Do reproductive characteristics of the species explain differences in the investment in weapon size present in males?

Sexually selected traits, such as male weapons, are highly variable in shape and size across species. However, little is known about the mechanisms that may govern this variation. Because males with greater investment in weapon size tend to win more fights, but also pay higher costs related to weapon development and maintenance, larger weapons should be expected only in species in which victory in male–male fights generate reproductive benefits that outweigh investment costs. Here, we hypothesized that the reproductive characteristics that increase the chances of winners to access females or to fertilize eggs will favor the evolution of larger weapons. To evaluate this hypothesis, we conducted a meta-analysis using arthropods as model organisms. To measure investment in weapon size, we gathered both Pearson correlation coefficient and the standardized (but non centralized) slope values for the relationship between weapon size and body size. We found that none of the reproductive characteristics we investigated was related to male weapon size. Thus, it seems that greater certainty of accessing a female or fertilizing female eggs with a victory does not modulate the investment in male weapon size. Perhaps the cost–benefit ratio between weapon size investment and reproductive success is not the main factor driving the variation in weapon size.     

Do migratory pathways affect the regional abundance of wintering birds? A test in northern Spain

Aim The abundance distribution of organisms at regional scales is commonly interpreted as the result of spatial variation in habitat suitability. However, the possibility that geography itself may affect patterns of distribution has received less attention. For example, the abundance of wintering bird populations might be influenced by the cost of reaching areas located far away from the main migratory pathways. We studied the abundance distribution of three common migratory passerines (meadow pipits, Anthus pratensis; common chaffinches, Fringilla coelebs; and European robins, Erithacus rubecula) wintering in farmlands located in the 600‐km long Cantabrian coastal sector of northern Spain, roughly perpendicular to the west Pyrenean migratory pathway that drives European migrant birds into the Iberian Peninsula. Location The study area occupies a belt located between the Atlantic coast and the Cantabrian Mountains in northern Spain. Methods We counted wintering and breeding birds and measured the structure of vegetation and environmental variables (altitude, rainfall, temperature) in 68 farmlands distributed at different distances from the west Pyrenean migratory flyway. We also studied the distribution of birds ringed in central and northern Europe and recovered in the study area between October and February. Analyses were based on single univariate statistics (chi‐square tests), ordination by principal components analysis and multiple regression. Results Controlling for the effects of climate, vegetation structure and local abundance of breeding conspecifics, the winter abundance of all three species decreased with the distance from their main migratory route in the western Pyrenees. Such patterns fitted well to the observed distribution of ringing recoveries. Main conclusions Our results support a link between the movements of birds along the Pyrenean migratory pathway and their winter abundance in northern Spain. According to this view, the sectors located near the migratory pathway seem to be more easily occupied by migrants, supporting the idea that proximity to passage areas may explain the fine‐grain regional patterning of species abundance in wintering grounds.     

An empirical analysis of mtSSRs: could microsatellite distribution patterns explain the evolution of mitogenomes in plants?

Functional & Integrative Genomics - Microsatellites (SSRs) are tandem repeat sequences in eukaryote genomes, including plant cytoplasmic genomes. The mitochondrial genome (mtDNA) has been shown...     

Do kleptoparasites reduce their own foraging effort in order to detect kleptoparasitic opportunities?: An empirical test of a key assumption of kleptoparasitic models

Determining if, or when, individuals trade off time spent personally feeding against time spent monitoring others for kleptoparasitism opportunities is essential to an understanding of the evolution of scrounging and usurpation behaviours. We provide a first field test of whether kleptoparasites reduce their personal foraging effort in situations where the frequency and rewards of kleptoparasitism increase. We provided experimental food patches for wild European blackbirds that varied in the distribution of prey and that had a potentially high rate of kleptoparasitism within pairs of blackbirds feeding in them. Although individuals differed in their rate of kleptoparasitism, they did not vary in the size of the reward that they gained from kleptoparasitism. As prey became more clumped, kleptoparasitism rate and its reward per incident increased on average. There was, however, no evidence that individuals that were kleptoparasitising more quickly and/or at a higher frequency had lower personal foraging effort. In contrast, foraging effort increased in both birds compared to when they were foraging alone, independent of dominance, kleptoparasitic opportunity or reward. Our evidence suggests that in some circumstances a kleptoparasite can detect kleptoparasitic opportunities without compromising its own personal foraging rate.     

Do delayed effects of overgrazing explain population cycles in voles?

Theoretical models predict that delayed density-dependent processes with a time-lag of approximately nine months are sufficient to generate regular 3–5 year fluctuations in densities of northern small rodents. To examine whether this time-lag could be generated by plant-herbivore interactions, we studied delayed effects of overgrazed food plants on voles. We introduced field voles ( Microtus agrestis ) in four large predator-proof enclosures that had suffered heavy grazing during the preceding autumn and winter, and compared them with voles introduced to previously ungrazed control areas. We found no detrimental effects of previous grazing on population growth, reproduction or body condition of voles. Chemical analyses did not show consistent effects of grazing on nutritional components of common food plants (grasses). These results suggest that short-term population cycles of Microtus voles in grassland habitats are not primarily driven by delayed effects of plant-herbivore interactions.     

Do carotenoids buffer testosterone-induced immunosuppression? An experimental test in a colourful songbird

Testosterone (T) is hypothesized to be an important honesty reinforcer of animal sexual signals. Owing to its immunosuppressive effects, only those individuals that can immunologically withstand high T levels can develop the most exaggerated traits. To date, few studies have isolated phenotypic or genotypic buffers that provide 'high-quality' animals with such an advantage. Dietary carotenoid pigments may in fact confer such a benefit because when in high supply carotenoids boost immunocompetence and coloration in animals like birds and fishes. We examined the experimental effect of T elevation on carotenoid and immune status in male and female zebra finches (Taeniopygia guttata) and found that T was immunostimulatory in a generalized cell-mediated challenge. We also detected a significant interaction between T treatment and the change in plasma carotenoids that occurred during the immune challenge; the relationship between blood carotenoid change and immunity was positive in controls and negative in T-implanted birds. This suggests that, while correlationally birds with high carotenoid stores were inherently better at mounting strong immune responses, experimentally administered T induced birds to deplete carotenoids for maximizing their health. Our findings highlight a nutrient-specific mechanism by which animals escape high immune costs of T elevation and thus can still elevate ornamentation.     

Is military incompetence adaptive?: An empirical test with risk-taking behaviour in modern warfare

In battles, opponents exhibit positive illusions in both believing they can win. With great costs of failure and uncertain success, this represents extreme risk-taking behaviour. Conflict may be expected if one side is cornered, a sacrificial pawn in an overall war strategy, or demanded into action by politicians. However, in many cases even patently weaker forces fight despite nonviolent options. This is “military incompetence”, a failure in the assessment of winning probability. Previous explanations (stupidity, psychological deviance and cognitive constraints) have been rejected. Recently, Wrangham [Evol. Hum. Behav. 20 (1999) 3.] proposed that such risk-taking could be adaptive through one of two effects: (1) Performance Enhancement through exaggerated resolve or (2) Opponent Deception by bluffing. Although adaptive if they confer a tendency to win, both processes promote risk-taking behaviour and are therefore potentially responsible for military incompetence. These hypotheses can be distinguished because the Performance Enhancement hypothesis predicts positive illusions in any type of conflict. In contrast, the Opponent Deception hypothesis predicts them in battles but not in surprise attacks, where lack of communication disables any bluff. We conducted a test of these hypotheses using data collected by the US Historical Evaluation Research Organisation, mainly from the Arab–Israeli and Second World Wars. The Opponent Deception hypothesis is supported over the Performance Enhancement hypothesis, but other explanations are not ruled out.     

Does the spatial variation in selective pressures explain among-site differences in seed mass? A test with Buxus balearica

Positive effects of seed size on germination and survival can be offset by a greater probability of predation or a poorer dispersal of larger seeds. We hypothesized that spatial variation in local selective pressures acting on seed mass may lead to differences in both optimal and observed seed mass among discrete populations. We first examined the variation in seed mass across a total of 14 populations of Buxus balearica; in six of such populations, we further examined the temporal variation by measuring seed mass during three consecutive years. Second, we evaluated seed mass effects on different recruitment phases (seed rain to seedling establishment) in three populations. Lastly, for these three populations, we estimated the resulting overall phenotypic selection on seed mass during recruitment and compared the observed and the predicted optimal seed masses. Most variation (c. 70%) in seed mass occurred among populations, and although we found inter-annual variation in seed mass, the differences in seed mass among populations were consistent over time. Conflicting selective pressures on seed mass appeared during recruitment, and their direction and strength varied among populations, depending on the relative local importance of seed predation vs. germination and establishment. Observed seed mass matched predicted optimal seed mass in two of the three examined populations, suggesting local adaptative responses to the spatial mosaic of selective pressures.     

Do neophobia and dietary wariness explain ecological flexibility? An analysis with two seed‐eating birds of contrasting habits

The neophobia threshold hypothesis (NTH) suggests that the acquisition and maintenance of a high behavioral and ecological flexibility in the evolutionary and adaptive history of a species is the consequence of lower levels of neophobia towards new micro‐habitats and of dietary wariness of novel foods. To test this idea we assessed the degree of neophobia and dietary wariness in two seed‐eating bird species with contrasting degrees of ecological flexibility that inhabit the central Monte desert (Argentina): a grass‐seed specialist, the many‐colored chaco‐finch, and a generalist feeder, the rufous‐collared sparrow. We expected that both species would exhibit neophobia and wariness when faced with new foraging opportunities but that the rufous‐collared sparrow would be less neophobic and less wary than the specialized many‐colored chaco‐finch. Experimental indicators of neophobia and dietary wariness included willingness to eat near novel objects and willingness to eat novel seeds, respectively. Both species showed similar levels of reluctance to novelty, although the sparrow could be slightly more reluctant than the finch. Contrary to our predictions, the sparrow was neither less hesitant nor faster or greedier than the finch. This experimental evidence does not support a negative relationship between neophobia/wariness and ecological flexibility in these two seed‐eating birds and it coincides with the growing evidence that challenges the NTH. Some of our results provide support for the dangerous niche hypothesis, especially as the rufous‐collared sparrow, that feeds on more diverse and potentially dangerous food, showed higher levels of neophobia in some cases. Although the idea of neophobia and wariness being plausible causes of ecological specialization sounds attractive, the current situation calls for further research so that the causes of ecological flexibility in granivorous birds can be better understood.     

Can wind help explain seasonal differences in avian migration speed?

A bird's ground speed is influenced by the wind conditions it encounters. Wind conditions, although variable, are not entirely random. Instead, wind exhibits persistent spatial and temporal dynamics described by the general circulation of the atmosphere. As such, in certain geographical areas wind's assistance (or hindrance) on migratory flight is also persistent, being dependent upon the bird's migratory direction in relation to prevailing wind conditions. We propose that, considering the western migration route of nocturnal migrants through Europe, winds should be more supportive in spring than in autumn. Thus, we expect higher ground speeds, contributing to higher overall migration speeds, in spring. To test whether winds were more supportive in spring than autumn, we quantified monthly wind conditions within western Europe relative to the seasonal direction of migration using 30 years (1978–2008) of wind data from the NCEP/NCAR Reanalysis dataset. We found that supporting winds were significantly more frequent for spring migration compared to autumn and up to twice as frequent at higher altitudes. We then analyzed three years (2006–2008) of nocturnal migratory ground speeds measured with radar in the Netherlands which confirmed higher ground speeds in spring than autumn. This seasonal difference in ground speed suggests a 16.9% increase in migration speed from autumn to spring. These results stress the importance of considering the specific wind conditions experienced by birds when interpreting migration speed. We provide a simple methodological approach enabling researchers to quantify regional wind conditions for any geographic area and time period of interest.     

Can the scaling of plant nitrogen to phosphorus be altered by global change? An empirical test

全球变化可能引起土壤中各种养分的供应失衡,进而导致陆地植物中氮(N)和磷(P)元素化学计量学的不平衡。前人的研究报道了全球变化因子对植物N、P含量和N:P的影响,但鲜有研究探讨全球变化因子是否影响以及如何影响这两种元素之间的异速关系。本研究利用改变降水(增雨和干旱)、增温和N添加处理的野外控制试验,结合异速函数 N = βP^α (或对数转换后的线性关系：Log N = Log β + α Log P),检验了中国西北地区黄土高原半干旱草原植物N和P化学计量特征及N和P之间的异速关系对这些全球变化因子的响应。研究结果表明,干旱和增温均降低了植物P浓度,N添加增加了植物N浓度,这些都导致植物N:P增加。干旱、增温和N添加没有显著改变植物N和P浓度之间异速关系的斜率(即N和P浓度对数转换后的线性关系斜率α),但是显著增加了其截距(Log β)。这些结果表明,全球变化因子可能不会影响植物N和P之间的协同变化关系,植物中N和P之间可能存在着紧密的耦合关系。这些结果将有助于我们更好地理解全球变化背景下的植物养分动态和元素平衡。     

Do density‐driven mating system differences explain reproductive incompatibilities between populations of a placental fish?

Matrotrophy, the provisioning of embryos between fertilization and birth, creates the potential for conflict between mothers and embryos over the level of maternal investment. This conflict is predicted to drive the evolution of reproductive isolation between populations with different mating systems. In this study, we examine whether density‐driven mating system differences explain the patterns of asymmetric reproductive isolation observed in previous studies involving four populations of the matrotrophic least killifish, Heterandria formosa. Minimum sire number reconstructions suggested that two populations characterized by low densities had lower levels of concurrent multiple paternity than two populations characterized by high densities. However, low levels of genetic variation in the low‐density populations greatly reduced our probability of detecting multiple mating in them. Once we took the lower level of genetic variation into account in our estimations, high levels of multiple paternity appeared the rule in all four populations. In the population where we had the greatest power of detecting multiple mating, we found that multiple paternity in H. formosa typically involves multiple sires contributing to offspring within the same brood instead of different fathers contributing to distinct, simultaneously provisioned broods. Paternity was often skewed towards one sire. Our results suggest that differences between H. formosa populations in the levels of multiple paternity are not sufficient to explain the reproductive isolation seen in previous studies. We suggest that other influences on maternal–foetal conflict may contribute to the pattern of reproductive isolation observed previously. Alternatively, the asymmetric reproductive isolation seen in previous studies might reflect the disruption of maternal–foetal coadaptation.     

Do physical plant litter traits explain non‐additivity in litter mixtures? A test of the improved microenvironmental conditions theory

The decomposition rates of plant litter mixtures often deviate from the averaged rates of monocultures of their component litter species. The mechanisms behind these non‐additive effects in decomposition of litter mixtures are lively debated. One plausible explanation for non‐additive effects is given by the improved microenvironmental condition (IMC) theory. According to this theory, plant litter species, whose physical characteristics improve the microclimatic conditions for decomposers, will promote the decomposition of their co‐occurring litter species. We tested the IMC theory in relation to leaf litter and soil moisture in two contrasting moisture conditions in a dry subarctic mountain birch forest with vascular plant leaf litters of poor and high quality. The non‐additive effects in mass loss of litter mixtures increased when moisture conditions in litter and soil became more favourable for plant litter decomposition. The sign of this increase (antagonistic or synergistic) in non‐additive effects was more predictable for litter mixtures of poor litter quality. Although the specific mechanisms underlying the IMC theory depended on the litter quality of the litter mixtures, a standardized water holding capacity (WHC) was the litter trait most closely related to the non‐additive effects in mixtures of both poor and high quality litter types. Furthermore, we found that higher dissimilarity in WHC traits between the component litter species in a mixture increased synergistic effects in litter mixtures under limiting moisture conditions. However, under improved moisture conditions, increased antagonistic effects were observed. Thus, we found clear support for the IMC theory and showed that climatic conditions and leaf litter physical traits determine whether the non‐additive effects in litter mixtures are antagonistic or synergistic. Our study emphasizes the need to include litter physical traits into predictive models of mixing effects on plant litter decomposition and in general suggests climate specificity into these models.     

Can differences in phosphorus uptake kinetics explain the distribution of cattail and sawgrass in the Florida Everglades?

Background  

Cattail (Typha domingensis) has been spreading in phosphorus (P) enriched areas of the oligotrophic Florida Everglades at the expense of sawgrass (Cladium mariscus spp. jamaicense). Abundant evidence in the literature explains how the opportunistic features of Typha might lead to a complete dominance in P-enriched areas. Less clear is how Typha can grow and acquire P at extremely low P levels, which prevail in the unimpacted areas of the Everglades.     

Are population differences in plant quality reflected in the preference and performance of two endoparasitoid wasps?

In recent years, increasing attention has been paid in exploring the role of direct plant defence, through the production of allelochemicals, on the performance of parasitoid wasps and their hosts. However, few studies have determined if parasitoids can detect differences in plant quality and thus preferentially attack hosts on which their progeny develop most successfully. In this study we examined the development and preference of two endoparasitoids, Diadegma semiclausum and Cotesia glomerata , developing in larvae of their respective hosts, Plutella xylostella and Pieris brassicae . In turn, these were reared on different wild populations of black mustard Brassica nigra originating in the Netherlands and Sicily (Italy), as well as single cultivated strains of B. nigra and brown mustard, B. juncea . Chemical analyses of foliar glucosinolates and volatile emissions by P. xylostella -damaged plants revealed large differences between B. nigra and B. juncea plants, with smaller differences among the B. nigra populations. The four mustard populations differentially affected development time and body mass of the herbivores and parasitoids. Contrasts among the means revealed significant differences mainly between B. nigra and B. juncea . Both parasitoids, however, preferred to alight on plants in which their progeny developed most successfully. In behavioural bioassays, D. semiclausum did not discriminate among the B. nigra populations and preferred to alight on B. juncea , which was the best plant population for parasitoid development. By contrast, C. glomerata females exhibited the lowest preference for Italian B. nigra populations, on which adult parasitoid size was the smallest. These results reveal that parasitoids can detect even small differences in plant quality presumably through their volatile blends and that plant preference and offspring performance in the two species are 'optimally synchronized'.     

Do spatial patterns of clonal fragments and architectural responses to defoliation depend on the structural blue-print? An experimental test with two rhizomatous Cyperaceae

Clonal architecture is involved in performance of clonal fragments, as it determines spatial distribution of ramets. It is expected to rely on the species-specific expression of several architectural traits (structural blue-print). However, in contrasting environments, realized clonal architectures may differ, due to phenotypic plasticity. In this paper, we compared clonal architectures between two rhizomatous ecologically close Cyperaceae (Carex divisa and Eleocharis palustris) in non-defoliated and defoliated conditions. Two questions were addressed. (1) How much do the structural blue-print and resulting colonization and occupation of space differ between both species? (2) Does the structural blue-print constrain plastic responses of clonal architecture to defoliation? Traits related to performance, spatial pattern, architecture and biomass allocation of clonal fragments were monitored through an original non-destructive mapping method. In non-defoliated conditions, both species showed similar biomass but contrasting architectures and patterns of biomass allocation to rhizomes that resulted in different spatial patterns. The rhizome network of C. divisa, which consisted in only two primary rhizomes but several branches, was involved in resource storage rather than in spatial colonization. Conversely, E. palustris produced on average six primary rhizomes that grew in the whole horizontal plane, maximizing both occupation and colonization of space. These differences in structural constraints coupled with allometric relationships, resulted in differential responses to defoliation. In C. divisa, the costs associated to defoliation caused a decrease in branching, limiting the area occupied and number of ramets produced by clonal fragments, but increasing ramet density. Conversely, the weakly branched rhizome network of E. palustris was not affected by defoliation. Both spatial strategies (consolidation vs. colonization) are likely to provide ecological advantages allowing their coexistence in grazed meadows.     

Does attraction to conspecifics explain the patch‐size effect? An experimental test

Recent theory suggests that attraction to conspecifics during habitat selection can be one potential, yet untested, mechanism for animal sensitivity to habitat fragmentation. The least flycatcher Empidonax minimus , a highly territorial migratory bird, has previously been shown to be attracted to conspecifics and sensitive to patch size by avoiding small patches of riparian forest in Montana, USA. I used a large-scale field experiment in this region to test the conspecific attraction hypothesis for explaining sensitivity to patch size, and I supplemented this experiment by estimating whether vegetation structure, nest predation, or nest parasitism rates could better explain patterns of sensitivity to patch size. Vegetation structure did not vary consistently with patch size, based on a random sample of patches across 150  km of the Madison and Missouri Rivers, Montana. Nest predation and parasitism rates by brown-headed cowbirds Molothrus ater also did not vary with patch size during the experiment. However, when conspecific cues were simulated across a gradient of patch sizes, flycatchers settled in all patches – and their sensitivity to patch size vanished – providing strong support for the conspecific attraction hypothesis. These results provide the first experimental evidence that attraction to conspecifics can indeed help explain area sensitivity in nature and highlight how understanding the role of animal behavior in heterogeneous landscapes can aid in interpreting pressing conservation issues.     

Are pollen records from small sites appropriate for REVEALS model-based quantitative reconstructions of past regional vegetation? An empirical test in southern Sweden

In this paper we test the performance of the Regional Estimates of VEgetation Abundance from Large Sites (REVEALS) model using pollen records from multiple small sites. We use Holocene pollen records from large and small sites in southern Sweden to identify what is/are the most significant variable(s) affecting the REVEALS-based reconstructions, i.e. type of site (lakes and/or bogs), number of sites, site size, site location in relation to vegetation zones, and/or distance between small sites and large sites. To achieve this objective we grouped the small sites according to (i) the two major modern vegetation zones of the study region, and (ii) the distance between the small sites and large lakes, i.e. small sites within 50, 100, 150, or 200 km of the large lakes. The REVEALS-based reconstructions were performed using 24 pollen taxa. Redundancy analysis was performed on the results from all REVEALS-model runs using the groups within (i) and (ii) separately, and on the results from all runs using the groups within (ii) together. The explanatory power and significance of the variables were identified using forward selection and Monte Carlo permutation tests. The results show that (a) although the REVEALS model was designed for pollen data from large lakes, it also performs well with pollen data from multiple small sites in reconstructing the percentage cover of groups of plant taxa (e.g. open land taxa, summer-green trees, evergreen trees) or individual plant taxa; however, in the case of this study area, the reconstruction of the percentage cover of Calluna vulgaris, Cyperaceae, and Betula may be problematic when using small bogs; (b) standard errors of multiple small-site REVEALS estimates will generally be larger than those obtained using pollen records from large lakes, and they will decrease with increasing size of pollen counts and increasing number of small sites; (c) small lakes are better to use than small bogs if the total number of small sites is low; and (d) the size of small sites and the distance between them do not play a major role, but the distance between the small sites and landscape/vegetation boundaries is a determinant factor for the accuracy of the vegetation reconstructions.