitoes – an alternative experimental design to study mosquito bite reactions

Until now a standardized method to evoke reproducible, qualitatively comparable, statistically analysable and precisely timed and located mosquito bites for experimental purposes was lacking. We here propose a simple procedure that meets all these requirements. In two studies, single Aedes aegypti and Anopheles stephensi mosquitoes of the same age that had been raised under defined conditions were placed in gauze-covered test tubes in direct contact to the human skin on predetermined sites. In most instances, the mosquitoes approached the skin in less than one minute, started probing through the gauze and took blood for a few minutes until they were fully engorged. In all cases, we succeeded in performing simultaneous bites of single mosquitoes, two of the same species on the same volunteer. Skin reactions resulting from the mosquito bites were tested within a clinical trial by comparing verum-treated and placebo-treated biting sites. In both studies, a reactive efflorescence appeared after each bite. The applied method for single-bite exposition allows for timing of the bites and predetermination of the biting sites. It is reliable, reproducible to a high degree, and easy to perform.     

Experimental increase of predation risk induces breeding dispersal of Tengmalm's owl

Nest predation and its avoidance are critical components of an individual's fitness and play an important role in life history evolution. Almost all studies on this topic have been observational, and thus have not been able to separate the effects of individual quality, habitat selection and predation risk of given nest sites from each other. More experimental studies on nest predation and breeding dispersal, therefore, are needed to avoid confusing interpretations of the results. In western Finland, pine marten (Martes martes) predation risk was experimentally simulated at the nests of Tengmalm's owls (Aegolius funereus) by using a caged American mink (Mustela vison) as a predator. Nests without exposure to a mink served as controls. In accordance with our predictions and earlier observational studies, males exposed to simulated predation risk increased nest-hole shift and breeding dispersal distances compared to control males. Nest-hole shift and long breeding dispersal distances probably decrease the risk of nest predation, because pine martens are known to revisit nest-holes they have found.     

Parasitoids on the loose – experimental lack of support of the parasitoid movement hypothesis

Forest fragmentation can disrupt important ecological processes both within and among species. Of particular interest is the extent to which fragmentation decouples economically important pest species from their parasites. In an influential paper, Roland and Taylor (1997) advanced the hypothesis that forest fragmentation at a relatively fine spatial scale would disrupt the ability of parasitoids to aggregate in response to host density, thereby facilitating the build-up of host populations at isolated sites. Our study provides the first experimental test of this notion. We augmented existing densities of forest tent caterpillar individuals at two types of sites: isolated forest fragments and continuous tracts of forest. Of the four parasitoid species studied by Roland and Taylor (1997), two were encountered in our experiment. For the sarcophagid fly Arachnidomyia aldrichi and the tachinid Carcelia malacosomae, we found no difference in parasitism rates among forest fragments and continuous forest tracts. A third tachinid species unique to this study, Lespesia frenchii , is known to be a broad generalist with respect to host choice. In this species, we observed significantly elevated parasitism in forest fragments, and higher parasitism at the forest edge than within the forest. All of these findings suggest complex effects of forest fragmentation on parasitoid movement, and warn against broad generalizations from observational studies. Forest fragmentation may affect movement patterns in a multitude of ways, significantly obscuring or modifying the simplest mechanisms proposed by Roland and Taylor (1997).     

Grazer diversity affects resistance to multiple stressors in an experimental seagrass ecosystem

When multiple stressors act simultaneously, their effects on ecosystems become more difficult to predict. In the face of multiple stressors, diverse ecosystems may be more stable if species respond differently to stressors or if functionally similar species can compensate for stressor effects on focal species. Many habitats around the globe are threatened by multiple stressors, including highly productive seagrass habitats. For example, in Chesapeake Bay, USA, regional climate change predictions suggest that elevated temperature and freshwater inputs are likely to be increasingly important stressors. Using seagrass mesocosms as a model system, we tested whether species richness of crustacean grazers buffers ecosystem properties against the impacts of elevated temperature and freshwater pulse stressors in a fully factorial experiment. Grazer species responded to pulsed salinity changes differently; abundance of Elasmopus levis responded negatively to freshwater pulses, whereas abundance of Gammarus mucronatus and Erichsonella attenuata responded positively or neutrally. Consistent with the hypothesis that biodiversity provides resistance stability, biomass of epiphytic algae that form the base of the food web was less affected by stressors in species‐rich grazer treatments than in single‐species grazer treatments. Stochastic (among‐replicate) variation of sessile invertebrate biomass within treatments was also reduced in more diverse grazer treatments. Therefore, grazer species richness tended to increase the resistance stability of both major components of the seagrass fouling community, algae and invertebrates, in the face of environmental stressors. Finally, in our model system, multi‐stressor impacts suggested a pattern of antagonism contrary to previous assumptions of synergistic stressor effects. Overall, our results confirm that invertebrate grazer species are functionally diverse in their response to environmental stressors, but are largely functionally redundant in their grazing effects leading to greater resistance stability of certain ecosystem properties in diverse grazer assemblages even when influenced by multiple environmental stressors.     

Leaf expansion – an integrating plant behaviour

Leaves expand to intercept light for photosynthesis, to take up carbon dioxide, and to transpire water for cooling and circulation. The extent to which they expand is determined partly by genetic constraints, and partly by environmental conditions signalling the plant to expand more or less leaf surface area. Leaves have evolved sophisticated sensory mechanisms for detecting these cues and responding with their own growth and function as well as influencing a variety of whole-plant behaviours. Leaf expansion itself is an integrating behaviour that ultimately determines canopy development and function, allocation of materials determining relative shoot : root volume, and the onset of reproduction. To understand leaf development, and in particular, how leaf expansion is regulated, we must know at the molecular level which biochemical processes accomplish cell growth. Physiological experimentation focusing on ion fluxes across the plasmamembrane is providing new molecular information on how light stimulates cell expansion in some dicotyledonous species. Genetic analyses in Arabidopsis, corn, and other species are rapidly generating a list of mutations and enzyme activities associated with leaf development and expansion. Combination of these approaches, using informed physiological interpretations of phenotypic variation will allow us in the future to identify genes encoding both the processes causing cell expansion, and the regulators of these events.